C9.25 vs Skywatcher 200P; Planetary AP and visual use

[Tundra]

I currently own a 200p on an EQ-5, motorized on one axis, and a ZWO ASI224.  I've had this scope for over a year now, and I am very pleased with it and the views I can see.  That being said, I suspect that my optics aren't diffraction limited and I want something higher quality and slightly bigger, but still portable. I plan to upgrade to an EQ-6, which I will use mostly for planetary AP for now but I will probably try some DSOs later on. The C9.25 seems like a good option, but I have some concerns which might make me want to stay with my 200p.  I'm not to worried about the focal length and not being able to fix in large objects, I think that the reducer/coma corrector could fix this problem for AP, and I can just use my 35mm eyepiece for visual (possibly with the reducer too).

Central Obstruction:  I know that many people think this value is talked about to much, but I have heard that it can be a large contributor to things like loss of contrast, image sharpness, and maximum theoretical resolution. 

Light Transmission:  Does the addition of a corrector plate cause more light to be absorbed (I think I read for Celestron only like 76 percent of the light makes it through) and would that make the 200p deliver more photons, even though I has a smaller aperture?

 

Basically, does the quality improve? Do I get better seeing and resolution?  Is that at the expense of contrast?  Is the issue of cooling down really something I need to worry about, I store my scopes out on the porch most of the time?  Any other general advice about the C9.25 would be amazing too!  Thank you so much in advance to helping me, I really appreciate it.

 

 

P.S.  How does one change the finder scope on the C9.25?  Is it possible to add another for autoguiding if I get into that later?

P.P.S  I'm almost certain that it would be diffraction limited, but confirmation would be nice... 

Edited July 8, 2023 by Tundra

[omo]

I don't image but do use a Baader finder shoe on my C9,25

[Mr Spock]

I used to have a C9.25 on an EQ6 and it's a perfect match. The only thing to be aware of is the C9.25 (like most SCTs) is really tetchy when it comes to seeing conditions. The majority of times I had the scope out it wouldn't last long before I brought it in due to disappointment over the image quality due to seeing. I reckon also the best magnification for seeing detail on the moon was x235 whereas my 12" StellaLyra Newt can maintain better image quality at x460.

The central obstruction does matter. It has no effect on the moon but Jupiter's belts are quite soft and difficult to make out compared to a Newt. It has no relevance to planetary imaging though - just take a look at what Damien Peach can do with one!

You'll find tracking for DSO imaging at 2350mm challenging.

[Tundra]

22 hours ago, Mr Spock said:

 The only thing to be aware of is the C9.25 (like most SCTs) is really tetchy when it comes to seeing conditions. The majority of times I had the scope out it wouldn't last long before I brought it in due to disappointment over the image quality due to seeing.

You'll find tracking for DSO imaging at 2350mm challenging.

Is the difficulty with seeing conditions from from the long focal length?  I would assume that the increased aperture compared to my current scope would mean that it would be more tolerant to seeing conditions...

If the issue is the focal length, for both the problems, would the reducer help fix the problem?   Thanks again!

[Mr Spock]

6 hours ago, Tundra said:

Is the difficulty with seeing conditions from from the long focal length?  I would assume that the increased aperture compared to my current scope would mean that it would be more tolerant to seeing conditions...

If the issue is the focal length, for both the problems, would the reducer help fix the problem?   Thanks again!

It's the design not the focal length. I'm not actually sure why. I know SCTs suffer from trapped currents inside the tube. As that doesn't affect refractors I assume it's due to the stubby tube.

[kirkster501]

A combination of their long focal length and, as Michael stated, tube currents and the system reaching thermal equilibrium with the surroundings.  Also, the design of the SCT with the large central obstruction causes a softness of the image.  Larger SCT's need to live outside in an observatory IMO to maintain this thermal equilibrium.  In the UK bringing them outside from the house or even a cooler garage they sometimes never reach thermal equilibrium for that night's observing.   So why are SCT's so popular many ask?  They are quite cheap to make and offer a lot of bang for the buck.  I love my C925.

I think I have had better views of Jupiter through my TEC140 - much smaller aperture - than I ever have with my C925 or even my Meade 14" whilst I owned it.  The image, whilst small in the case of Jupiter, is razor sharp.  That's why there is a strong market for large premium refractors despite their smaller aperture than a cheaper SCT; the views and ease of use for visual and imaging is just so good.

[takurrito]

I've done quite a bit of planetary observing with my 10" Meade ACF SCT and have compared it to the APM 140 (140mm doublet apo), and have had the opposite experience. My 10" showed a substantial amount more on Jupiter than I've ever seen through the APM 140 (or my 4" Takahashi). One steady night with the binoviewers, my observing buddy and I counted 14 distinct bands on Jupiter and detail within the GRS. 

I insulated my SCT with Reflectix, which almost eliminates the need for cool-down. There's extensive threads on insulating SCTs and it works as advertised. 

[davidc135]

Hi Tundra. There is a 9.25 XLT with a few accessories on Astobuysell asking £925, Cambridge area, if you are near.

I'll second use of reflectix to remove or very much reduce thermals in closed scopes.

Another option would be to get a first class mirror. Around 800Eu for an 8'', 1000ish for 10'' from Mirro-sphere in France who I think are very good. They would refigure your existing mirror if it's known to be faulty.

David