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Old 09-03-2016, 08:34 PM
HenryNZ
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The elusive 1000 mm+ focal length threshold

I have been involved with AP for a while now, and it has not been easy. I think I have finally tamed short focal length AP, but medium to long focal length AP remains problematic. Initially the problem has been with the mount and auto guiding, but after hyper tuning my NEQ6 and getting good OAG set up with a Lodestar, I think the problem with auto guiding is now behind me. I can now run PHD at 0.6 arc sec RMS error all night long using an OAG setup on my 6" RC at native 1370 mm focal length. You would think that the worst problem is over - but no! I have hit another stumbling block - the optics. To get the 1000+ mm focal length, I have a GSO 6" RC at f/9. Not only is the RC slow but it is also a bear to collimate. I could get semi-decent result when I was using small sensor like the Atik 314L which I now sold; but I simply could not get the stars round at the far edges / corners when using a DSLR.

I am seriously thinking about changing the RC to something a bit friendlier to use. But cannot decide which path to go down:

8" newtonian f/4 for 800 mm or f/5 for 1000 mm; or 10" newtonian f/4 for 1000mm
A 120 mm refractor at f/7.5 like SW120ED for 900 mm
A truss RC (smallest is 10" I think) at f/8 with fL 2000mm, hopefully easier to collimate as I think the primary and focuser are decoupled.

What do you use for 1000 mm + focal length and why?
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Old 09-03-2016, 08:55 PM
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Somnium (Aidan)
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with any RC or Newt you will need some sort of corrector to fix up those corner stars. as far as collimation goes, i am not sure a truss RC would be easier to collimate. with a good laser, Newts are pretty straight forward to collimate. but if it were me i would stick with your 6"RC if the only concerns are collimation. stick to it, maybe get a TAK collimating scope and invest in a corrector.
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Old 09-03-2016, 09:05 PM
HenryNZ
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Here is an image (resized heavily to fit the 200kb limit - dropbox link for a not so compressed version https://dl.dropboxusercontent.com/u/...GC2467_6RC.JPG) taken tonight as a test after a week long collimation attempt. It is the best result I had so far, but even then once you get about 1/2 way across the field, I start getting elongated stars and by 80% away from centre the elongation becomes quite objectionable. The elongation is what I would think "balanced", if one subscribe to the "DSI" dogma. I don't know whether this is the best that the RC can do, or whether collimation requires further improvement.
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Old 09-03-2016, 09:13 PM
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i am sure someone with more RC experience will chime in but it looks like you need some sort of corrector, the scope looks reasonably well collimated
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Old 09-03-2016, 09:49 PM
raymo
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The elongation is all in one direction, and worse in the lower right hand side, leading me to think that your camera is not absolutely perpendicular to the light path.
raymo
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Old 09-03-2016, 09:54 PM
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Quote:
Originally Posted by raymo View Post
The elongation is all in one direction, and worse in the lower right hand side, leading me to think that your camera is not absolutely perpendicular to the light path.
raymo
It is also likely to in part be the 0.67x reducer, the RC will natively have field curvature if uncorrected, having a reducer will make the situation even worse. It may be slow without the reducer but you'll get much better looking stars.
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Old 10-03-2016, 02:50 AM
HenryNZ
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Hi atmos, that's without the reducer as I was testing collimation.
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Old 10-03-2016, 04:47 AM
HenryNZ
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Still interested what other use for > 1000 mm focal length. Will obviously keep trying on my RC but doesn't hurt to start looking elsewhere
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Old 10-03-2016, 08:37 AM
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The problem you have here is field curvature. For some reason people think RC's have a field corrected optics. This is not true and as a result the optics still need a flattener/reducer to flatten out the field. Your image looks well collimate to the point that any mis-collimation is barely noticeable.

You are using a large sensor on a field that could only support a smaller sensor like you had before and hence why you were getting "semi decent" results before.

To get around the problem you need to use a flattener/reducer. If you use a reducer it must have flattening abilities.
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