10” RC scope?

[tooth_dr]

Guys I looking into a longer focal length scope, and the 10” F8 RCs look pretty nice.  

Couple of questions - is there much field curvature?  How big of an imaging circle can be imaged?  What do star shapes look like - same as regular Newtonians?

The 0.75x reducer would give 1500mm/F6 which seems ideal for me.  But is it enough of an upgrade from 1200mm/F4.7.

My guiding with the 1200mm scope using OAG is usually around 0.5-.6 but could probably be improved, as this is just standard setting in PHD2 and I’ve made no attempt to fine tune it.  My camera has a pixel size of 5.4um.

 

Any input welcome

Adam  

[kirkster501]

I find imaging with anything than refractors exceptionally frustrating.  There are enough variables at play with DSO imaging without adding collimation to the mix as well.  0.5arcsec/p is going to be very challenging mate on your guiding in NW Europe. I have tried it and gave up. YMMV though and maybe you might fair better than me.

 

[Ikonnikov]

I have the old Altair Astro solid tube version of this scope and like it a lot, although it took quite a bit of work to get it up to scratch. I strongly recommend getting a version that has the primary mirror cell decoupled from the focuser (I think this is the case for the newer truss tube versions) otherwise collimation is very tough. Once you get the collimation close with tools (e.g. a cheshire) you can fine tune it with your camera attached by inspecting star shapes (see the DSI method on the web) to get it pretty accurate. Then it seems to hold pretty well.

In answer to your questions, I think APS-C is generally accepted as the largest chip size that you can use with the GSO-type RC10 scopes (using mirrors only) without encountering significant field curvature and degeneration of star shapes. I find using my KAF16200 CCD (APS-H 6 micron pixels) that stars are noticeably distorted in the corners without a corrector, but using a 2" RC flattener from Teleskop Service, (rated up to 44mm image circle) the whole field is pretty well corrected. I image at bin 1x1 or 2x2 depending on seeing although 1x1 is a bit on the over-sampled side 95% of the time.

The basic GSO 0.75x reducer has no field flattening capability so it reduces the usable image circle to down to 15mm or so; reducer/flatteners that work well on RCs (e.g. the TS 0.8x RC reducer which I am tempted to try) are unfortunately a lot more expensive but supposedly give well corrected 44mm or bigger fields.

My main complaint with the scope is that I don't get many nights where conditions are good enough to make the most of its resolution.

 

This is a recent image with the RC10 plus TS 2" RC flattener using 2x2 subs (~1.2" per pixel) and very rough and ready processing (without preserving stars properly).

 

[tooth_dr]

1 hour ago, kirkster501 said:

I find imaging with anything than refractors exceptionally frustrating.  There are enough variables at play with DSO imaging without adding collimation to the mix as well.  0.5arcsec/p is going to be very challenging mate on your guiding in NW Europe. I have tried it and gave up. YMMV though and maybe you might fair better than me.

 

Steve I appreciate the honest and experienced answer. It’s a beautiful scope to look at but I fear that you are correct - perhaps I’ll just stick with my old reflector for now. 

[tooth_dr]

12 minutes ago, Ikonnikov said:

I have the old Altair Astro solid tube version of this scope and like it a lot, although it took quite a bit of work to get it up to scratch. I strongly recommend getting a version that has the primary mirror cell decoupled from the focuser (I think this is the case for the newer truss tube versions) otherwise collimation is very tough. Once you get the collimation close with tools (e.g. a cheshire) you can fine tune it with your camera attached by inspecting star shapes (see the DSI method on the web) to get it pretty accurate. Then it seems to hold pretty well.

In answer to your questions, I think APS-C is generally accepted as the largest chip size that you can use with the GSO-type RC10 scopes (using mirrors only) without encountering significant field curvature and degeneration of star shapes. I find using my KAF16200 CCD (APS-H 6 micron pixels) that stars are noticeably distorted in the corners without a corrector, but using a 2" RC flattener from Teleskop Service, (rated up to 44mm image circle) the whole field is pretty well corrected. I image at bin 1x1 or 2x2 depending on seeing although 1x1 is a bit on the over-sampled side 95% of the time.

The basic GSO 0.75x reducer has no field flattening capability so it reduces the usable image circle to down to 15mm or so; reducer/flatteners that work well on RCs (e.g. the TS 0.8x RC reducer which I am tempted to try) are unfortunately a lot more expensive but supposedly give well corrected 44mm or bigger fields.

My main complaint with the scope is that I don't get many nights where conditions are good enough to make the most of its resolution.

 

This is a recent image with the RC10 plus TS 2" RC flattener using 2x2 subs (~1.2" per pixel) and very rough and ready processing (without preserving stars properly).

 

Thanks for the detailed replied. As much as I don’t mind collimating now (since becoming owner of Tak Epsilon a year ago) I don’t like the thought of having to fine tune the collimation.  I had hoped at F8 it may not be as critical but I am wrong it seems.

What are the alternatives?

[Ikonnikov]

26 minutes ago, tooth_dr said:

Thanks for the detailed replied. As much as I don’t mind collimating now (since becoming owner of Tak Epsilon a year ago) I don’t like the thought of having to fine tune the collimation.  I had hoped at F8 it may not be as critical but I am wrong it seems.

What are the alternatives?

Once you've done the fine tuning and are happy with it, it should hold collimation pretty well for months/years; I dismantle my setup after every imaging session and I've only had to redo it once after accidentally bashing the end of the scope on a door bringing it in the house.

[tooth_dr]

9 hours ago, Ikonnikov said:

Once you've done the fine tuning and are happy with it, it should hold collimation pretty well for months/years; I dismantle my setup after every imaging session and I've only had to redo it once after accidentally bashing the end of the scope on a door bringing it in the house.

Thanks again!  I wonder is dewing on the mirrors much of a problem, and are heaters needed?

[Ikonnikov]

7 hours ago, tooth_dr said:

Thanks again!  I wonder is dewing on the mirrors much of a problem, and are heaters needed?

No worries! I've had the secondary dew up a couple of times after several hours when it's been particularly damp and now use a large dew strap placed around the outside of the tube just above the primary (as recommended by folks on Cloudy Nights) on a low heat setting which works well without noticeably affecting the image quality.

This isn't an option for the truss tube version if you're interested in that but you can get a dedicated secondary heater which fits under the mirror with the power cables going across one of the spider veins (to prevent them from creating unwanted diffraction effects).

[Whirlwind]

20 hours ago, kirkster501 said:

I find imaging with anything than refractors exceptionally frustrating.  There are enough variables at play with DSO imaging without adding collimation to the mix as well.  0.5arcsec/p is going to be very challenging mate on your guiding in NW Europe. I have tried it and gave up. YMMV though and maybe you might fair better than me.

 

You'd probably be better to bin the pixels to something more manageable.  That way you are still getting the benefit of the aperture with 'less' guiding issues.