Borg DG-L 0.85 reducer (#7887) vs WO 0.8 Flattener III

[citivolus]

My new flattener arrived today. I plan on doing a little review on it compared to the WO Flattener III. For now, here is a shot of the two side by side.

Initial impressions on the Megrez 90: very promising for astigmatism. More on that later.

Eric

[citivolus]

OK, I did a bit of preliminary work on this tonight, and shot the attached images.

I picked a magnitude 1 or so star through the clouds and shot a bunch of 1/10 second images with it both at the centre and top left of the frame. As I was not fully set up tonight, this was done on a camera tripod hence the very short exposures.

Attached we have the following images:

1. Star at centre of frame, Borg flattener

2. Star at top left of frame, Borg flattener

3. Star at top left of frame, WO flattener

As you can see, there is no appreciable difference in star frame at the top left of the frame vs the centre with the Borg flattener. Even the dimmer star to the right in the frames has good shape and no size change.

The WO flattener is a different story. While the star size is roughly the same, the astigmatism skews the light out into a diamond shape. Unfortunately the dimmer star was out of the top of the frame in these shots so I can't compare it.

I plan to do a full comparison this weekend if weather permits, with some wide star field shots.

Regards,
Eric

[iceman]

hmm looks nice Eric. What's the price difference?

[citivolus]

The total on the Borg DG-L including a properly sized Canon EF t-adapter, 2" focuser nose piece, exchange/credit card fees, and shipping from Hutech in the USA was $476 AU. The WO Flattener III with a Canon EF t-adapter is currently about $270 from Andrews, so we're looking at a $200 price difference, which isn't too bad when taken as a percentage of the cost of the complete system.

[[1ponders]]

Interesting Eric. The WO II seems to do a much better job than the III (See review) (http://www.iceinspace.com.au/index.php?id=93,458,0,0,1,0)

[citivolus]

Tonight's testing was interesting. The attached shot is a single 60s exposure, flat calibrated, no dark, stretched. You can find a higher resolution version here: http://img.regolith.net/astro/ngc3372.jpg

This was taken with the Borg flattener on the Megrez 90. You can see that it is nice and flat, with no evidence of coma or astigmatism.

Unfortunately my testing of the WO FF III was rushed by some clouds so I didn't really get usable data, but oddly it seems that the astigmatism can be worked around if you get critical focus, as I did one test shot with the WO that didn't have astigmatism. I'll have to test this out again later, especially on some brighter targets. The area of sky I was using when testing the WO was lacking in sufficient bright stars to really abuse it.

Eric

[citivolus]

Attached is an image of the curvature of the Megrez 90 without a reducer. It is significantly more severe than the ED80 from your test shots, which I suspect has a lot to do with why the WO Flattener III doesn't work so well with it (at least not for me :) ). The WO Flattener III may do well on the ED80 due to the difference in the curvature.

[[1ponders]]

Wow. It would be interesting to see how the WOII would work out.

[glasseyes]

Hi Eric,

I read your comparison tests of focal reducers with great interest as I have both an ED80 and a Megrez 90. I have yet to decide on which focal reducer to buy, but the Borg looks good, apart from being the most expensive.
Paul's comments left me wondering whether the WO2 might actually perform better than the WO3 when used on the Megrez 90? I had read that the WO3 was designed for use with 80mm and 90mm scopes, so I would have expected it to perform better on the Megrez 90 than the WO2.
Have you actually proved which one works better with the Megrez 90?

John

[citivolus]

I have not tried a Flattener II. My understanding is that it may correct better, but that it has more vignetting due to the smaller clear field. I also understand that the version III was designed for the 80-110mm scopes, so it may actually perform better at one end of that range. Additionally, the curvature of the ED80 is likely different so I can't say how well it would work with those.

I hope to try the WO Flattener II some time. If I do I'll try to post some comparison shots.

Regards,
Eric