C9.25 vs C11 for planetary imaging

[Robert_T]

Hi All, no doubt this is a very subjective question, but would welcome any opinions as to which scope C9.25 or C11 you feel is best for planetary imaging. I guess all else being equal, aperture would win, but I understand that the 9.25 is configured in a slightly different way that provides some specific advantages - certainly I haven't seen any planetary images posted on the various forums I frequent from C11s that appear any better or more detailed than any 9.25 images (sometimes the reverse). I wondered whether thiis is really due to the design of the 9.25 or more due to the limitations of seeing that restrict times when the C11 will really shine and also perhaps the fact that more people have 9.25's by virtue of their more compact size and price?

cheers,

[JohnH]

Looking at what you have already I wonder why this question? FWIW I have just researched buying a new scope and considered the C9.25, it has a flatter field that suits it better to photographic work than the C11 but less light grasp, I did not really consider an 11 due to size/weight limits so I cannot say much more. For planetary work you want the greatest focal length (for magnification without having to resort to exotic eyepieces) and smallest central obstruction possible (or none) for best contrast. Thus a long FL APO refractor or a MAK are best...

[Robert_T]

Thanks John, bit difficult to justify getting the 9.25 when I already have a good 8.25 inch cassegrain. I have a touch of arpeture fever to get that little extra detail/resolution that (reasonable quality) aperture can deliver when the seeing permits. I'm also just plain greedy!:)

cheers,

[bird]

Robert, in general terms the C11 will be harder to use for planetary work than the C9.25 because it has a larger mirror that requres more careful cooling, and holds more air inside the closed OTA that also has to be cooled.

The C14 would be more difficult again than the C11, and requires either perfect thermal environment or a lot of hard work to get it into thermal equilibrium.

There may be specific optical differences as well with the 9.25, I have heard people comment on that but I don't know what they are.

regards, Bird

[Robert_T]

Thanks Bird, what can be done to assist a SCT to reach thermal equilibrium? - I've read that Maksutov Cassegrains are particularly bad in this respect, but generally they have fans fitted to assist.


cheers,

[bird]

The Lymax "cat cooler" would be a good start, mind you I don't own an sct so I might not be the best person to ask :-)

I have thought about variations on my peltier cooler which would fit an sct, but never done anything about designing or building one. Anything that moves the air around will help in transferring heat out of the tube.

I guess the other thing you could do is make sure you live somewhere with very stable temperatures :-)

regards, Bird

[Stu]

Hey Bird, Is the peltier cooler fitted directly to the mirror? Or attached to something else that then cools the mirror?

Wouldn't you get a cool spot?

[bird]

G'day Stu - here's a link to my website that talks a bit about the peltier cooling:

http://www.acquerra.com.au/astro/

Yes, I get a cold spot, means I have to wait about half an hour after I turn it off before the mirror has all evened out to the same temp.

regards, Bird

[davidpretorius]

Nice website Bird and great gear. I can't wait for the 13.1"

What would your mirror maker charge for a 16". (in all seriousness)

Where do you put your sensors. I am taking your advise and getting some sensors. You have two for ambient, two for mirror?????

Can you get sensors to measure surface temp or any cheap sensor that is touching something will measure that temp??? ie does it need to be directional???

[Robert_T]

just returning to the thread for a mo, anyone have anything else to add on the C9.25 versus C11 - someone out there must have played around with both?

cheers,

[rumples riot]

Robert, I have spent months research this very issue. Albeit slightly different. I am in two minds between the 9.25 and the 250 mewlon. Cost is a significant factor and ease of service is another.

That being said, I would rather the 9.25 mainly because people like Damien Peach, do use this scope and get fantastic results. In fact I would have to say that he is the best planetary imager in the world. His images of Jupiter are the best I have ever seen. Bird is right though, cooling makes a significant difference. My scope is permanently mounted outside with a cover over it. I have noted that this keeps the scope permacooled and helps with my imaging efforts.

I think the 11 has some advantages like image scale, but cannot compete with the specific optical design of the 9.25. Chris Go of Singapore is now using his 11 and I think the images look blurred. The 9.25 is I believe the better tool for planetary work.

Although, many things like collimation, and seeing make so much difference to an image. Personally, I think the Mewlon is rated very highly and would love to own one over the Meade I have. I am sure that given practice and the right camera your images could rival the best. Don't buy the 9.25, buy a better camera and a day star and your images will be out of site. The images of Jupiter that you exhibited are very good, but with the right camera they could be brilliant.

Keep the Mewlon you are onto a winner.

[bird]

I'd have to say that I would lean toward the 9.25 over the C11 but it would be a close thing. I just feel that the 9.25 would be easier to work with and gives results almost as good (in practice probably identical).

The mewlon would have the best rep of them all, so seriously consider that as the best option.

Davo, the sensors are tiny things, they come in a TO-92 package and look just like a low-power transistor. I have mine stuck onto the mirror with a tiny drop of thermal paste and an insulating cap made of styrofoam to keep them away from the air. One sensor on the back of the mirror and another on the side up as close to the front as I was game.

I'll take some pictures tomorrow and stick up on my website while the scope is still dismantled.

Bird

[Robert_T]

Thanks Bird and Rumples! Paul, especially appreciate your thoughts and benefits of the research you've been doing. Going the route of better camera etc was what I'd originally planned, though I'm still exploring the limits of the neximage having only just implemeneted raw mode. I may still go this way though I havent seen much improvement in Chris Pellier's images with the Mewlon 210 and the Lumenera. I guess I've been lured a little myself by the C9.25 efforts of Damian Peach. I've also noticed too that Go's C11 shots aren't any improved over his earlier C8 shots - what concerns me a little here is I think a fair bit depends on the quality of the particular OTA you get.

There was another factor, I've just packaged the Mewlon up to be sent off for some servicing and am likely to be without it for at least a couple of months:scared:. Had thought to get either the 9.25 or C11 in the interim to keep me sane, run both together for a while and then sell the one that wasn't getting used... getting another scope also gave me an excuse to get a more substantial or tricked up mount that the old eq5 I'm using now. Alternatively, had considered getting a 10in newtonian on the EQ6 as a cheaper option to a sct for something sufficiently different to warrant keeping both it and the Mewlon (while giving me a decent mount to use with both).

ahhh - the Mewlon's only been in it's box for one night and already I can feel the DT's...

[iceman]

Just remember that the image scale will be much smaller with a 10" newt, as if you get a mass produced one the FL will be about 1250mm. Obviously if you get a custom made mirror like bird, you can ask for a slightly longer FL but of course then you need a longer tube to put it in which can mean transport and/or mount problems.

I can't see how a C925 would be better than your mewlon, but if you have the option of having both for a period, it would be a great comparison to see the images you produce out of both - assuming all other things equal (good collimation, equal settings, equal seeing etc).