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TECHNIQUES

Image-train for TS-Optics 130 with ATREDT30



Image-train for TS-Optics 130 with AT2FF



Image-train for SV70T with SFFR-70APO



Image-train for Camera Assembly


* 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:

• Scopes with short focal-lengths have more field curvature.
• Faster scopes (lower F-ratio) make field curvature more visible, because more light rays are out of focus.
• Spacing is much more critical with Focal-Reducer/Field-Flatteners than with straight Field-Flatteners.
• Start at 55 mm or recommended spacing.
• Increase spacing if the corner stars look "pointy", decrease if they look "flat" or have a funny "inside out" appearance from too much correction.

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:

• The collimation instructions in the manual are not good for imaging and can actually make things worse.
• The same goes when attempting to use the standard way of collimating a reflector or SCT.
• There is a lot of misleading info on the internet (nawh!)
• The adjustments are not intuitive.

After much frustration, I finally embarked on a step-by-step experiment and discovered the following:

Secondary Mirror:

• Adjusting the secondary mirror mainly affects the oblong stars at the edge of the image.
• Tightening a secondary mirror screw (and loosening the other 2 screws) reduces star elongation of the corresponding edge.
• This is best seen by viewing an in-focus image of at least 1 minute exposure time.

Primary Mirror:

• Adjusting the primary mirror mainly affects the coma of stars in the image.
• When de-focused OUTWARD, loosening a primary mirror black-screw (and tightening its mating silver-screw) reduces the coma flare or "fat" of the corresponding side of the star-donut. This also reduces the brightness of the other side of the donut.
• This is best seen by viewing a slightly out of focus star-donut at the center of the image.

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

Preparation:

• First try using a proven field-flattener such as the Astro-Tech ATFF2 or the Astro-Physics CCDT67. The results may be so good you'll forget all about collimating!
• Aim the scope toward a suitable field of stars such as a large bright open cluster.
• Attach the camera and guiding setup.
• If you use a field-flattener or focal reducer, it's important to attach it as well because they seem to shift the field curvature as well as flatten it, hence requiring different adjustments.
• Rotate the camera so that the top of the image corresponds with the top of the scope. For DSLRs, the top of the camera should be aligned with the bottom of the scope. For most CCDs, the top should be aligned 90 deg to the right. (CCD cameras are designed this way so that Dec movements correspond to up/down on the image.)
• Confirm the rotation by de-focusing OUTWARD, taking an image with your hand in front of the top part of the scope, and looking for a gap on the top part of the star-donuts.

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.

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:
http://astroimages.weebly.com/collimating-an-at8rc.html

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:


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:

If using an OAG, or the new On-Axis-Guider from Innovations Foresight (see Links):


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:

Comments welcome: mark.eby@twc.com