fastar

[Astroblagger]

Hello Iv recently looked at afew vids of folk imaging with the fastar system at f2 it sounds too good to be true, but if it was that good everyone would be using it, are there any drawbacks to the system??

[Peter Drew]

My understanding is that it is potentially very good but a bit tricky to get working well due to being particularly sensitive to positioning and collimation. The removal and replacement of the secondary also becomes a bit of a put off for the fainthearted.  

[ollypenrice]

Like most things that seem too good to be true... it probably is!

What are the drawbacks? F2 produces an incredibly steep light cone so that being out by a few microns will see you thoroughly out of focus. The focus mechanism is the less than perfect moving mirror sytem designed, originally, to work with the very shallow and forgiving light cone of the standard F10 optics. This is not a good combination. The steep light cone and shallow depth of field also make the system hypersensitive to chip orthogonality. If the chip is not perfectly square to the light cone (if it has a little tilt) it will be in focus on one side but out of focus on the other. Next up is the problem created by stars as photographic targets. They are very challenging to control yet we all want our stars as small and as tight as possible. If you look at broadband Hyperstar images they tend to look large and soft. (Some people will have mastered this issue but I rarely like Hyperstar stars in posted images.) Narrowband imaging makes the stars easier to control becaue they are inherently smaller and we used to seem some fine NB images from Greek Anthony on here at one time from a 14 inch Hyperstar.

Then there's the central location of the camera. DSLRs will produce diffraction artefacts because they overlap the corrector plate. Small CCDs won't do that. If you use a mono CCD then the problem is that you have to change filters using a slide drawer, there being no room for a filterwheel. This can make flats tricky because you may vary the filter orientation when changing them.

No free lunch. Quite a few people go for speed via the dual rig approach. Given that decent little apos are so affordable this is a serious alternative in the search for fast data.

Olly