widest angle of view from plossl

I recently bought a 32mm plossl for the same reason. It does help, but certainly doesn't become an alternative to the spotter scope. Based on the advice from here, I have a Telrad on the way from the US. I got it on eBay - €43 incl. delivery.

Ok, a few more numbers to help (or confuse you)… ‘cos I’m bored!

First, I presume you have an f/10 SCT – either 8”, 10” or 11” aperture.

8” f/10 Focal Length(FL)=80”=2032mm
20mm Plossl = 101.6x magnification and yields a 0.49degree field of view (based on 50mm fov for a Plossl)
32mm Plossl = 63.5x and yields 0.79degree fov
Lowest useful magnification (7mm exit pupil) = 29x with a 70mm eyepiece – good luck finding one!

8” f/6.3 (f/10 with focal reducer) Effective Focal Length = 50.4” = 1280.16mm
20mm Plossl = 64x and 0.78deg fov
32mm Plossl = 40x and 1.25deg fov – now you’re talking!

Also…

10” f/6.3 EFL=63”=1600.2mm
20mm Plossl = 80x and 0.63deg fov
32mm Plossl = 50x and 1deg fov

11” f/6.3 EFL=69.2”=1760.22mm
20mm Plossl = 88x and 0.57deg fov
32mm Plossl = 55x and 0.9deg fov

The longest focal length eyepieces I have come across are an 85mm Plossl from Russell Optics and a 52mm Erfle from Antares. Both used to advertise in Sky&Tel and are on the web. You’ll have to hunt down the FOV of the Erfle for yourself!

NB: You will get more vignetting (light loss) towards the edge of the field of view when using the focal reducer but this will probably only be noticeable when you are imaging… and then only on a largish CCD chip. You probably won’t notice the vignetting visually. The other problem with the focal reducer is that you will have to screw it on every time you want a wide field for star hopping – it doesn’t attach like a Barlow. Swings and roundabouts!

Hope you’re not too confused now

Philip.

PS – if you haven’t got an f/10 SCT, and you want the numbers crunched for your scope, let me know what you do have J

Hi,

What kind of finder do you have??

I have acouple a 10x50 and a 6x30. I also have a 6x30 spare. If thats any good to you , you're welcome to it FOC.

You also need to remember that finders can be a tricky item to get used to. Maybe you just need more practice? For example I can only use a straight through one, I cannot, for the life of me, use a right angled finder.