coma corrector for cpc 1100

[hubble space telescope]

hello everyone

im trying to find a coma corrector for my cpc 1100 gps xlt telescope. i found a field flattner from celestron that will be good for my telescope but i dont find anywhere a coma corrector that will be good for my cpc 1100 gps xlt telescope (a Schmidt–Cassegrain telescope), anyone know where i can find it?

thank you!

[alanjgreen]

Cpc1100 is f10 speed ( very slow speed ), it does not need a coma corrector!

coma corrector is only needed on very fast scopes like my f3.6 dob.

why do you think you need a coma corrector?

[YKSE]

Coma in CPC 1100 should be very minor, does it bother you very much?

[TheLookingGlass]

Not needed.

[hubble space telescope]

i dont know... i heard that you need to buy coma corrector in order to make your images looks even better. if you say that i dont need to buy, so maybe i really dont, i dont really understand it very well

thank you!

[Louis D]

4 hours ago, hubble space telescope said:

i dont know... i heard that you need to buy coma corrector in order to make your images looks even better. if you say that i dont need to buy, so maybe i really dont, i dont really understand it very well

thank you!

You mostly just need a field flattener.  Stars at the corners of a non-flattened field will tend to look comatic, but it is not due to actual coma.

[hubble space telescope]

so just a field flattener!

thank you very much! you just saved me some money 

[Axunator]

It's not my intention to make you feel like you need something you may not need, but some of the answers above were technically, hmmm.., inaccurate enough that I felt the need to chime in.

First of all, the optical performance of an f/10 SCT design is not comparable to f/10 Newtonian of same aperture, not in terms of coma and definitely not in terms of field curvature. It has a lot more of both. The amount of coma, as well as the width of diffraction limited (i.e. coma-free) field of view in f/10 SCT is about the same as in f/4.5-5 Newtonian of same aperture. Whether that is acceptable or even noticeable, is a matter of personal opinion (many are happy with f/5 Newtonians without a coma corrector, whereas others definitely feel the need for CC). What makes the coma in SCT even more visible than in f/5 Newtonian, however, is the field curvature. Because of FC, the stars at the edge of field have coma AND are out of focus at the same time (if you focus on the center of the field), which makes the coma even more visible. (If someone wants to delve more deeply into the theoretical side of this, e.g. Smith, Ceragioli & Berry: Telescopes, Eyepieces, Astrographs, published by Willman-Bell, is an excellent treatise on the subject, IMO).

This phenomenon is the very reason, why Celestron produces Edge HD versions, with in-baffle corrector which both flattens the field AND corrects the coma.

Now, since you have a standard XLT version, what can you do about it?

For visual use, IF you don't find the view satisfactory (I am NOT saying that you can't find it satisfactory, many many observers do and have done so for decades, that's why SCTs are so popular!!), the only thing you can do is to get the focal reducer/field flattener, which improves the field (but shortens the f.l. at the same time, and may also induce vignetting in 2" EPs with wide field stops). Edge HDs work fine for visual out of the box, but you cannot transform your existing XLT into the Edge; it's a different scope. Coma correctors designed for Newtonians may or may not work in an SCT, but they are definitely not designed for it, and getting one would be a huge gamble and likely end up in huge disappointment.

For imaging, you may get fancier, if you like. For instance, Starizona makes a field flattener/focal reducer/coma corrector for imaging:

http://starizona.com/acb/Starizona-SCT-Corrector-LF-P3689C805.aspx

It is very expensive (599 USD), but if imaging with large sensors is what you're up to, C11 XLT + Starizona LF corrector are still quite a lot cheaper than C11 Edge + Celestron focal reducer together. The problem with using the Starizona LF corrector for visual is that the distance from the corrector to the focal plane has to be very accurately set for it to work, which will be difficult or next to impossible for 2" diagonals and eyepieces (and why would you want to use it with narrower 1.25" EPs to begin with?).

Again, let me say that if you are happy with your current setup, do not waste your money in something you do not need. But I thought that you may want to know what are the optical realities of your scope, and what options you have if you want to squeeze that last bit of performance out of it 

Finally, a disclaimer: I DO NOT have a C11 (neither Edge nor XLT) myself. However, the above is correct to the best of my knowledge...

[Peter Drew]

I have 8", 12" and two 16" F10 Sct's, I can't say I've ever noticed coma or field curvature to be of a significant enough level to spoil the view. Imaging might well be improved with added correction. 

[Axunator]

8 minutes ago, Peter Drew said:

I have 8", 12" and two 16" F10 Sct's, I can't say I've ever noticed coma or field curvature to be of a significant enough level to spoil the view. Imaging might well be improved with added correction. 

Exactly. And the OP should enjoy his scope and not worry too much about technical details  HOWEVER, that being said, it is incorrect to claim that standard SCTs do not have coma. They do, about as much as f/5 Newts, and even more FC. How much that bothers an individual observer is another thing, as always. Pages after pages have been written by people arguing whether Edge HDs are "worth it" for purely visual - many think they are, while others could not care less.

Furthermore, the OP did not mention whether visual or AP is his primary interest, and for AP it is definitely helpful to realize that FC is not the only aberration affecting the image in SCTs...

[Louis D]

15 hours ago, Axunator said:

Exactly. And the OP should enjoy his scope and not worry too much about technical details  HOWEVER, that being said, it is incorrect to claim that standard SCTs do not have coma. They do, about as much as f/5 Newts, and even more FC. How much that bothers an individual observer is another thing, as always. Pages after pages have been written by people arguing whether Edge HDs are "worth it" for purely visual - many think they are, while others could not care less.

Furthermore, the OP did not mention whether visual or AP is his primary interest, and for AP it is definitely helpful to realize that FC is not the only aberration affecting the image in SCTs...

I will add I looked at Jupiter through a brand new 8" Edge HD with a 10mm Delos eyepiece at a star party last summer, and the combination rivaled the best views through 5 and 6 inch ED refractors on the field that night.  Regular SCTs were mushy by comparison.  It made me rethink my opinion of SCTs.  The edges were quite well corrected as well, but what struck me was how much the on axis view was improved over a regular SCT.