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| I am fairly new to this subject, and have been imaging with a 102AR refractor, with AM5 mount, ASI294MM and fairly good guiding. I feel the urge to upgrade my telescope to a longer focal length and more aperture to reach the deep, dim, galaxies and the wispy detail (like that in Stephan's Quintet). I have been trying to analyze the limits of focal length, aperature, seeing, etc. and have concluded (without much certainty) that maximum aperature with a focal length of 1000 is ideal. I'm also trying to decide what type of telescope is best for this. I want to keep the weight below 25 lbs. Has anyone looked at the same issue and can offer some guidance/"rules of thumb"? |
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| 250mm f/4 newtonian or 200 f/5 newtonian. But i'd go with the former. |
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| There is an on going thread "what is your galaxy imaging set up" in Generic Equipment Discussions that you might find useful. |
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I think it's almost universally true that in all situations one would want the maximum aperture possible (given quality/cost/weight limitations) for the focal length that gives you the field of view you need. That's true whether you are imaging the entire Milky Way, large galaxy clusters, or the smallest planetary nebula. People seem to worry a lot about focal ratio, much more than they probably should. One should think instead about what focal length gets them the desired field of view, then maximize aperture. At least, that's how I see things. For small galaxy clusters like Stephen's Quintet, I think ideal would probably be no less than 1,500mm. As pixels have gotten smaller and smaller, the need for focal length has diminished, but I'd still want > 1,000mm for tiny objects like distant galaxies, planetary nebulae, etc. That might well result in oversampling for poor seeing conditions, field of view is a complete non-issue for situations like this, so the oversampling won't hurt any, and on nights of good seeing it might help a bit. Do look at the other thread mentioned. It brought up several scopes that are a good match for galaxy season. |
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Jared Willson: This is the way I have always thought of things. Aperture dictates light collection, focal length determines field of view for a given sensor size. I'd also say that one of the first questions to as is: do your seeing conditions support what it is you want to image? It is all well and fine to say I want x focal length or y pixel size and worry about over or undersampling, but none of these things help if your seeing fundamentally limits resolution. If your seeing is poor 90% of the time, maybe the thing to do is reset your goals and expectations rather than invest many thousands in scopes, cameras, and mounts that will only do what you want them to a fraction of the time. |
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| Thanks, that's very good advice. We have a rural site with seeing in the "good" category (2" to 4"), and I go to star parties where seeing is better (probably 1" to 2"). I'm considering the 8" R-C (FL 1624), but I understand that guide scope guiding is perhaps limited to shorter focal lengths. I just want to get the best possible reach of dim galaxies. Would a 6" R-C or SCT do just as well at these seeing conditions? Or perhaps 6 or 8" newtonian? All things being equal, should I be able to get as good results with a 6" vs. 8" if I can expose longer with good guiding? |
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I’ll throw this into the mix… The small galaxies are dim, and you want potential resolution. The only way to achieve either one is aperture. (In a perfect world). I decided on an edge hd 8 because of the focal length and the lack of a spider. Your 102 is 700mm-ish right? An 8 inch newt is what, 1000mm. An rc 8 is about the same as an edge w/reducer at 1500ish-mm. The difference is diffraction spikes. Check out https://astronomy.tools/calculators/field_of_view/ |
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I use both a SCT and a RC. RC telescopes are touchy. Collimation is super critical for getting a RC to work correctly. Collimation is much easier with a SCT. A C8 or C9.25 in the Edge family are fantastic telescopes. The 8" would be my choice with your mount. Look at images from various scopes on the site. They are the best way to visualize how your gear and skill level will translate to an upgraded rig... Good luck! CS Tim |
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| One reason I was leaning toward the R-C is the difference in FL. The RC is 1624mm which is at the upper end of my target without using a reducer. Other than extra cost, do you think using the reducer is a negative? |
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If you are looking for something small and dim, your only choice should be something like a sharp newtonian F5 and below. Dont worry about RCs, unless you live in La Palma, Chile or places with sub arcsecond seeing they are practically obsolete compared to the newtonian counterparts (for same aperture). Ideally something like a 200mm F/4 or 250mm F/4. This is of course assuming you are shooting at like 1"/px or close to that sampling. |
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Don, I am not sure which scope type - reducer pairing you are asking about? I use the Celestron .7x reducer/flattener for the C11EdgeHD. I would use a reducer on your SCT no matter the aperture. I also use a .85x reducer/flattener from a Skywatcher 100ED refractor on my RC10. I image with 2600's and 533's from ZWO with 3.75micron pixels, round stars to the corners on all cameras with both scopes. The RC10 with that reducer is imaging at approximately F7 and my subs plate solve at 1771mm of FL. I do know that my Orion/GSO RC would not be useable at all without several modifications. The internal baffle tube was replaced and the secondary mirror holder was converted from an center bolt/spring and 3 screws pushing on the back of the secondary mirror holder to a system that the allows the secondary mirror to pivot while remaining centered and the 3 screws now act like Bob's knobs on a SCT. Focuser replaced. Out of the Box the Celestron is ready to go. Add a reducer and get the focus motor. I would get the reducer to image at F/7 and you have a nice scope to play with the planets if you are so incline... I hope my verbose answer was some help? CS Tim |
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I do agree with the Newtonian folks! If all you want is 1000mm of FL? A 200mm F/5 newtonian is the way to go. I am an old guy from back when folks like Robert Cox and other amateurs ground and tested the mirrors we used to build scopes with back in the 70's. They always said that it takes a lot of skill to produce a great F/4 mirror vs a F/5. Why some of the super fast quality newtonians are priced the way they are. The slightly smaller secondary mirror of the F/5 should not be an issue imaging galaxies... Tim |
