Borg DGL 0.85 reducer (#7887)

[citivolus]

Has anyone done any testing of this reducer on medium focal length refractors? I'm specifically interested in flatness and astigmatism when coupled to a refractor such as my Megrez 90 (90mm f/6.9 FD). I know Avandonk has one of these, but isn't primarily using it on a refractor. Hopefully a few other people have run into some information on these, as Google seems a bit sparse on information.

The 5D/101ED image at the bottom of this page looks promising:

http://www.sciencecenter.net/hutech/borg/parts/html/7887.htm

as do the spot sizes in the graphs.

This being a significantly more expensive flattener than the one that I currently have, I thought I would ask around before picking one up ;)

Regards,
Eric

[avandonk]

Eric the #7887 works extremely well as a field flattener. The reduction is only 0.85. I have used it on everything from a 80ED, 100ED, 150mm diameter F5 Achro and the TAL200k. There is a thin Aluminium ring between the two sets of elements that you change for different thickness ones depending on the focal lenth of the OTA. This FR/FF works very well on the full frame Canon 5DH with all these scopes.

I now use an Astro Physics FR/FF on the 100ED only because it gives me a focal reduction of 0.75. For this you also need an Astro Physics 2.7" Focuser.

This image was taken with a Skywatcher 150mm F5 Achro with the Hutech #7887 FR/FF so it is now a f/4.5. I also used a Hutech nebula filter. Taken with Canon 5DH.
Large image 1.0MB
http://members.optusnet.com.au/~cheekyfish/ic4628.jpg

As you can see it is good to the corners.

Here is an early thread on the #7887 with an 80ED.

http://www.iceinspace.com.au/forum/showthread.php?t=7979&highlight=7887

Bert

[citivolus]

Thanks for the info avandonk. I've just ordered one of these. As the Borg is somewhat modular, I'm also picking up a #7425 and a #5005, so that I can use it on my Canon and in a 2" focuser. Ted from Hutech reminded me that the threads on this are not t-threads, so the Borg t-ring becomes a necessity. Given how large the clear aperture of this beast is, I should be happy with it for some time to come.

I'll do some side by sides with my WO Flattener III when it arrives, and then likely put the WO up for sale. I was being cautious as all up this is going to be over twice the price of the itself-not-cheap WO, to remove the slight amount of astigmatism I am seeing with my scope. I do like Hutech's 30 day return policy.

The earlier test shot I had posted using the WO had a mistakenly inserted spacer in the image train that was exacerbating the astigmatism by increasing the reducer to sensor distance to 63mm, but it is still there (albeit to a lesser extent) when imaging with the proper 55mm reducer to sensor distance.

Regards,
Eric