Starfield 0.8x 2.5" adjustable flattener

[pipnina]

With such a massive range of flatteners about, it's hard to say which ones, or at which price ranges, are any good. I certainly found that the TS Photoline 0.79 I used with my APO was not very good (chroma, undercorrection etc).

Not knowing how much I might need to spend, I decided to go for a "cheaper" option on FLO first: The Starfield comes in at only £220 which is quite low-end for big-scope flatteners (WO sell one at £700+, the small Riccardi sets you back nearly £500). But it also comes with the benefit of a big set of lenses and rare feature of adjuatable backspacing!

I got my old kit off the back of my scope, assembled the focuser with the new corrector, and a WO rotator I picked up to allow me to continue adjusting the camera rotation.

Immediately I found that this corrector moves the focal plane a lot more than the Photoline. So much so that even though the corrector sits in a similar place to the Photoline, my focuser ran out of travel! I had to remove the rotator to give me enough slack, and I only have a few mm left either way.

Impressions once I got my focuser re-straightened for the new kit were quite good. Stars seem quite tight around the field, with a small bit of coma right at the corner of my APS-C frame, however this may be related to backspacing which also needs some discussion.

I started off with it at 0 on the adjustable scale. I noticed it might be a little short so I moved it, in two goes, about 5mm further out. Surprisingly this had little affect on the image! My Photoline flattener was so intolerant of backspacing error that missing 5mm of backspacing produced a warp-speed effect! This one almost doesn't seem to care, but I will need some more testing to be sure.

As for a more "final result" here's a stack and a single-frame (3-sub R-G-B) colour image of the results:

I think the biggest issues with the corner stars here come from the optics of my OTA and not the corrector, I think my focuser is still slightly mis-aligned or maybe my front cell now isn't 100% tilted properly because the focuser has moved. Either way I think this is a big improvement over the photoline in various ways (less vignetting and nowhere near as much chroma spread at the corners for one, better tolerance for poorly set backspace for another. And the price is quite reasonable. This is on an APS-C sensor.

Photo taken with the Photoline for comparison:

[Coco]

Looks like spacing is wrong to me, not the scope or the flattener, smaller stars come from better optics.