Imaging Tips

Dark Frames

- reflects the camera-induced noise,
- temperature, exposure time and ISO setting -sensitive,
- camera-specific,
- aperture, telescope etc. have no effect so they are not needed.

- no light coming in (use lens or body cap) and block the viewfinder as well,
- exposure time and ISO setting the same as for light frames,
- near the same temperature as light frames,
- take as many as you can and average-combine them.

- whenever the temperature, ISO and exposure time is the same,
- for the camera they were taken with (with whatever scope, filters, etc),
- ImagesPlus Auto dark calibration matches across exposure times and
also somewhat across ISO and temperature,
- if you have perfect tracking (=autoguiding) you may need upto 5 times
as many darks as light frames,
- with nonperfect tracking (some movement between each light frame),
or dithering between each light frame even 10 darks are quite enough,
- I have been trying different approaches but have not yet settled on
one (a huge number of darks collected over longer time vs 8-20 darks
collected after each imaging session), but in both cases the Auto
feature is great.

Flats Frames

- records the uneven illumination of the image,
- dependent on the camera (sensor size and type) and the optical system
(scope, including barlow lenses, extension tubes etc),
- even filters can change it (usually not their color but their size
and position),
- dust in the sensor is also recorded.

- use an evenly illuminated source of light (evening sky opposite the
setting sun, double T-shirt in front of the lens, a lightbox etc),
- preferably light should be bright enough to keep the exposure time
under one second (otherwise, stack more or blur the noise out),
- check that the scope/lens is focused to infinity (no need to be perfect
but should be very near),
- use the lowest sensitivity (usually ISO 100),
- use camera autoexposure (Av mode in Canon),
- take a few (4-10) and average-combine them,
- for lenses, one set for each aperture setting,
- for scopes, one set for each combination of filters/extension tubes/barlows
etc, in some cases even for different camera orientations,
- I prefer to keep the camera sensor clean - especially when taking
the flat frames, to get the dust out of the equation,
- the ImagesPlus from version 2.75 does not need changing flats into
grayscale any more, but other programs usually need that.

- whenever the optical chain is the same (scope, filters etc),
- some telescope/camera combinations sag, so you may need to make sure
the orientation is the same,
- for lenses use the one with the same aperture,
- keep the sensor clean also when imaging,
- I have taken four sky flats, rotated 90 degrees, for each lens and
aperture (upto 1 stop closed down for 10D, full frame cameras require
more) and averaged each set together.

Bias Frames

- reflects the camera zero level,
- different for each pixel, usually has a pattern over the image area,
- camera-dependent,
- ISO sensitive,
- not dependent on optics.

- cap the lens (or the camera without a lens),
- use the same ISO setting as for light frames,
- pick the shortest possible exposure,
- take a few (1-10) and average-combine them.

- usually not required, because unscaled dark-calibration subtracts
the bias as well,
- if you have a remaining weak fixed pattern all over the image, bias
frames may help,
- you can always reuse them when the ISO setting is the same (for the
same camera),
- I have taken one set (10 images for each ISO, average-combined) which
I use repeatedly.

Dark and bias frames change slowly over the time, so you probably should
not use anything more than one year old.

Balance your equatorial mount for imaging

Determine which side of the meridian you are going to be imaging on. If on the east, weight the scope to be heavy enough to sink to a hard stop on the east, so that the worm gear will be working against the unbalanced heavy weight. If imaging on the west, weight the scope to rise up so that the unbalanced weight again will be putting a load on the worm gear. If two weights are used it makes it easy to mark the 2nd one with tape so the weights can be moved to match east or west position of the scope.

Drift Aligning:

Rough align first using the polar alignment scope built into the mount. When done install a 9mm illuminated reticule eye piece with cross hairs into the C8 which gives 222 magnification.

Find a star directly south on the meridian at near 0 degrees declination. Manually drive the mount in RA and adjust the cross hairs in the ep to be parallel with movement of the star.Next determine the direction of star travel in the eye piece. To determine west turn the drive tracking off and the star will drift west. Note this direction which on my set up up = west and down = east.

To determine south nudge the tube in declination towards north and star will move south. Note this direction which on my set up right = north and left = south.

This phase will be for adjusting the azimuth of the mount. Now point to a star close to due south and at 0 degrees declination. Line it up right in the center of the cross hairs. Now wait for movement north or south. Ignore east west movement as this is drive RA periodic error. If the star drifts south, the mount is to far east. Move the azimuth west which is counter clockwise on my G11 looking from the east into the adjusting knob. If the star drifts north the mount is to far west. Move the azimuth east which is clockwise on my G11 looking from the east into the adjusting knob. After the azimuth adjustment re-center the star and keep repeating until it stays right in the center for 15 - 30 minutes.

This phase will be for adjusting the altitude of the mount. Now point a star close to due east and at 0 degrees declination. Line it up right in the center of the cross hairs. Now wait for movement north or south. Ignore east west movement as this is drive RA periodic error. If the star drifts south, the polar axis is to low and needs to be elevated. Move the altitude up which is clockwise on my G11 looking from the north into the adjusting knob. If the star drifts north the polar axis is to high and needs to lowered. Move the altitude down which is counterclockwise on my G11 looking from the north into the adjusting knob. After the altitude adjustment re-center the star and keep repeating until the star stays right in the center for 15 - 30 minutes.

If fine accuracy is required repeat the azimuth and altitude alignment again

CCD Drift Method

By Robert Vice

This article was original published in the September 2005 issue of AstroPhoto Insight™ Magazine. No portion of this article may be copied, reposted, duplicated or otherwise used without the express written approval of the author and AstroPhoto Insight. © 2005 Professional Insight

For many years I have spent time learning the Star drift method of alignment for my telescope. Although tedious, it has turned out to be quite beneficial. However, after wasting night after night attempting to align the telescope properly, I have found an easier way of alignment.

By modifying the photographic method of alignment to incorporate the newer technology such as CCD cameras and DSLR cameras, one can achieve an accurate alignment in just a matter of minutes instead of hours or days.

Here is what you do.
Setup and align your telescope normally.

  1. Set your telescope to point due south and at 0 degrees DEC.
  2. Find a semi-bright star. A 6th magnitude star works perfectly, but a dimmer star can be used.
  3. Insert your CCD or DSLR camera into the eyepiece holder or attach via the t-adapter.
  4. Focus the star for the CCD or DSLR.
  5. Once focused, move the star to the right hand side of the camera sensor.
  6. Set your telescope to its lowest drive speed. Typically a guide rate mode.
  7. Set your camera software to take an exposure of 125 seconds. The first 5 seconds is used to create a point of reference on the image.
  8. As soon as the first five seconds have elapsed, then press the W on the telescope keypad to cause the star to move to the opposite side of the sensor.
  9. For the first minute continue to move the telescope West. As soon as the first minute has elapsed, immediately reverse the telescope direction.
  10. When the second minute has finished, stop moving the telescope.
  11. After the image has downloaded, you should have something that looks similar to the image below.
  12. This is an initial image taken. What you see is the angle of deviation. What we are trying to do is to make the < a solid line. In order to correct this, we have to make some adjustments to the azimuth on the telescope mount. Notice that the initial star point is lower than where the exposure finished. This tells us that the telescope if pointing too far West. So to fix this, make a correction to the azimuth control to move the telescope East.

    Now, follow the same steps again. When the image has downloaded, it should show that the angle of deviation has decreased.

    Now you can see that the angle of deviation has decreased even further. We’ll make further corrections to the azimuth.

    As we continue to make corrections, the angle of deviation has decreased greatly. However, we are still not finished. Continue to make a few more corrections to the azimuth.

    Here is a final correction. The star trail is a single solid line. The angle of deviation is now 0.

    Once you have the azimuth fixed, you have to fix the altitude. To do this you simply move the telescope to a star along the Eastern or Western horizon and at 0 degrees Dec.

    The only difference this time is that we adjust the altitude instead of the azimuth. The images will be identical when adjusting the altitude. However, you will have to adjust the altitude accordingly. Here you will either raise or lower the altitude until the star trail is a single solid line. If done correctly, you will have a very accurately aligned telescope.

    Now that you are done with the altitude adjustment go back and double check your azimuth alignment. If everything checks out ok, then you are finished.

    The images used are a two minute exposure done for demonstration purposes. You can increase the time exposures to increase the accuracy.

    With the size of CMOS sensor and the FOV for my 8" f/6.3, you can see that I have a large area to play with. Two minute exposures only use a small area on my camera/telescope combination. If necessary I could increase the exposure times and go for about 10 minutes to cover the entire frame. That would create a much more accurate alignment than a two minute exposure.

    So be daring and experiment with exposures to see how long you can go and how accurate you can make your system.

    Note: This method was used with a fork mounted telescope. Those who use a GEM must make different corrections, however this method will work with them as well and will increase their accuracy just the same.



Easy-Fast drift aligning method for the Astrotract:

I use Backyard EOS, but you can use any program really. Its nice to have the ability to blow images up to 200 or 400 percent.

So my technique is -after getting close with an inclinometer and a compass (or just using my iphone's compass and inclinometer apps if I don't have a compass or inclinometer)- as follows

step 1: point camera with either my scope (ED45) or lens toward the meridian [South 0 degrees Dec), just as if you were drift aligning an equatorial mount. Start the astrotrac tracking.

step 2: take a one minute image (after focusing)-doesnt matter what ISO

step 3: inspect the image-you should see star trails at any focal length above 50 mm

step 4: move the wedge one way in azimuth. (pick a direction at random. Important you keep track of which way you moved the mount to avoid frustration!)

step 5: take another 1 minute image. Inspect. If the star trails are shorter, you have gone the right way. If not reverse the direction in which you moved.

step 6: repeat till you have minimised or eliminated star trails

step 7: now repeat with 2 minute exposures, then when you have eliminated star trails step up to 3 minute exposures. Repeat as necessary at longer exposures(I have gone as long as five minute exposures and managed to get zero trails with my Borg ED45 in this step). You really need to go only a bit longer than you plan to actually image.

Step 8 ; Now point the camera/scope at the east or western horizon, and start again with one minute exposures, making adjustments in altitude this time till you get rid of star trails

then go to 2 minutes, and repeat. Go longer depending on how patient you are :) and how long you plan your imaging subs are going to be.

When you have zero trails, you are polar aligned,

Theoretically you should go back and check the drift when pointed again at the meridian but in practice I havent bothered..instead I image with exposures that are a little shorter than the drift alignment exposures. mainly because I am getting impatient and want to collect photons.

It does take time but it works. But you can get quite good alignment in 30 minutes or long as you don't start off miles away from the pole and you don't forget which way you made each adjustment :) and have to start all over again-(been there done that)

This method works just fine with my Vixen polarie as well: (I have had the astrotrac and polarie running side by side recently and aligned them both using this method)

Drift aligning using the DSLR is easier than it sounds in this long winded explanation.

Critical though:
the tripod/wedge must be level before you start polar alignment.

and its really important to hang a weight off the tripod to improve stability and keep the centre of gravity low

With the images I posted I used the kit zoom lens to drift align using this method at 300 mm focal length. I drift aligned till I got round stars at 2 minutes.

Then for actual imaging I zoomed out to 135mm and imaged at 3 minutes..nice round stars. Probably could have gone longer


Photoshop Tips


“Shrinking Stars” trick (from Jack attributed to Dan Verschatse):
1.                  Open the image file
2.                  Select > Color Range
3.                  Choose a bright bloated star
4.                  Exit Color Range
5.                  Select Modify Expand
6.                  Choose a reasonable expansion to include halos (say 8 to 10 pixels)
7.                  Set Feather to about 3 pixels plus or minus 1 pixel
8.                  Filter > Other > Minimize > radius (1 pixel) > OK
Halo removal (From Ken Crawford):
1.                  Use the ellipse tool to circle the halo with about 10 pixels to spare
2.                  Turn on feathering at 5 pixels
3.                  Cntl-H to hide the selection
4.                  Choose the color layer to work on first (e.g. Blue)
5.                  Image > Adjust > Replace Color
6.                  On image, select part of the halo
7.                  Adjust fuzziness to clearly see the area selected
8.                  Adjust lightness to match background
9.                  Accept and go back to RGB to see result
10.              Work on the remaining colors As needed
11.              Finally select background as color used to replace final adjustment in RGB
12.              Cntl-H to see selection
13.              Cntl-D to deselect
14.              save result
Highlighting Dark Lanes (from Ken Crawford):
1.                  Create a new layer by dragging background layer to new layer icon below
2.                  Rename new layer something like “Hilite”
3.                  Filter > Other > Hipass (6.1 pixels)
4.                  switch layer mode from normal to overlay
5.                  Now create a layer mask
6.                  Layer > Add layer mask  > hide all
7.                  select brush tool B
8.                  use brush (white) to selectively unhide areas
9.                  use brush (black) to revert to hidden if needed
10.              Once satisfied – Layer Flatten Layers and save
11.              save result
HaR Combinations (from Don Goldman):
1.                  Create RGB and Ha Tiff files in MaxImDL
2.                  Process both in PS CS until about 80% finished
3.                  Split channels in the RGB
4.                  Layer Ha over R channel with normal mode and 30% opacity
5.                  Use HaR as red channel and combine RGB
6.                  Finish RGB processing
7.                  Finish Ha as Luminance
8.                  Saturate RGB by 30%
9.                  Use GS if needed
10.              Paste in Luminance (Ha) using luminosity mode
11.              Hipass processing to highlight if needed
12.              Duplicate the RGB and use as top layer in soft light mode with 20% opacity

Star Selection (from Russ Croman): 

  1. Make a grayscale copy of the image. I'll call this image #2.
  2. High-pass filter image #2 with a radius of one pixel.
  3. Apply a Gaussian blur to image #2 with a radius of one pixel.
  4. Invoke Image->Adjust->Threshold.
  5. Adjust the Threshold Level one click at a time until just the stars are white and everything else is black.
  6. In the original image, in the Channels Palette, create a new channel. Name it "Stars." Choose "color indicates masked areas."
  7. Paste image #2 into this channel.
  8. Make just the RGB channels visible (i.e. make the Stars channel invisible).
  9. Discard image #2.
  10. In the original image, invoke Select->Load Selection. Choose the Stars channel you just created.
  11. Invoke Select->Expand and expand the selection by a few pixels (e.g., three).
  12. Voila!
Layered Contrast Stretching (from Jay Gabany at AIC 2006):
1.                  Create a two new layers
2.                  Rename layer 1 to “Screen” and set blend mode to Screen
3.                  Rename layer 2 to "Soft Light" and set blend mode to Soft Light
4.                  Create layer masks for both layers set to Hide All
5.                  Select brush tool B set opcity to 20% and flow to 20% very soft
8.                  use brush (white) to selectively unhide areas
9.                  use brush (black) to revert to hidden if needed
10.              Once satisfied – Layer Flatten Layers and save
11.              save result
Adding Ha highlights to galaxies (from Don Goldman's M74 technique):
  1. Use a program such as Registar to register, resize and crop the H-a TIF file to match the master LRGB file.
  2. Open the LRGB file in Photoshop
  3. Open the H-a image and process using curves, levels, etc.
  4. Copy and paste the H-a image over the LRGB image.
  5. Create a clipping layer mask of the H-a layer by creating a Hue/Saturation adjustment layer, clicking colorize, sliding the hue all the way to the right (360 for pure red; 350 or so for some magenta to simulate some H-beta), saturation to 100% and intensity to -50.
  6. You will now have this adjustment layer on top of the H-a layer.  You will have 3 layers at this point. Right-click on the adjustment layer and select "clipping layer mask".  You will now see a downward-pointing arrow  in this layer.
  7. Change the blending mode of the H-a layer (not the adjustment layer) to saturation. You will see alot of red in the core and arms of the galaxy. We do not want the core or stars affected.  The core is bright in the H-a image mostly due to broadband light being picked up by the H-a filter.  So, we will "paint-in" only those obvious HII regions with a layer mask.
  8. Select the H-a layer and create a "hide all" layer mask from the Layers menu.  A black square will appear in the H-a layer.  You will see the red H-a color disappear in your image.  We will now selectively paint in the red HII regions. But, where to paint, because at this point, you are looking at the master LRGB?   We have hidden the red-colorized H-a data. You need a map of where to paint.
  9. Repaste the same, processed H-a data as the top layer with the normal blending mode, BUT keep the layer mask that you just created in the colorized H-a layer below active. You will now have 4 layers and the black square in the second layer from the bottom should be active.
  10. Make sure that the foreground is set to white (white box in tools palette) and select the paint brush.  Size it to the scale of the HII regions. 
  11. At this point, you will be painting in reddish color with the layer mask, i.e. painting lets the color come through the hide-all mask AND you will be using the B/W H-a image in the top as your guide. Again make sure the top layer is NOT active. 
  12. Start painting. Avoid obvious stars and much of the core.  The HII regions should be obviious.  You can either paint continuously or just click repeatedly.  To see your progress, just click the "eyeball" on the top B/W H-a image to hide it. You will now see the areas that you painted with additional red saturation.  Don't worry about the color being too strong at this point.  Just color all the HII regions that you have or want to accentuate. 
  13. When you are satisfied, hide or delete the top B/W H-a image and then adjust the opacity of the H-a layer with the layer mask (below the clipping layer mask) to achieve the level of saturation that you want.
  14. Again, if you don't delete the top H-al layer, you will have 4 layers -  from bottom to top, the LRGB layer, the H-a layer with its "hide all" layer mask, the hue/saturation clipping layer and the B/W H-a layer that you used to guide your painting into the "hide-all" layer mask.
  15. Compress the image into one layer (CTL-SHFT-E) and save into whatever format for posting your image.
 Cleaning up irregular stars:
1.  Start in 16 bit mode:
2.  Use the Elliptical markee tool to surround the star
3.  Feather the selection to about 3-4 pixels
4.  Use Filter / Blur / Radial blur / set to spin, best and about 40% to 60% or so
5.  Finish in 8 bit mode:
6.  Use the Eliptical markee tool to surround the star
7.  Feather the selection to about 3-4 pixels
8.  Use Filter / Distort / sphereize / at about 20%-60%

Last updated on July 2, 2012