How can I get a WIDE field?

[koputai]

Guys,
I'm wondering how someone might get a wide field view, preferably with aperture, for visual use.
It seems that the bigger the scope, the less width of field you get.
Now I think there are probably two options:

1) Meade 5000 80mm refractor. Fitted with a 35mm Panoptic eyepiece, would give a field of a bit over 5 degrees, similar to binoculars. The aperture is a bit small though.

2) 8" SCT with f3.3 focal reducer. Fitted with a 35mm Panoptic eyepiece, would give a field of 3.6 degrees.

Can you see any major issue with either of these set ups? I know the f3.3 is said to be for photographic use only, but surely if a camera records the image well, then the eye must be able to see it?

Cheers,
Jason.

[skwinty]

Hi Jason
My understanding is that the focal length is the fov decider rather than aperture.:thumbsup:
The eye does not integrate the photons the way the camera does.
The eye is wysiwyg whereas the camera keeps adding the photons up.

[rogerg]

Hmm, yes, a common quest but not entirely easy!



The same would go for any 80mm refractor like the ED80's I'd think, they're relatively wide field and are comparable to binoculars when using wide field eyepieces.



I wouldn't recommend that option. I believe you'd get an extremely vinyetted and curved ugly view. Nothing pleasant. F/3.3 reducers have limited uses even with photography these days, and were never suited to visual use. SCT really isn't the direction you want to go, they are long focal length from the start.

If you're after 8" you're going to get a wider FOV and better view from a quality 8" DOB with focal ratio of about F/4 or 4.5. I think they are reasonably common but I'm not a newtonian guy :)

I think youre best bet is going to be wide field refractor (gets expensive, but like the William Optics FLT 110's), or "fast" DOB like in the F/4.5 range (of 8", 10", etc).

I'm pretty sure that in general wide field with aperture = $$$

[Zuts]

Hi Jason,

A small high quality refractor, say a televue or takahashi 85/80mm will give pleaseing widefield views with say a 26 mm Nagler (23 mag).

Usually though the view will not be so pleasing in a cheaper ED80. The cheaper scopes have a great deal of field curvature which is fine for taking photo's as it can be corrected with a field flattener but not so fine for viewing where the smaller focal length eyepieces tend to give better results.

Paul

[davewaldo]

Hi Jason,

Just to continue (or rather add) to what Roger was saying about Newtonians....

The GSO 8" F4 Newtonian would give quite a wide view but also has the aperature to show you some faint objects.

You would need a coma corrector like the Baader MPCC with any newt at F4, but all up this setup would only cost around $700 -$800, A LOT less than a premium refractor.

This Newtonian has a focal length of 800mm, which is similar to most wide refractors so would give similar wide views with the same EP.

Just another option... :)

Dave

[davewaldo]

Also... this calculator will show you the true field of View for many scope and EP combos.

http://www.csgnetwork.com/telefov.html

The GSO 8" Scope with a panoptic 35mm EP would yield almost 3 degrees FOV. More with a Nagler!

[Zuts]

Hi Dave,

I still think for widefield and portability a small premium refractor with unobstructed aperture will give better sharper views than a newtonian. At significant price premium though.

Paul

[davewaldo]

I agree Paul that a premium refractor would be wonderful, however I was giving another option which gives wide views "with aperture" as Jason asked. Also I'm not sure that portability is a concern as it wasn't mentioned in the original post.

[Bassnut]

I understand an 80mm aperture refractor would not be suitable for visual, aperture counts with visual, or your viewable objects would be limited to the very bight.
I would say more aperture with a reducer would be more satisfiying.,, even if there was some vignetting..

[koputai]

Guys,
Many thanks for the ideas thus far. I should qualify things by saying that it would be for satellite spotting, so basically point source objects, but down to mag 10 or 11 would be good. It would also be mounted on an alt/az motorised mount, so small would be better than large. A right-way-up, right-way-around image would be highly preferable. Think motorised big binoculars to follow fast moving objects (but not bino's, only a single OTA).
Is it possible to take a Newtonian OTA and add gadgets to give a correctly oriented image?

Cheers,
Jason.

[AstralTraveller]

Jason,

As my grandmother would have said, 'you want your cake and eat it too'. You and most astronomers! ;) Getting good wide-field views is fighting laws of nature. But, hey, what are laws for? :P

I think the best 'wide-field scope with aperture' may be a schmidt-newtonian. According to the manufacturers, a f/4 s-n has the off-axis coma of an f/5.6 newt.

Consider the effects of vignetting in any scope but particularly newts where the secondary size is often the limiting factor. But of course you want to keep the secondary diameter <25% of the aperture or you lose too much contrast (20% obstruction is best).

Remember the limitations of eyepieces. My 30mm 80 degree fov eyepiece is sold as the widest possible true field (in 2" barrels). On a 20cm f/4 that gives 3 degrees.

{EDIT: If the exit pupil is larger than your pupil not all the light will get into your eye and so in 20cm scope you will only 'use' 18cm or 15cm or 10cm.... of it's aperture. The diameter of the exit pupil is aperture/magnification and your dark-adapted pupil will be between 7mm and 5mm depending on age. This gives a usable minimum magnification of 28x to 40x. A 30mm eyepiece in a 20cm f/4 gives 26.7x and so is just usable.}

If you really want 5 degree fov why not get some good binos and a good stand and chair. I enjoy my 15x80 binos a lot.

Cheers