What camera lens

[chris connolly]

I have C9.25 with hyperstar and a Atik 428ex, I also have SW ED80 and Canon 1000d on the same mount.

My problem is they both give the same FOV.  

If I was to sell SW ED80 and go for a camera lens to give wide FOV what lens should I go for spending no more than I would get for ED80

[Digz]

I'm not sure what you would get for your ED80 but what I would say is check your measurements carefully. By that I mean, you may find it difficult to use a camera lens, CCD camera and filter wheel if you don't have a OSC. It all depends on the back focus of the CCD and lens and the thickness of adapter required to connect the toe to get focus.

Quite bizarre to see this post as I spent all last night researching the use of camera lenses and CCDs.

[LDUNN1]

I think Chris may have meant the C9.25 with hyperstar & 428ex is giving similar FoV to the Canon 1000d on the ED80 & his question is not about using a camera lens with the 428ex, but using a camera lens on the 1000d - correct me if I am wrong Chris.
 

It seems to me that the other option you have is to use the C9.25 without the hyperstar & with the 428EX, this giving you a different FoV from the 1000d on the ED80, just a thought, but I guess you've discounted that already for other reasons.

When it comes to camera lenses for astro use, be careful is what I'd say. It is not just FoV you want to check, but also how the camera lens handles high contrast point light sources (aka stars). I have Canon 5d2 & 40d & have tried a range of camera lenses out with them ranging from Canon's 15mm fisheye to an old Olympus manual focus Zuiko 600mm f6.5 & not all handle stars well, & it doesn't seem to correlate to lens price very well either.

For example, Canon's L series 24mm f1.4, on the face of it, would seem to be on paper an awesome astro wide angle lens, with its wide angle and fast focal ratio. However, stars are handled so poorly in the edges & corners when used at anything close to wide open, that you would have to stop down to f4 or slower to tighten the star shape up (they end up looking like a distant flock of seagulls with coma/astigmatism shaped 'w' instead of tight point light sources.

To make matters worse, few camera lens review sites touch on this. The only one I know is http://www.lenstip.com/ , which use a red diode to simulate a point light source on their coma/astigmatism page in their lens reviews, this would be a good starting point once you know what focal length you are after.

The Canon 50mm f1.4 is not particularly great near the edges either.

I have found the Canon 15mm Fisheye to be acceptable close to wide open, if you don't mind the fisheye look, or don't mind processing it out in software (the star shapes suffer a little around the edge when you do though). I use a 70-200L f2.8 IS throughout its focal range, wide open & it handles stars pretty well. I have not tried, but I know the Canon 200mm f2.8 fixed focal length lens also works very well for astro too. Telephotos seem to work better with stars than standard or wide angles. I've recently tried a Zeiss 21mm f2.8 distagon in Eos mount, & while not perfect on the stars, it wasn't too bad, however wide open it suffers from bad vignetting. It is known for this on daylight use, but it seemed to be even more of an issue in astro images. It is so extreme that the edges on the full frame 5d2 where like shooting at f5.6 or slower, while the centre is at f2.8! Not a great astro lens then. I've also been trying the Canon 24mm TS-E II out - this has good star shapes wide open & doesn't suffer vignetting (used un-shifted & un-tilted of course), & is very sharp, but is only f3.5, but I prefer this to the Zeiss due to the vignetting.

I've also heard that the Samyang 14mm & 24mm f1.4 handle stars pretty well (see lenstip, but note the different versons of the 24mm), but there is a bit of a question mark on their quality control from what I've read online & they are not particularly sharp across the frame (vs a good Canon lens).

You probably want to get something like the occulars plug in for Stellarium & enter your camera & various lens details in, pick a target like M42 & then try the different options out in Stellarium with the occulars plugin running so you can get a good idea of the FoV the C9.25 & 428 gives you vs the 1000d & various Camera lenses. Carte du Ciel has something similar too in the display settings section. There are also free FoV websites that you can enter your camera & lens details in & it shows you roughly what you'd get.

 



 

[LDUNN1]

Oh, & if you did mean using a camera lens with the Atik, you will have to watch the back focus. Canon EF & EF-S lenses are design with a 44mm lens flange to sensor distance. The Atik 428EX will use up 13mm backfocus, which leaves 31mm for your adapter, which should be okay........unless your Atik 428EX is the mono version, in which case, you might want/need to squeeze in a filter or more conveniently/likely, a filter wheel in that 31mm too, in which case, suddenly, that 31mm gets gobbled up fast.
I've yet to test it out, but I've managed to mount an eos bayonet on an Atik EFW2 & after the necessary adapters on the camera end (I have an Atik 460EX mono) to filterwheel, & then the filterwheel to Eos bayonet, I think I might JUST have scrapped in, but I haven't got a millimetre to spare if I have, it really is that tight. 
 

[chris connolly]

 Sorry I have not thanked members for there help. 

[chris connolly]

Sorry I have not thanked members for there help.   Lawrence was right I use the Atik 428ex only on the C9.25 with hyperstar, I do not like altering this setup .  I use my Canon on SW ed80. I have attached a image of the Rosette Nebula I took recently showing why I need a larger field of view. Whilst there loads of objects that fit my fov it would be great to have a greater choice. The idea is to shoot the same object at the same time, so having two fov.

[LDUNN1]

nice little Apple app from Atik here Chris:
https://itunes.apple.com/us/app/ccd-field-calculator/id779440109

and there is a windown CCD FoV program here:-
 

http://www.newastro.com/downloads/ccdcalc/ccdcalcfull.exe

and an online one here:-

http://www.12dstring.me.uk/fov.htm

Enter your two scope & camera details into these & you can get a visual representation of what you would get on the Rosette Neb Caldwell 49 (or others), & you can 'play' around with different focal lengths on your cameras to see what you would be happy with.