T-mount spacing distance/rings: Megrez 90 + FFv3

[rogerg]

G'day all,

I have a Megrez 90 and the William Optics field flattener v3 which I'm trying to combine to produce acceptable results (so far without success).

From reading and asking on the WO Yahoo! group I am under the impression I need to change the distance between my FFv3 and my Canon 350D's chip.

A suggestion was these:
http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=2761

Does anyone have suggestions on products available in Australia which will allow me to vary the distance between FFv3 and chip at millimeter accuracy?
:help:


I'm familiar with t-ring spacer tubes of 10mm, 25mm, 50mm lengths etc that come with Meade focal reducers, but not anything smaller than those.

Thanks,
Roger.

[citivolus]

Baader makes some in 7.5mm, 15mm, and 40mm as well:

http://www.baader-planetarium.de/sektion/s08/s08.htm#+25abc

(see http://www.optcorp.com/productList.aspx?mid=225&uid=105-182-967 for some english).

Somewhere around here I'm sure I have a few of those, and could try them with the Flat3 on the Megrez 90 if I can dig them up. I purchased them when trying, like you, to get the M90 working better with the Flat3. I'll let you know if I can find them. I seem to recall the 7.5mm being within 1mm of optimal for coupling the Flat3 to a Canon EF, but it has been a while so I can't confirm that.

Anything adjustable will unfortunately likely introduce flexure, which would be counter-productive. I used to have a variable Meade adapter, however it was way too big for the spacing you are looking for.

Regards,
Eric

[rogerg]

Eric,

Did you end up having any success with the Megrez 90 + FFv3 + Canon?

Thanks,
Roger.

[citivolus]

Nope, I did not have success. I ended up getting the Borg 0.85 flattener for comparison and it blew the Flat 3 away. I still have the Flat 3, so when I track down the rings (and if it ever stops raining) I'll tinker a bit more with it. I did find a receipt saying that I purchased the Baader 7.5mm T-2 adapter, so it is around here somewhere. I can't confirm if I ever used it or not.

Consensus seems to be that the Flat 3 leaves too much astigmatism in the edges when paired with the Megrez 90, so you would need to throw out 1/3 of your image when shooting with a 1.6 crop DSLR in order to have round stars. I've spoken with at least four Megrez 90 / Flat 3 owners now who ended up ditching the Flat 3 due to the same issue, or just accepted that they would need to crop.

The USD $300 Flat 4 (http://www.williamoptics.com/wo_shop/catalog/product_info.php?cPath=26_42&products_id=436) may fix the problem as it adds a tuning adjustment to optimise it for varying focal lengths. I have yet to see a review on it though, as it is only a few months old. The parts I needed to make it work in my case were Borg #7887, 7425, and 5005, which totaled around USD $415 + shipping if I recall correctly, so the Flat 4 would still be a viable option if it really works.

Here are some threads on the subject with some example shots from my testing:

http://www.iceinspace.com.au/forum/showthread.php?t=28918
http://www.iceinspace.com.au/forum/showthread.php?p=304314
http://www.iceinspace.com.au/forum/showthread.php?p=301109

Somewhere there was another with sample images comparing the Flat 3 and Borg DGL, but I can't find it right now.

Regards,
Eric