Quote:
Originally Posted by RobF
I ended up a millimetre or two shorter than what I calculated for theoretical lightpath distance of adaptors etc Troy. I've read somewhere that filters in the light path can "increase" distance. Could be bollox, but seemed to be the case for myself.
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G'day Rob. Yes, I'd read similar and took the filters into account. The QSI website is awesome in their technical section, and they've got a special page (PDF?) just on backfocus, and even have worked out how much to add for my Astronomiks filters as well as some other types. I'm thinking my tweaking is necessary due to the tolerances rather than the 0.33mm extra the filter adds anyway.
Quote:
Originally Posted by RickS
Not "bollox" at all 
A typical filter will have an optical thickness about 1/3 less than its physical thickness. The focus shift is T x (n-1)/n where T is the thickness of the filter and n is the refractive index of the glass.
The SX AO unit I just received has an optical length which is 4mm less than the physical distance from front to back. I hope I got the calculations right...
Cheers,
Rick. |
Thanks for the confirmation, mate.
Quote:
Originally Posted by pmrid
This is interesting. I'm setting up an all-SX imaging train for my astrograph and that mmeans either a straight MPCC (55mm) or one of those 90mm jobs Baader also sell.
At the moment, my best calculation of the physical backfocus from MPCC flange to imaging train in 57mm and I recall reading from Baader that the MPCC tolerances are only +/- 1mm. But from this thread, I see that my actual optical distance could be more than that 57mm because of the filters in between. Have I got that right?
Peter
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G'day Peter. Think I'd stick with the straight 55mm MPCC. Sounds like you have it. From my reading the common increase in lightpath is 1/2 to 1/3 of the filter thickness. My Astronomiks filters are 1mm thick, so talking 0.5 to 0.33mm extra. I've worked on the 1/3, but it's really splitting hairs and as I'm discovering, the tolerances in the order of millimetres or 2 take that out of the equation somewhat anyway.
Quote:
Originally Posted by Tandum
Yes, the wheel thickness should be the actual wheel thickness + 1/3 filter thickness but that 1/3 should be trivial, under a mm.
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G'day Robin
Yes, that's what I'm finding as per above.
Quote:
Originally Posted by kinetic
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Thanks Steve. I've got a Moonlite focuser so the collimation of that is sorted.
I think I may have a bit of CCD tilt going on as well. The MPCC has that indentation around it where the focuser's compression ring, I assume, is supposed to sit. But I think it doesn't give a chance for the camera to sit square and think I'd prefer it if the part of the MPCC that sits in the focuser was smooth. I was going to post this question in another thread.
Cool adapter for spacing there. I'm just using a stock, not adjustable, 20mm long T thread extension tube. To allow for some fine tuning, I'm thinking about getting a 15mm, 2mm, and 3mm extension tubes. In combos I should be able to get 15, 17, 18, 20, 22, and 23mm extensions and hopefully somewhere in there will be the sweet spot