IMAGING

Astro photography is one of the more interesting, and frustrating, ways to expand the hobby. Colors and details you cannot see with your eye are easily picked up in a photo. And, if you can achieve consistent results, and have the time, many university programs need amateurs to collect data for various projects.

As is painfully obvious with the images I have on this web site, I am far from an expert. I learn something new every time I take a camera out and my images keep getting better.

This is presented as a "What I Have Learned", not as an expert reference. Just some hints and suggestions that I have either learned the hard way, or been given by others.  I suggest scanning through the entire page, as I have grouped things by Subject, instead of constantly repeating what is in other sections.

Photo Type

Equipment

 

Processing


Wide Field

Afocal

Prime Focus
       Webcam
SLR
CCD

Scopes

Mount

Camera

Software

Focusing

Guiding

Collimation

Seeing

Image Size

Image Processing

Webcam

CCD & Digital

All Images

 

WIDE FIELD

The simplest astro images are simple photographs of constellations, asterisms, star trails, etc.   Point a camera in the sky and open the shutter for an extended time.  Experimentation will tell you how long you must keep the shutter open on a digital camera to minimize noise in the image.  Film will eventually fail, there are tables on the web that list reciprocity failure times for different films.

Want to eliminate the star trails and have nice, sharp star images?  Either mount the camera on a telescope mount, or a cheaper (maybe free) way is to build a Barn Door Tracker.  Do a Google search and you will find hundreds of pages with info on building and using one. 

Barn Door Tracker

A two-arm Barn Door Tracker I built many years ago.  The two-arm version has less tangent error than a single arm resulting in sharper images on longer exposures.  The hinge line has to be at the same angle as your latitude and aimed directly at Polaris.  The camera can be mounted on a ball mount and point anywhere in the sky.

Barn Door Tracker 

The threaded rod is rotated at 1-rpm.  I usually turn it 1/2 turn every 30 seconds.  Longer lenses will require a shorter time period, I have had fair results at 200mm and 1/4 turn every 15 seconds.  Be careful you don't shake the setup when you turn the rod.  The dimensions you build the tracker to are very critical so that 1-rpm on the rod matches the rotational movement of the stars.  Lazy?  Mount a 1-rpm motor and have continuous tracking with no intervention on your part. 

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AFOCAL

A simple way to begin using a telescope and camera for astro photography is with a process called "Afocal Photography". You simply hold a camera, with lens, to the eyepiece and snap a photo. This can work amazingly well with bright objects that require a very short exposure. Longer exposures require some means of holding the camera steady to the eyepiece. Televue, Scopetronics, Orion, and others make a variety of adapters to either hold your camera up to the eyepiece, or better yet, attach the eyepiece firmly to the filter ring of your camera lens.

AFOCAL

AFOCAL

These two pictures show a simple Afocal camera mount I built to hold virtually any camera to a 1 1/4" eyepiece.  It could easily be modified for 2" eyepieces.  The main drawback to this system is the weight.  Orion has a lighter weight version, but isn't as versatile.   My page on building this camera mount. 

AFOCAL

Scopetronics eyepiece adapter attached to a Celestron Nexstar 25mm Plossl and the same camera as above. 

Scoptronics has been in and out of business, currently they seem to be gone for good.  Televue, and some others, make the same style adapters for their eyepieces that attach to a standard T-Ring for your SLR and/or adapters for digi-cams.   

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PRIME FOCUS

In "Prime Focus" photography, you attach a camera body directly to the focuser or visual back. No lens or eyepiece is normally used.

Webcam

Many off the shelf webcams can be easily adapted to a telescope for imaging of bright objects, usually planets. Meade, Celestron, Orion, and others market planetary cameras that are little more than glorified webcams. These are a great way to get your feet wet in imaging and can take fantastic planetary images.

Webcams are not normally sensitive enough to image deep sky objects, though if you search the web you will find some folks that have modified webcams and have taken some pretty good images of faint fuzzies.

The best way to image with a webcam is to take a 20-30 second video. I prefer AmCap to run the camera. Windows can capture video but it is painful. Maxim DL Essentials for webcams is even worse. K3CCDTools is also very good, but is not free, though it is the most versatile webcam control program I have seen.

Webcam

Orion StarShoot Solar System Color Imager II 

SLR - Film or Digital

Disclaimer - I rarely use an SLR for astro imaging.  Having a couple of SBIG cameras I prefer to do my deep sky imaging with them.  However, I do pull out the Nikon for bright comets, lunar photos, planets, and such.  Mostly due to the much faster shutter times I can run versus the SBIG.

You will need a T-mount ring for your camera.  One side fits the bayonet mount on your camera body, the other side is a standard M42x0.75 internal thread.  Many focusers and visual backs have a T-thread on the end, others don't.  If not then you can get a 1.25 or 2" nosepiece with a T-thread on it.   There is also a T2-thread, which is an external thread, mostly used on Baader and Televue imaging accessories.  Don't mix them up when ordering parts...

Conventional astro filters can also be used on your camera.  Mostly available in 1.25 and 2 inch offerings.  Using one will require the use of a nosepiece on your t-ring, instead of threading the t-ring directly on the visual back or focuser.  Exposures will need to be increased significantly, but many difficult to image nebulas will jump right out with the proper filter.  The biggest problem is going to be focusing.  SLRs are difficult enough to focus on a scope, adding a filter, like an Oxygen III that passes a very narrow bandwidth, will make it almost impossible.  And they will change the focus, so focusing without one and then putting the filter on will not work.  DSLR users have a significant advantage here - boost the ISO as high as possible and chimp the image after each focus adjustment, just remember to lower the ISO before taking the final images.

Use a remote release to trip the shutter, touching the camera while imaging is bad.  If your camera has a mirror lock-up feature, use it.  The mirror banging back and forth causes vibrations that may cause blurring, depending on the focal length and exposure time.  If you cannot lock-up the mirror, then a hat placed in front of the scope prior to opening or closing the shutter will solve the problem.

Either use one of the built-in shutter times or watch your clock.  If you are going to stack multiple images they all really need to be close to the same exposure time.  Exposures of differing times can be used to enhance some details, but requires extra processing steps to achieve what you want.

Normal SLR camera focusing screens are dim enough that you may not be able to see anything in them. A flip mirror that you have carefully focused to your camera in daylight can be a major help here. If your camera has interchangeable screens, there are focus screens made specifically for astrophotography that can be a major improvement. Other SLRs will require you to send the camera to a specialist for a focus screen replacement. Keep in mind that changing the screen may change your light meter and it will require recalibration for normal photography.

Don't forget that your SLR lenses are little more than fancy telescopes.  Many objects can easily be imaged with lenses that you have, and the moon is bright enough that spot metering will usually enable all of the cameras auto functions to make it even easier.   One of my favorite SLR only images is Comet Holmes. 

Film

Exposure time is limited by the film reaching its reciprocity point - essentially when the film starts to loose contrast and washes out. Tables are available that list the maximum exposures for various films.  Multiple exposures can be stacked in the darkroom, or digitally, to boost the image.

Film can be "Pushed" or "Hypered" to increase sensitivity, but unless you can do your own processing, or have a true photo lab in your area, these are mostly options of the past. 

Today, most photo labs only do automated processing.  Their machine tries to print a normal, daytime type of contrast, so don't be surprised if your prints look bad, really bad.  Most astronomers use fast slide film, scan the slide, and press on with Photoshop.  If you have access to a darkroom, or can find an old fashioned lab, then you can get decent chemical prints.

Digital

Digital exposures are limited by your patience, object brightness and mount quality.  Usually anything over a couple minutes is too much.  However, multiple exposures can be stacked to boost the signal-noise ratio and obtain a good image.  Total exposure times of 30 minutes or more is common for high quality final images.

DSLRs have a long wavelength filter in front of the chip.  The bad news is that it absorbs much of the hydrogen color lines.  If you want to take great photos of nebulas and such you will need to have this filter replaced (Baeder makes a very popular replacement for most cameras).  The camera will continue to work fine for normal use, just with a significantly increased red sensitivity that you may be able to reduce in the camera setup menus.  If this is going to be an astro only camera, you may get by with just having the filter removed.  Canon made the 20a for a few years, which was optimized for astro use.  They are a hot item on the classified postings.

If your camera includes a RAW format use it. You will find that you need as much original data as possible to process a good image; a JPG has already had too much corruption done to it by the camera trying to make it into a normal light image. Remember, these cameras are optimized to take great, normal light pictures.

Noise Management

Avoid using ANY on-board noise reduction, as it will usually remove fainter stars and reduce the image sharpness - ensure any noise reduction options are OFF.   Instead, immediately after taking your images, put the lens cap on and take a couple of dark images at the same exposure length as the light images.  You need at least two for an "Average" combined dark, and three or more for a "Median".  Median combined dark frames tend to work better.  The camera needs to be at the same temperature as when taking the light images so don't go inside to do this - noise is temperature dependent. 

Some expert DSLR imagers recommend taking a couple of dark frames immediately after each light image - they state that DSLRs continue to increase in chip temperature the longer they are turned on, and waiting to take your dark frames at the end of the night won't give you as good of a dark calibration.

Keep your ISO as low as possible, the higher the ISO, the more noise the image will have. It is usually better to take two 60 second images at ISO 400 than one 60 second image at ISO 800.  And four 60 second images at ISO 200 will be even better.  However, some objects will require a higher ISO to pull anything out of the noise.  Experiment, experiment, experiment....

Astro photography pushes Digital SLRs way beyond their design limits, but with practice and patience you can take some great images with them.

DSLR

Nikon D-70s attached to the visual back with a T-ring

CCD

This is the realm most astro photographers aspire to. Not many amateurs have the patience, or quality mount, to take good astro photos with film. Film generally requires one long exposure; a CCD can be automated to take numerous shorter exposures and then combine them to boost the signal-noise ratio to a decent image level.  Digital SLRs present their own unique problems.  With a CCD, and a reasonable mount, anyone can take good images of deep sky faint fuzzies.

There is a very steep learning curve to these beasts.  Don't buy one and expect to take publication quality images the first night out.  Take a lot of time to read the manuals for the camera and software.  Surf the web - there are many sites dedicated to imaging with a CCD.  Best bet is to seek out help from a fellow CCD imager. 

A CCD camera has none of the image quality drawbacks that digital SLRs have - no long wavelength filter, no image distorting noise reduction, etc.  On the other hand, they do require you to pack along a computer, and one or two deep cycle batteries.

A quality CCD camera has cooling. I personally wouldnít buy any of the cheaper cameras that are advertised for deep sky use that are not cooled. CCD chips generate heat, which generates electronic noise.  The longer the image, the better the signal - noise ratio.  Cooled cameras have significantly lower noise levels allowing for a shorter overall exposure time.  Adjust the cooler temperature to be running at 80-90% power (CCD programs have a window that displays the cooler temp and current power draw).  Less than that and you will have noise you don't need, more than that can cause some noise issues too, not to mention running your battery down quicker.  Summer nights I can easily run at -20C and on cold winter nights I have occasionally reached -40C.

Anti-Bloom (AB) versus Non-Anti-Bloom (NAB):  My ST-8E is NAB.  Many images I can only run 60 seconds before a bright star causes a serious bloom to erupt across the chip.  We went NAB because they are significantly more sensitive than AB cameras, and if you want data from the image - magnitude, photometry, etc. - then you MUST use a NAB camera.  I would really like to add an AB camera to the arsenal just for taking pretty pictures.

While single-shot color cameras may look appealing, they donít have near the resolution or sensitivity of a monochrome camera. If all you want is pretty pictures, they are a viable option. For data or capturing real faint fuzzies a monochrome camera is the way to go. Color images are obtained with monochrome cameras by taking a series of images through color filters. Some cameras may require you to manually install each filter as needed, other cameras have color wheel options available and will automatically switch the filters as needed. The most common color filters are red, blue and green (RGB). Some folks use cyan, magenta and yellow filters (CMY) instead. Processing images taken with CMY is more involved than RGB images. Which you use primarily depends on what your personal preference is, though CMY filters can work better with many nebulas.

CCD cameras are real difficult to image planets with. I have to use a variable polarizer maxed out to get the brightness down to a point the camera can handle. The ST-8E has a minimum exposure of 0.11 seconds and without substantially reducing the brightness it instantly blooms. Running a tri-color image of Jupiter doesnít often work because the rapid rotation of Jupiter will smear the final compiled image. The best bet for planets is to use a normal camera or a webcam.

ST-8E

ST-8E

ST-8E / LX-200

SBIG ST-8E with colorwheel, and JMI NGF-S focuser on the 10" f6.3 LX-200. Lots of cables, lots of length, lots of weight

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IMAGE SIZE

The image size is determined by the focal length of the scope and the physical size of the imaging chip. If you need to increase image size, an eyepiece projection adapter (a camera adapter which holds an eyepiece inside the tube) can be used. The length of the tube can either be adjustable or fixed. Barlows and Powermates can also be used with good results.

Field of View

  SBIG ST-8E
9 x 9 micron pixel
1530 x 1020 pixels
Size - 13.8 x 9.2mm
Nikon D-70s
7 x 9 micron pixel
3008 x 2000 pixels
Size - 21.1 x 18mm
Orion SSCI II
5.2 x 5.2 micron pixel
1280 x 1024 pixels
Size - 6.45 x 7.95mm
Scope Focal
Length
Image Scale
(arcsec/pixel)
Field of View
(arcmin)
Image Scale
(arcsec/pixel)
Field of View
(arcmin)
Image Scale
(arcsec/pixel)
Field of View
(arcmin)
TV-60is 360 5.15 87.6 x 131.3 4.58 171.7 x 200.8 2.98 50.8 x 63.5
ST-80 400 4.64 78.8 X 118.2 4.12 154.5 x 180.7 2.68 45.7 x 57.1
ETX-90 1242 1.49 25.4 X 18.1 1.33 49.8 x 58.2 0.86 14.7 x 18.4
LX-200 10" f6.3 1575 1.18 20 x 30 1.05 39.2 x 45.9 0.68 11.6 x 14.5
Celestron C8 2000 0.93 15.8 x 23.6 0.82 30.9 x 36.1 0.54 9.1 x 11.4
NGT-18 2025 0.92 15.6 x 23.3 0.81 30.5 x 35.7 0.53 9 x 11.3

For afocal imaging, the image size is a combination of the scope, eyepiece and camera lens focal length. Zoom lenses will allow adjustment of the image size without changing eyepiece or lens. Many digital cameras also have a digital zoom. Do not use the digital zoom! They will significantly degrade the image.

OK, not all of the combinations listed above are practical.  However, I have taken some images with the ST-8E and ST-80.  What a fun challenge.  And the ETX does not make a good imaging platform without  a lot of patience and tinkering.

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GUIDING

If your images are going to run in excess of one minute, most mounts will require some form of guiding. Images of less than a minute can usually be taken on the average amateur mount without much difficulty. Either way, the mount must be accurately polar aligned.

An off-axis guider, which has an eyepiece and a small prism to allow you to look at a guide star near the image light path, may be used to manually guide a long exposure. But, in todayís electronic age there is little reason to spend hours squinting through a very dim guider to manually guide the scope.

Even a simple webcam can be setup to autoguide on a bright star using a second, inexpensive scope, mounted on the primary instrument. Both scopes need to be closely aligned to get accurate guiding.

CCD cameras may use a mode called "Track and Accumulate" where the software switches the camera between guide and image modes at a user set interval and adjusts the mount, or in some cameras there are two image chips - one for imaging and the other runs full time as a guider.

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MOUNT

Nothing will demonstrate just how poor consumer grade mounts are until you try to image with one. The typical LX-xxx, Nextstar, and such are just barely usable and will require some adjustment and tuning before you can begin to achieve consistent results. These mounts are great for visual use, but fall short for great images. Iíve spent hours working on our LX-200. One night it tracks flawlessly, the next two nights it will be all over the chip. Arghhhhh! Iíve also got a Celeston Advanced GT mount (german equatorial) and it gives the same results.  

I finally bit the bullet and ordered an AP900.  Wow, is it sweet.  No random wild jumps, mild periodic error that is easily corrected with PEC.  A lot of money, but worth every penny.

The first thing you will learn is that you need to frequently clean and lube the gears. A quality lube that is fairly temperature indifferent is needed. Over time, the lube gets sticky, chunky, squeezed out, full of dirt, etc., creating wild random tracking errors.  Lithium based grease seems to work the best.  I prefer Pro-Gold, a gun grease available at many gun shops.

Dynamic balance is a major help.  If you are running a piggyback scope on a fork mount, the weight needs to be the same distance from the axis as the piggyback setup is, otherwise you won't be able to balance the scope vertically.  Point the scope straight up and adjust the weights until it will stay vertical.  Move the scope to a horizontal position and slide the weights, or telescope, back and forth along the axis until the scope stays horizontal.  Recheck the vertical.  Etc.   Add a little weight back to the East to keep the RA gears lightly loaded, otherwise the gears may tend to oscillate the image as the scope bounces back and forth.

Good polar alignment is a must. I find the polar alignment routine on the LX-200 works rather well, provided I run it about 5 times with several minutes between iterations. The GT mount does a lousy job of aligning itself Ė about the only way to polar align it is to do a drift alignment.  AP has excellent (accurate) markings on the mount and a good alignment routine built-in, Roland also provides instructions for alternate alignment methods that work real well.  Never thought a carpenters level could be so useful...

Visual drift alignment requires an illuminated reticule eyepiece, you cannot get it close enough without one.  A CCD image of one minute or more can be used instead of using an eyepiece.  Some of the latest CCD software can "solve" where it is aimed by comparing the image to star catalogs.  It can also "solve" polar alignment adjustments, telling you how much to adjust the scope.  Most written instructions for drift alignment I've seen are very confusing.  Here is an excellent animation of drift aligning I found.

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SEEING

If the image is bouncing around in the eyepiece, donít waste time trying to image. Even nights that look good may have enough turbulence that your stars are bloated instead of nice pinpoints.

You will notice most of my star images are bloated, thatís the price for living along the Colorado Front Range. Thereís terrible turbulence coming over the mountains almost every night. I either have to drive about 80 miles east to get away from it, or go high in the mountains to where the airflow is more laminar.

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COLLIMATION

You eye is easy to fool, a camera isnít. Take the time to check and refine your collimation whenever you image. The gross collimation instructions in virtually every telescope manual works OK for visual use, but for imaging it has to be tighter.  Off by the least and your stars will look funny.  Use the highest power eyepiece that you can bring to a decent focus, installed in the scope just like the camera will be mounted. In other words, if you are not using a diagonal with the camera, donít use a diagonal when collimating.  Take a centered, bright star just out of focus so you can see the diffraction rings and adjust the collimation to make the rings concentric. Center the star after every adjustment before evaluating if you need to make more adjustments.  If seeing allows you to get around 600x make the final tweaks using the in focus airy rings.

It is easy to tell when the scope needs collimating when Iím imaging with the ST-8E; the stars on the guider chip are all shaped like gullsÖ

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FOCUSING

Focusing the camera is the hardest thing to do while imaging.

I have taken images with a cheap rack and pinion focuser, but it isn't easy. They typically have a lot of flex in them, and the smallest turn of the knob can zoom right through focus and out the other side, while bouncing your object all over and probably out of the frame. A quality focuser, properly adjusted, can make it so much easier. Good ones will have no image shift while focusing back and forth. A single speed manual focus is usable and a two speed is sweet, but better yet is one that you can focus electrically so you are not touching and jiggling the scope.

SCT scopes have horrible image shift when you reverse the focuser; an add-on Crayford, such as the JMI NGF-S, makes focusing much easier. Meade has finally addressed the image shift issue by including a Crayford for fine focusing on their latest SCTs. Another SCT issue, not really related to obtaining focus, but rather maintaining focus and image location in the frame, is due to the mirror shifting. As the scope tracks across the sky it is inevitable the mirror will shift during one or more exposures. Many SCTs have a shipping hold down bolt; if your scope does then a long bolt, nut, washer and spring can be installed there to maintain some tension on the mirror, yet not interfere with your focusing the scope.  Other SCTs will require you to tear the back end down and install springs internally to tension the mirror.

SCT mirror tension

Mirror tension device on a 10" LX-200.  If your scope does not have a shipping hold down bolt this system won't work.

Seeing conditions will often have afocal and webcam images dancing around. Make small adjustments when you think you are close, then sit back and watch for a bit. Make another adjustment, watch and decide if you gained or lost anything. Often, I will take a series of exposures, each with a slightly different focus to ensure I was right on at least one of them. The brightness and contrast of the monitor will often fool you on setting a decent exposure, so I always bracket my exposures a couple of f-stops or shutter speeds.

CCD focusing is actually the easiest of the bunch. Most camera control software has a focus mode and displays some sort of data (in Maxim DL it is called "Inspection"). Several different values are displayed - what you are initially after is the "max value" to be at the highest number you can get (adjust your exposure to be about 1/3 of the max value your camera can handle). When close to focus, the "max value" will start jumping around, so watch the "FWHM" (full width half max) to dial in the smallest number. If you canít decide between a couple of focus positions, take 3 quick images at each focus and average the values of one of the stars, highest average is the best focus position.

There are also software/hardware solutions to automate focus. I modified our JMI NGF-S focuser with a Robo-Focus.  Wow.  Once you get the proper settings (step size, optic configuration) set in Maxim it only takes a minute or two for it to focus better than I usually do by hand.  Well worth the money.

Another option is to use a manual focus aid. Newtonians have a spider, or on a non-Newtonian, try an "X" of masking tape across the dew shield, or a cardboard mask with two or three holes cut in it, or Ö Any of these methods will either create diffraction spikes or a double/triple image. Adjust the focus until the spikes are single or the multiple images merge into one.

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SCOPES

Everyone wants to take the ultimate, up-close, publication quality image. The reality is these are extremely difficult to do. Increased focal length, through the use of a longer scope, barlow or eyepiece projection, will make the image more sensitive to tracking errors and seeing conditions. Match the focal length to your conditions and you will have a better rate of success.  My LX-200, at 1600mm, will usually have bloated stars. My C-8 and NGT-18, at 2000 & 2025mm, are even worse. When seeing permits, they all take some great pictures, the rest of the time I am rarely satisfied with the end result.

Short refractors are hard to beat. I have taken some nice pics with a Short Tube 80 (400mm), though the brighter stars have a violet halo around them, the rest are fairly decent pinpoints. After a few years I got a Televue 60is and WOW, does it work well.  No halos, sharp stars, easy to focus. 

Short focal length scopes tend to distort the field so some form of a field flattener is required, especially if you are going to try to assemble multiple images into a mosaic.  Longer scopes can be outfit with a focal reducer to increase the field of view and decrease exposure times.  The down side is they tend to vignette the field a fair degree so again issues with assembling mosaics will be encountered.  Occasionally longer scopes will also have a degree of field distortion.  Contact the  manufacture and inquire as to the size of the flat field of any optical tube you are interested in.  Using a scope with a larger flat field than your imaging area will result in distortion free images.

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PROCESSING

Okay, you now have a bunch of horrible looking images. What now?

Webcam

Registax is your friend, and it is FREE! Documentation is very weak, some good tutorials and such on their web site though. Lots of options, lot of experimenting to see what works.

I try to set the Quality Filter at 90, sometimes I have to lower it to 85 or 80 to get enough frames to make a quality image

Adjust the levels (histogram) and brightness/contrast before adjusting the wavelets.

CCD & Digital Camera/SLR

I prefer Maxim DL for processing images and then I run them through Photoshop. There are no hard and fast rules, just keep experimenting with them until you get the best image you can. I go back and reprocess images I took a few years ago as I learn new techniques and hints. 

Sorry, I can't provide hints for CCDOPS or Astroart or whatever.  Maxim DL, Registax, and PS are the limits of my astro software knowledge.  (I also have AIP for Windows, but I haven't played with it on a long time.  No real reason, it is very good stuff, and the book is priceless for learning the hows and whys of image processing)

The astro software you are using should have good image processing instructions. Also, read Ron Wodaskiís New CCD Astrophotography Ė lots of good hints. Another source I use a lot is Jerry Lodriguss' web site.

Things that have made the biggest improvements in my images:

A quality mount.  Not only does my AP900 look nice, it works extremely well.  It has much less periodic error, random error, etc.  Stars are much tighter and rounder.  Sweet.

Exposures

Take as many as possible

Make the exposure time as long as possible.  One 5 minute exposure will have a better signal - noise ratio than five 1 minute  exposures stacked.

Remove gradients

 Gradients can be caused by either uneven sky illumination (sky  glow) or internal causes your camera.  Bias and flat frames can remove a lot of gradients automatically, but you still will often still have additional gradients to remove.   Your imaging software should have some form of gradient removal process.

Process your frames together, but remove gradients prior to color combining if possible. 

Maxim DL

The auto-remove gradient process works fairly well, but it has issues with frames with multiple gradients.  In that case I frequently just combine the colors and mess with it in Photoshop.  I have not had a lot of luck manually removing gradients in Maxim, probably an issue with me, not Maxim.

Photoshop

I open two copies of the frame.  Really boost the levels of one so you can easily see the gradients.  On the other frame set a mask and use the gradient (gray) that best fits the direction and style of the gradient.  Adjust the level of the mask to even the frame out the best you can.  Create another mask level and do it again for the next gradient.

Color balance

If you are using a normal RGB filter set

The filter manufacture can probably tell you what the proper offsets are.  For instance, SBIG states that my filter set has offsets of R-1, G-1.5, B-3.  I take equal exposure sets with each filter and in Maxim I enter the offsets in the color combining process and they come out real nice.  You could also adjust your exposures by the proper offset and not enter an offset when you combine the colors.

If you are using a one-shot color camera

I will let you know, there is a new SBIG on the brown truck.

G2V color balance

If all else fails, or you really want to tweak the best color from your setup, you can run an image set on a G2V star and then adjust your settings to get the proper color balance.  A real good tutorial I found is here - http://www.astrodon.com/Orphan/g2v_tutorial/

Personal preference

There is nothing wrong with adjusting the color balance to suit your purpose.  I do think if you are going to sell or otherwise represent the image as "real" then the colors ought to be right, otherwise have some fun.

Manually normalize the backgrounds prior to combining color images.  The actual ADU value is not super critical, but all of the RGB frames need to be at the same value, and should never exceed the levels of the Luminance frame.  I find the Luminance frame is not as critical, but still needs to be in the ballpark of the RGB frames.  Many experts suggest 50 ADU as a good starting point.

In Maxim DL open the Information dialogue.  Set it to Area.  Pull a box in a couple of background areas and write down the average values.  Ensure there are no stars in the areas you select.
Open the Pixel Math process.  Leave the Operation set to None, and Add Constant to -xxx, whatever the value you need to subtract .

I honestly don't have clue how to do this in Photoshop.  I figure there is a way, but it is so quick and easy in Maxim I haven't found a need to do it in PS.

Deconvolution should only be run on the Luminance frame or a straight monochrome image. Lucy-Richardson tends to work best on nebulas & galaxies, Max Entropy works great on stars and clusters

Digital Darkroom or FFT filters work well on combined images, monochrome or color

Major Hint:  Invert and Multiply the individual frames together in Photoshop to really drive the signal - noise ratio up!!!!!  Works best with low contrast images, or you can sometimes salvage an imaging session if you didn't take enough frames for a quality image.  I do this frequently with the Luminance frames, save as a TIFF and load back into Maxim, then combine with the color frames. 

All Images

Get images looking as good as possible in the CCD program, but donít get wrapped around the axle trying to prefect it. CCD programs continue to improve, but they are still weaker than a photo application for final tweaking.

Save the image as a stretched 16-bit TIFF and load it into a photo program, like Photoshop, to set levels, contrast, color balance, noise reduction, sharpness. A slight Gaussian blur layer added to the original image, followed by sharpening will remove a lot of noise while preserving good detail. A dedicated noise filter, like Noiseware, works even better.

Keep notes on each image Ė what worked, what didnít.

Allow a couple of uninterrupted hours and a couple of cold adult beverages for each image.

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