C11, Orion 8in F/3.9, or XT8 on Atlas + SSAG + Orion StarShoot Pro V2/Meade DSI II Pro [or ES 82* 24mm, 11mm, 6.7mm] http://www.flickr.com/photos/95673005@N06/
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Quote:Yes, I was also suspect the asymmetric coma. Come to think of it, I should do collimation more religiously since all the moving-around involved with mounting XT8 every time. However, I do have another shot at the same session which show symmetric coma:m81_60s The forcus was off probably due to the dew on primary and I cannot make it better. But the coma was around.People at Beginner Imaging forum thinks FR and Newt (even f/6) will never work. I do like to hear people's opinion on this since I try to find what to do next.Should I buy a better FR (if that ever possible to work with XT8) or just get a F/4 Newt. The power of large F/# demonstrate itself very well in my video session.
Quote:On the other hand, combining a CC and FR altogether is difficult and I've never tried it.Usually CC needs to be fairly closed to focal plane (about 55mm inside) while FR sits much deeper inside. I *think* CC should take place first then followed by a FR. However per my earlier described optical sequence, it is not in that order.Clear Skies!ccs_hello
Quote:For fast Newt (or slow Newt with FR added), coma will always be there unless you have a matched Coma Corrector.
For an even faster Newt with its entire image circle shines (due to the use of a FR) on a CCD imager, the edge distortion will be there.
I.e., there is no free lunch anywhere in the optical world. High quality fast lens or OTA costs quite a bit. Newt being an entry level product should be a hint.
Quote:Lambda Research still offers a free download for their optics design software called Oslo .There is a comprehensive user manual, tutorials , and a truckload of examples both in the user support area of their website and in the actual software download . I recall several years ago they had a few working examples of newtonian reducers and correctors including Ross, Wynne and a combination of focal reducer plus coma corrector in one . Their demo software was at that time limited in the number of surfaces but the limit varied depending on version (never less than 10 surfaces) so any version allows analysing a combined netwonian reducer plus coma corrector .I don't know how it works now because I have the paid version which I haven't upgraded in a few years but I see they still have the free limited edition.
Quote:The degree of coma depends also on sensor size. The little video chips having widths of ~6mm--in addition to their small pixel counts--are rather less afflicted by coma (and field curvature) than larger CCDs. An additional advantage of the wee chips is that you can apply more aggressive focal reduction before aberrations and vignetting become offensive.An f/4 Newt with reduction via inexpensive 1.25", so-called 0.5X reducer spaced to give 0.7X (and *perhaps* as low as 0.6X) on a video cam should deliver acceptable images. At f/2.8 (0.7X), that's an image whose surface brightness is 2X (a full f/stop) brighter than at f/4.
Quote:Thanks to all the help and ideas.Last night I collimated my XT8 before mounting. After mounting, it was off a little bit, so I re-collimated with Glatter Laser kit. And I triple checked the FR was threaded correctly. I have also tried different 2"-to-1.25" adapter, including Howie's parallizer. And there is no dew last night.Unfortunately, I still see the same distortion at edges. So I will try to get Antare's 0.5x FR just as a double check since it is not too expensive.And I will be go overseas for several months so I will not have chance to work on this issue until July. Maybe I'll get a F/4 scope at that time. Thanks to all the help and I will keep you updated later.