refractor advice

[alistairsam]

Hi,

just wanted some advice on a suitable refractor for visual use which i can use to view nebulae and galaxies from dark sites.
i already have two 8" reflectors (one for visual, and one F4 for AP), but i wanted something that i can keep in the car or take with me when i'm travelling by plane, also use filters and 2" eyepieces.
i'm mainly interested in observing DSO's.
i guess 80mm would be a minimum, but i wasnt sure on the focal length.
if i chose a short tube refractor, would objects look too small? galaxies, especially? what would be a reasonable focal length for this purpose for either 80mm or 102mm aperture and 25mm 2" standard eyepiece?

also, anyone tried out the long perng scopes at andrews? there is an 80mm F6 for a good price. "Achromatic 80mm f/6 OTA doublet refractor $399.00 AUD".
i'm not familiar with refractors, but are there limits on which scopes would take 2" eyepieces?

[andrew2008]

You keep sayinng DSO's are your intended targets but what are your expectations? Something in the 80-100mm range is going to be limited when it comes to most objects and as long as you are aware of this and aren't expecting to see near as much as an 8" dob you should be up for years of fun.

I have a Megrez 90 that i use for grab n go on both planets and DSO's and have lots of fun observing with it. Globs are disappointing but nebula like Orion, Eta and tarantula are all visible with lots of detail from suburban skies. Open clusters are also a fave of mine. Unfortunately i haven't had the chance to look at many galaxies yet but the Leo triplet was an easy and interesting target under dark skies.

My preference would also be to go for something that will give a wider TFOV. The 100ED F9 scopes are great value but will only give a small improvement in FOV over the 8" dob. Max is about 2.5 degrees. The Megrez 90 is F6.2 and gives max FOV around 4.2 degress. Unfortunately my current EP only allow me up to 2.8. Excellent for wide sweeping views of the milky way and large clusters like M45.

[alistairsam]

Thanks for the reply.

not sure how to word this, but if i go with a 102mm refractor, how do i calculate what focal length i'd need to say have eta across 60% of FOV with a 25mm EP?

I don't really understand the relation between eyepiece FOV, telescope FOV and other parameters.

i guess what i'm looking for is to be able to look at say M83 reasonably well from a dark site, or generally DSO's at a reasonable size with a 25mm EP. would this mean i'll need longer focal length apart from a wider objective?

I recently saw sombrero with an 8" F4 Dob and a 25mm EP, and it was tiny.
when you say globs are dissapointing, is it because they're too small in the Megrez 90?

[andrew2008]

I'll just do a rough calculation for you. A 102mm F7 will have a focal length of 714mm. 714/25mm EP will give a magnification of 28X. To work out the FOV you'll need to know the FOV of the EP. For example if it was a plossl with FOV of 52degrees you divide that by 28 to get a 1.85 degree AFOV or 111 arcmins. Eta carina neb is 120' so it would just fit, you would likely need extremely dark skies to make the nebulosity out as well to that size in a 102mm.

Globs are disappointing because they just don't resolve well. With the exception of M13, 47TUC and Omega Centauri it is very difficult to resolve individual stars with this apperture. Not that you can't see the glob but it is just that, a hazy patch.

[alistairsam]

thanks for that. makes sense now.

is the exit pupil only related to eyepieces, is it related to FOV of the scope or the eyepiece?

[andrew2008]

Exit pupil is EP focal length divided by the scopes F/ratio. So your 25mm EP will give an exit pupil of 3.5mm in an F7 scope.

[dannat (Daniel)]

GC's will be disappointing in 90mm, a 90mm scope will be decent on planets & show the bright dso (M objects)..if you want more dso than a 5-6" mak would be the way to go, on alt/az I reckon

The 25mm ep is a bit long, 9-13mm is better for dso I reckon (except bigger ones) in shortish f/l scopes (<1000mm)

To calculate what objects takes up of the fov first get the size of the object eg 30 a.sec by 20 a.sec. You will need at least half a degree or 30 a.sec to fit in in. The use scope focal length to work out view size...eg 1000mm / 10mm ep = 100x at 50 deg gives 0.5deg at ep