Quote:
Originally Posted by Joshua Bunn
I would suggest focusing on the centre of the frame, then checking the corners - see what they look like. Then, move your focuser in (or out, depending on the curvature) so the corners are in focus and the centre out of focus. This will at least tell you if the collimation is good and if its really field curvature. what do you think?
Josh
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Yes good idea Josh. I will undertake this for further determination of what is going on.
Quote:
Originally Posted by ericwbenson
Hi Paul,
It's been a while...
+1 to what Joshua said. The stars in the corners when focused (so that the center is out) will show you the best that the flattener could hope to do. In order to fix anything else (mostly off-axis astig and coma from what I can see), the system needs a corrector. Remember an RC is not entirely coma free, it has a coma free zone, and only if the hyperboloids hit the prescription, and it is definitively prone to off-axis astigmatism, this may be the wall you are hitting. (Doug G. once mentioned to me that he could start to see the off-axis astig on his uncorrected 12.5 RCOS with his KAF6303, so the KAI11000 would definitively show it).
I believe changing the mirror spacing primarily affects the amount of spherical aberration in the system (SC's are like that, as are CDKs). There is a point where it is minimized. On my system it has to be better than +/-1 mm (so says PWI). PWI supplies a Ronchi eyepiece and spacer tube to adjust the mirror spacing until the Ronchi shows a null. I gather GSO doesn't supply this? If they can tell you the correct distance the Ronchi should be from the back plate, you could then borrow my Ronchi and cut a 1.25" ID tube to the right length.
The flattener can add it's own aberrations too unfortunately, to observe those rotate the flattener wrt the OTA and the camera (you sorta did that already), but keeping the camera fixed wrt the OTA might help in the diagnosis.
Best,
EB
p.s. Congrats on the Malin!
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Thanks Eric.
I now have the optimum spacing distance. I am told it is 687.89mm from centre of both mirrors. Not sure how I go about measuring that.
Good idea about the Ronchi eyepiece. I will let you know when I will need it.
Yes I was thinking the flattener would introduce aberrations if the mirrors were not at optimal. Ted and I were discussing this fact the other night.
LOL GSO does not provide such things. Remember cheap comes with its own issues, and this is one of them.
Quote:
Originally Posted by Logieberra
Paul
Did you consider this one as well?
http://www.teleskop-express.de/shop/...rkuerzung.html
2" corrector for f/8 RC telescopes - without altering the focal length
Suitable for all GSO RC telescopes from 6" to 16" and for all RC telescopes with F8
I was considering it...
Will you return yours, if it's not working? |
This is actually not the recommended one. It says it will work but limits the size of the sensor you can use. My Flattener is the 3" version which is supposed to work with larger sensors. Not planning on sending mine back. I am sure it works but need to sort distances etc. I am now told by TS that a figure between 106 and 109mm will work. I am there so now it must be either/or both distances of mirrors and degree of tilt.
Quote:
Originally Posted by Logieberra
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Short answer is no. Jim is great with reflecting surfaces. He admits that he is not a refracting expert. So GSO has no plans at all to manufacture correctors at all. It will need to be handled by other parties. Hence why we have seen all manner of correctors being used without any announcements by GSO to make one for their RC range. The one they provide for their other gear is to the best of my knowledge made from an outside source.