SkyDude Astronomy


Image-train for TS-Optics 130 with ATREDT30

TS-Optics 130
8 mm spacer
0.4 mm spacer
Camera Assembly (see below)

Image-train for TS-Optics 130 with AT2FF

TS-Optics 130
5 mm spacer
Camera Assembly (see below)

Image-train for SV70T with SFFR-70APO

8 mm spacer
0.4 mm spacer
Camera Assembly (see below)

Image-train for Camera Assembly

Orion Thin OAG
8 mm spacer, filed down to about 7.8 mm *
ZWO Filter Wheel
0.1 mm spacer **
ZWO ASI1600 Pro Camera

* The 8 mm spacer was filed down to get the correct OAG position, rotation-wise to the camera (OAG screws in a bit more so the pick-off prism lines up with the top of the camera sensor).
** The 0.1 mm spacer is to get the correct camera position rotation-wise on the filter wheel (camera is screwed in a bit less so the top of the camera lines up with the top of the filter wheel).

Fine-tuning the Spacing

Most telescope designs result in "off-axis aberrations", which is a fancy way of saying that the stars at the edge of the field-of-view are not round. For reflectors, this is called "coma", where the stars look like comets. For refractors, this is called "field curvature", and the stars look "pointy", which means elongated toward and away from the center of the image. Coma can be corrected by using a coma-corrector, while field curvature can be corrected by using a Field-Flattener or a combination Field-Flattener/Focal-Reducer. All of these require proper spacing between the corrector and the camera sensor. A very common spacing requirement is 55 mm.

For best results the spacing should be fine-tuned. This can be a long and tedious process: take an image, look at the corner stars, adjust the spacing, repeat. Here are some things that may be helpful:
The worst-case scenario would be a short fast scope using a Focal-Reducer/Field-Flattener, as in the case of the SV70T with the SFFR-70APO, where a spacing change of 0.1 mm makes a small but noticeable difference. Best case scenario would be a long slow refractor (say 1200 mm and F10) where a corrector may not even be needed. More forgiving as well is a straight Field-Flattener. With the TS-Optics 130 and AT2FF, spacing can be off by +/- 2 mm and still have good results.

An unexpected characteristic I've seen is that the rotational position of a corrector seems to have an impact as well. I first saw this while fine tuning the SV70T with the SFFR-70APO. When I got close to perfect spacing with corners stars almost perfectly round, I noticed that the stars were "out of balance". That is, the stars in one corner looked slightly different than the opposite corner. I continued to adjust the spacing in tiny amounts, +/- 0.1 mm, until the stars were as balanced as possible. But note that adjusting in those small increments doesn't change the spacing much, but it does change the rotational position of the corrector. My theory is that both the corrector and scope have a tiny bit of optical tilt., so depending on the rotational position of the two, the tilts tend to exaggerate or cancel each other.

AT8RC Collimation

It's easy to tell when a
Ritchy-Chretien scope is out of collimation. The tell-tale sign is oblong stars at one side of the image while the other side looks better. You may also notice a slight coma-flare on stars, most noticeable at the center of the image.

But collimating an Astro-Tech Ritchy-Chretien scope can be a daunting experience for the following reasons:
After much frustration, I finally embarked on a step-by-step experiment and discovered the following:

Secondary Mirror:
Primary Mirror:
Armed with this info, I arrived at the following procedure:

Warning1: If you de-focus inward, the donut gap will be on the bottom (opposite of where you placed your hand).
Warning2: Failure to confirm the rotation may result in adjusting the wrong screw!

Primary Mirror Collimation:
  1. Using the focus mode of your software, de-focus OUTWARD slightly so that the stars are very tiny donuts.
  2. Select a star near the center of the image, and determine which side has flare or is fatter. The other side will be brighter, which is sometimes easier to see.
  3. Loosen the primary black-screw that corresponds to the flare by no more than 1/16 of a turn and tighten its mating silver-screw.
  4. Repeat steps 1-3 until the star-donut is balanced.
Secondary Mirror Collimation:
  1. Focus the scope, switch on guiding, and take a 1-inute image.
  2. Examine the corner stars and choose the corner with the most oblong stars (stars stretched toward and away from the center of the image). If you are using a field-flattener, this may require some careful examination to determine the worst corner.
  3. Tighten the corresponding secondary screw by no more than 1/16 of a turn and loosen the other screws by the same amount.
  4. Repeat steps 1-3 until each corner is balanced with its opposite corner. Don't be surprised if one set of opposite corners has more elongation than the other set. This is normal for a Ritchy-Chretien.
Note1: If at any time the stars look like triangles, the secondary mirror is pinched. Loosening all 3 screws will fix it.
Note2: If you are not using a field-flattener, you may notice the secondary adjustment causes the center stars to become slightly oblong. Back off the adjustment a bit to reach a happy compromise.

After collimation is complete, and assuming you have one of the above mentioned field-flatteners, you should have well shaped stars across the entire image.

Example Images:

Primary Collimation: The right side of the donut is flared and the left is brighter. Loosen the right-hand black-screw and tighten its mating silver-screw.

Secondary Collimation: The stars at the top are the most elongated. Tighten the top screw and loosen the other 2 screws.

The good folks at Deep Sky Instruments offer a similar collimation procedure and some great info on Ritchy-Chretiens. The main difference between our methods is that I found it is easier to see oblong edge-stars on an image that is in-focus, rather than out-of-focus:

Another Astro-Tech RC user with a similar method, and great images:

Off-axis-guider (OAG) vs. Guide Scope

Back in the days of film imaging and manual guiding this was a great controversy. A guide-scope system was much easier to use but would typically have flexure problems that were difficult or even impossible to solve. OAGs have no flexure, but finding a guide-star was sometimes impossible, and guiding was sometimes too difficult to yield good results. I used an OAG system when I used to image with film.

But now we have digital cameras, computers, and specialized software. The result is that guide-stars are selected by point-and-click, mounts are guided automatically, and sub-frame exposure times are typically 5 minutes, which in most setups would not be long enough to present flexure problems. Modern
guide cameras such as the Starlight Xpress LodeStar or the QHYCCD QHY5L-IIM are so sensitive they can be used on an OAG with little difficulty in finding guide-stars. The decision between OAG or guide-scope is less of a controversy and more a matter of choice.

But oblong stars are still reported by guide-scope users, even when they image with a refractor (fixed optics) and have a very solid setup. I suspect that often overlooked is the fact that the guide-scope focal length is too short. There is a tendency these days to use short focal length guide-scopes, even finder-scopes, which degrades the ability of the software to calculate star position.

Through testing I have demonstrated the following equation in regard to the guide-scope and guide-camera:

Guiding Accuracy = Guide-scope-focal-length * Guide-star-image-quality

Yes, guiding software is capable of sub-pixel guiding down to 1/10 pixel, but that is only true if the star-image quality is large enough and good enough for the software to calculate the guide-star position accurately. Poor image = poor guiding, and you may not get a star image that is good enough when using a very short focal-length guide-scope.

If using a guide-scope:
Avoid guide-scope focal-lengths that are too short - no less than 1/4th of the imaging focal-length.
The longer the guide-scope focal length - the better.
The more sensitive the guide camera - the better.
Try to keep sub-frame exposure times to 5 minutes or less.
Understand that guiding accuracy may suffer when a dim guide-star is selected.

If using an OAG, or the new On-Axis-Guider from Innovations Foresight (see Links):
Use a very sensitive guide camera, such as the LodeStar or the QHY5L-IIM (see Links).
Sub-frame exposure times may be as long as you want.

Polar Alignment

I am about to enter a perilous area of controversy by stating the following:
I think in many cases the importance of precise polar-alignment is over emphasized. (gasp!)
Back in the days of imaging with film, polar alignment was very important. Exposure times were an hour or longer, and guide-scopes might have to be pointed a few degrees away from the center of the image in order to point to a bright enough guide-star. The combination of the two could result in stars that were shaped like an arc, unless the mount was perfectly polar-aligned.

But with digital imaging and guide cameras, typical exposure times are 5 minutes, and guide-scopes can remain at the same aim as the imaging camera. The importance of polar-alignment has been greatly reduced. I've even heard arguments that a small amount of polar-misalignment is better since it causes the mount to always perform DEC corrections in one direction, and avoid backlash problems. There is also the idea that a slight amount of polar-misalignment causes a little bit of dithering between images, and so reduces fixed-pattern-noise. For me, I just use the mount's polar-alignment scope. The typical accuracy of less than 0.25 degree it provides is good enough for most imaging.

But there are 2 important exceptions to this rule:
1. When imaging without any guiding at all, fairly precise polar-alignment is needed to keep the frames reasonably similar.
2. When imaging an object very close to the poles (10 deg or so), the mount may not be able to keep up with the needed R/A corrections. This occurs because DEC corrections cause increasingly larger R/A movements as the you get closer to the poles. Precise polar alignment keeps this problem to a minimum.

Comments welcome: