DSO maging with an 8-inch SCT at native focal length ?

ZWO ASIAIR Plus Equipment Reflectors Deep Sky
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SouthWestAstro avatar
SouthWestAstro avatar
Does anyone have experience with DSO imaging with an 8-inch Schmidt-Cassesgrain at native focal length ?

Most imaging done with these scopes uses an f6.3 focal reducer. I have done that but now I want to explore imaging at full FL. I have two particular issues that I want to understand.

If I get an APS-C camera like the ASI2600 will that be sufficient for plate solving, e.g. with the ASIAIR Plus ?

Will I really need a scope with corrected optics like the C8 Edge ?
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andrea tasselli avatar
andrea tasselli avatar
Yes, I used to. But smaller sensor than nowadays. Overall not great. But acceptable with good seeing. And cropping. No way it could compare with my 8" Mak-Cass which was light years better. So I let it go. In terms of field I expect a sensitive APS-C sensor should have enough stars to plate solve. Obviously, you'd need to bin it (anyway for more reasons than this one). And, yes, you do need corrected optics with largish sensors.
SouthWestAstro avatar
SouthWestAstro avatar
Thanks. I appreciate the response. I'm thing to start by getting and APS-C, which I also need for other projects, and think about upgrading to the C8 edge later if necessary.
Arnie avatar
Arnie avatar
Hi,
I imaged for years and years with a non-EdgeHD C8 at both native f/10 and f/6.3. I was using an SBIG ST2000 with 7.9 micron pixels which perfectly matched the long focal length. The story is different nowadays with CMOS camera with much smaller pixels. Using a typical value of many popular CMOS cameras from ZWO and QHY of 3.76 microns, one can determine what focal length works best under your imaging conditions. Under average seeing conditions of 2 - 4", you want 0.67 - 2"/pixel resolution. With a camera of pixel size 3.76um, the C8 with a 6.3 reducer gives you 0.61"/px, so a little over-sampled, but not bad. At native f/10, you get 0.38"/px, way over-sampled for the conditions. In this case, your image will be larger but you will get no more detail than if you imaged at f/6.3 and enlarged the image in post, yet guiding will be much easier at f/6.3. If you have much better seeing conditions, 1 - 2", then you want 0.33 - 1"/px resolution, in which case the native focal length makes more sense. Most of us don't image from a mountaintop or an island in the middle of the ocean, so seeing that good is rare. If you can determine your average seeing conditions, that will go a long way to letting you know if the increased difficulty with guiding at f/10 is worth it compared to f/6.2.

You can find calculators for this at https://astronomy.tools/calculators/ccd_suitability

Arnie
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SouthWestAstro avatar
SouthWestAstro avatar
Arnie:
Hi,
I imaged for years and years with a non-EdgeHD C8 at both native f/10 and f/6.3. I was using an SBIG ST2000 with 7.9 micron pixels which perfectly matched the long focal length. The story is different nowadays with CMOS camera with much smaller pixels. Using a typical value of many popular CMOS cameras from ZWO and QHY of 3.76 microns, one can determine what focal length works best under your imaging conditions. Under average seeing conditions of 2 - 4", you want 0.67 - 2"/pixel resolution. With a camera of pixel size 3.76um, the C8 with a 6.3 reducer gives you 0.61"/px, so a little over-sampled, but not bad. At native f/10, you get 0.38"/px, way over-sampled for the conditions. In this case, your image will be larger but you will get no more detail than if you imaged at f/6.3 and enlarged the image in post, yet guiding will be much easier at f/6.3. If you have much better seeing conditions, 1 - 2", then you want 0.33 - 1"/px resolution, in which case the native focal length makes more sense. Most of us don't image from a mountaintop or an island in the middle of the ocean, so seeing that good is rare. If you can determine your average seeing conditions, that will go a long way to letting you know if the increased difficulty with guiding at f/10 is worth it compared to f/6.2.

You can find calculators for this at https://astronomy.tools/calculators/ccd_suitability

Arnie

Thanks very much. These insights are really helpful.
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