Field of View

frank14

I hope I can make this short. This is my first post on an astronomy forum ) I have a 10" f.7 hand built clock driven reflector. I want to do some astrophotography. I have been looking at the Zwo 120. I have been doing some Field of View calculations and with a 4.8 x 3.6mm sensor for the Zwo 120, the FOV with my 1778mm FL mirror is 9 minutes. That might be perfect for DSO's like the Ring Nebula (3') and the Dumbell Nebula (6.7'), but it is not enough to get in M13 (20'). So here's my question. If I purchased Zwo's 60mm f.4.6 guide scope and put the Zwo 120 CCD on it, my calculations show a FOV of 60'. If I purchased their 80mm f7.5, then I would get a FOV of 27.5', perfect for M13. Am I missing something here? Are my calculations correct? FOV = size of the sensor divided by the FL of the mirror or lens, and that number times 3438 gives the FOV in minutes? If my calculations are correct, then a simple fast guide scope, with the small sensor of the CCD, could make a great auxilary scope to mount on my large telescope, and then I would use the CCD on the scope that best matches up with the FOV I need for a particular DSO. Can anyone confirm? Thanks, Frank.

einari

That looks pretty much right.

There are these online calculators which you can use and they show various DS targets as well.

Here's one:

http://www.blackwaterskies.co.uk/p/imagingtoolbox.html

Mind you, you drive should be pretty accurate to image in 1778mm focal length.

Do you have autoguiding possile in that mount ?

Also you could use a DSLR to get a bigger field of view.

frank14

Thank you, Einari! Now I know I am on the right track and my mind is spinning with the possibilities. I have concerns about the limited FOV on my 1778mm, even with my Nikon D7100 DSLR (45' FOV). I did a great job on building the clock drive, but you're right, at that FL, a 30 to 45 second exposure is about the most I can hope for. In addition, I would be using Live View, with all the attendant heat build up that brings, and no auto guiding is possible. With a smaller scope and a Zwo CCD, I can mount the whole thing on my Orion portable clock drive and auto guider. I am crunching the numbers now to see exactly what equipment I will purchase. Thank you again! And thanks too for the online calculator link. I know that is going to be extremely helpful! I'll keep you posted.

frank14

Thanks again, Einari! I made the decision for the Zwo 178MC-Cool and the Orion Short Tube 80 refractor. The web link you gave to the FOV calculator was incredible in enabling to see exactly what I would get with a variety of cameras and lenses/mirrors. Can't wait to get and use 'em!

Frank.

merlin66

Frank,

The other handy FOV calculator is CCDCalc....

http://www.newastro.com/book_new/camera_app.html

frank14

Thank you! I will check it out!

Frank.