field flattener required?

troypiggo (Troy) · #1 04-03-2009, 02:06 PM

I'm noticing the stars in corners of my images being distorted. See here: http://piggo.com/~troy/photos/2009/2009_02_28/m42.jpg or http://piggo.com/~troy/photos/2009/2009_02_28/m83.jpg

I understand this is due to the optics of my scope, ED80 Pro. A focal reducer or field flattener (same thing?) will stop this? But this will change the field of view? How much? I see there are .6x etc. Is that directly proportional to the size of field of view?

Is there another way to prevent this distortion, other than post-processing?

Sorry for all the questions.

gbeal · #2 04-03-2009, 06:17 PM

Troy,
hmmmmm, the finicky side says yes perhaps, but the realist in me didn't see too much to complain about, mind you the cracking M42 shot took my eye away from the corners.
A while back I bought an 80mm triplet, famous one too, very expensive. It requires, nay demands, a field flattener, and the usual/accepted fix is the Televue 0.8x reducer/flatenner. Price one and see why I winced. I tried my Baader MPCC and it works. Yes, I still went ahead and got the Televue, but the key to this that the TV one gives me a shorter (0.8x shorter) focal length, something I sometimes want, sometimes I don't. The MPCC doesn't change the focal length.
What am I suggesting here? Try an MPCC if you can, they are common enough that you could possibly borrow one. But like any of them it MUST be at the correct spacing to achieve what you ask of it, and in my case this necessitated a specific, custom made, adaptor.
Gary

troypiggo (Troy) · #3 05-03-2009, 03:27 PM

Thanks Gary,

Been doing a bit of searching. Those MPCCs read like they're for Newtonian reflectors. I've got an ED80 refractor. Is that why you mention adapters?

Tilt (Michael) · #4 08-03-2009, 05:51 PM

To deal with the distortion I use a William Optics 0.8x FF/FR (Pflat-2), it knocks the focal length down from 600mm to 480mm. It works quite well with the ED80.

Michael

[1ponders] (Paul) · #5 08-03-2009, 06:38 PM

Check out the Article here.

gbeal · #6 08-03-2009, 07:38 PM

Hi Troy,
yes, I suppose the core design is for a reflector, and a fast one at that, one which is likely to suffer coma.
BUT, try one, or the W/O that Michael suggests.
The key to my comment about adaptors is that the spacing chip to reducer of flattener is often critical, +/- mm or so.
Gary

troypiggo (Troy) · #7 08-03-2009, 09:15 PM

Thanks for the recommendations and the article. Good read.