tweaking MPCC spacings

[troypiggo]

I've calculated the spacings between my QSI583ws and MPCC and got a 20mm spacer. The calcs take into consideration all of the theoretical dimensions etc, and all I could do was average the tolerances. My images are close, but don't think the spacing is quite right. Still some coma in corners. I assume it's only out by a millimetre or 2.

Trying to figure out whether I need to go in or out. The coma that's appearing in the corners has the thicker "head" part towards the centre of the image, and the "tail" heading out. Can you tell whether my spacing needs to be increased or decreased by that, or is it just trial and error?

[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.

[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.

[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

[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.

[troypiggo]

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.



Thanks for the confirmation, mate.



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.



G'day Robin :) Yes, that's what I'm finding as per above.



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 :)

[DavidTrap]

You can see if it the distance is too short by unscrewing the adapter slightly, and pack the "gap" with some tape. Yes it's crude, but if your image get's better, at least you'll know if you're headed in the right direction. Bit hard to shorten the distance though!

The bit about optical filter effective thickness took me a bit of a while to get my head around too. Was quite nerve racking ordering adapters from Precise Parts at a specific thickness.

Just my 2 cents worth.

DT

[tlgerdes]

Peter Tan in HK can make you a variable spacer, cost me about $50, basically 2x 2" spacers that screw inside each other and a lock nut to make sure it doesnt move. I can infinately vary it across a 12mm spacing distance.

He will make it suit whatever prime spacing you need ie My QHY10 had the CCD approx 20mm from glass, so I had made a 29-41mm spacer.

[tlgerdes]

I must also give credit to Le Froginator (Marc) for coming up with the idea.;)