widest angle of view from plossl

im having trouble at the moment navigating around the sky. my finder scope is useless and my 20mm plossl doesnt have as wide an angle to see enough of the sky to allow me to starhop. will a 32 mm plossl help much or should i save the pennies for some super expensive wide angle eye piece?

Plossls have a afov of 50deg. (in general) so your fov when viewing will be =afov/magnification. A 32mm plossl will give you only marginal gains. I dont know if a wide field eyepiece would give you what you expect.

Do you have access to something like starry nights software where you can fiddle with the various parameters and compare fov's against a simulated starfield?

JMP.

no, not yet, but im sure i can buy the software. maby , also,it would be worth upgrading my finder scope to something that works! can you recommend any decent yet affordable finder scopes?

The actual (visual) field of view for any eyepiece is the published 'aparant' FOV divided by the magnification that the eyepiece yields in your telescope.

The magnification yielded by any eyepiece is the telescope focal length divided by the eyepiece focal length.

To get the widest (richest) field of view from your telescope you need to use a wide angle eyepiece that also gives you the lowest possible 'useful' magnification.

The lowest useful magnification for any telescope is calculated such that the eyepiece exit pupil is no more than 7mm - or less if you are old! This is because your eye's iris can only open to a maximum of 7mm if you are young and you are fully dark adapted. As you grow older (and in light polluted areas) your iris will not open up so far, so you can use an exit pupil of 6.5mm or 6mm. A larger exit pupil will simply result in light not being able to enter your eye past your own iris.

To work out the exit pupil for an eyepiece, divide your telescope's aperture by the magnification of the eyepiece.

For instance, with my 20" f/4 (FL=2032mm) I get my lowest, widest field of view with a 27mm Panoptic which yields a 0.9degree true field of view. With that and a Telrad, I don't need any optical finder at all - if I can place an object anywhere near the centre of the Telrad while star-hopping, it will be in the eyepiece field. Cool!

Sorry for being long winded, but I hope this helps.

Phil.

Hi

When you say your finder scope is useless do you mean that you cant see anything through it or is it that it is not aligned properly with the main telescope.

If it is not alligned then the best way of solving this is to go out (in the daytime if necessay) and centre a landmark in the viewfinder such as a chimney pot or a alarm box on a neighbours house. Then centre it in the telescope. The viewfinder will regulary go out of allignment and this is a skill that you will need to learn but is not too dificult once you get the hang of it and is more of a nuicence really then a problem.

You can of course replace the view finder with a red dot finder which may be easier to use if you dont know the sky but some of them have a habit of dewing up after a while.

Hope this helps.

Hi Fintan,

If I remember correctly it is an SCT you have?

With such a long focal length a 32mm plossl will still have a narrow fov.

You could use a reducer on your scope or go for a zero power finder like Dave mentioned.

I found that a red dot finder was essential on my Mak (f12.1) when I had it. I couldn't get used to the 6x30 at the time.

They are quite reasonable as well. You can pick them up for about 30-40 quid.

Trev