Skywatcher 27mm ExtraFlat or 32mm Super Plossl?

[enigma]

What would be the best choice between the above two eyepieces in a fast reflector with only a 1.25" focuser? The ExtraFlat costs approximately £50 and the Super Plossl £30.

[Ogri]

I've not tried the Extra Flat (though I have heard from a few sources now that they are a bit naff, as I was considering trying one myself a while back), but I picked up one of these Meade 4000 32mm SP's from here to try NEW! MEADE 32mm Series 4000 SUPER PLOSSL Eyepiece Lens on eBay (end time 26-Sep-10 03:28:32 BST) before shelling out on something expensive for my ST120 (I was specifically after one of those nice PanaView's for home use, and figured the 32mm SP might be nice to take touring with me anyway).

There's no getting away from it, it's a very nice eyepiece with a comfortable eye relief.

The problem is, my ST120 has a fl of 600mm, and the resulting magnification of 18.75 x results in pretty awful contrast and poor views as a result (far bettered by a cheap 20mm erfle design with a 66 degree fov, for 30 x mag).

I am quite confident that it should 'hit its stride' with my new 'scope when it arrives, which with a magnification of 37.5 x (fl 1200mm), should dramatically improve the contrast.

Edge to edge crispness of the 32mm SP in the ST120 semed fine, but I don't think I would want to use any 32mm EP in a 'scope with a fl of less than 900mm personally.

I suppose the obvious questions are, what magnification are you looking for, and what fl is your 'scope?

[steve_bham]

Ive used both the 32mm and 27mm ultra flat, both are good EP's. I had both in my collection but if id say the 27mm was used more.

[Ogri]

Your ETX has pretty long fl though doesn't it Steve?

[steve_bham]

It wasnt used on the ETX

[michael.h.f.wilkinson]

I see the 27mm extra flat has an FOV of just 53 deg, which is not a huge difference to a 32mm Plossl. I would work out the true field of view, and of course the size of the exit pupil: this is the focal length of the EP divided by the focal ratio of your scope. This number should be smaller than or equal to 7mm for younger observers (<35 y.o.) and smaller than or equal to 5mm for us older observers. If either EP satisfies the latter condition, go for the one with the larger true field of view (apparent FOV / magification).

[Ogri]

Another way (gives the same results I think, but might be easier for some to remember) is to divide the magnification into the diameter of the objective.

So a 200mm objective with an EP of 50 x magnification, would give 4mm.

For example a 24mm EP in a 200p dob with a fl of 1200 and F5.91, gives 50 x mag, for the 4mm.

Your method would be 24/5.91 for 4.06 (as near the same as makes no difference).

Funny how ratios work out like that isn't it?

[michael.h.f.wilkinson]

Another way (gives the same results I think, but might be easier for some to remember) is to divide the magnification into the diameter of the objective.

So a 200mm objective with an EP of 50 x magnification, would give 4mm.

For example a 24mm EP in a 200p dob with a fl of 1200 and F5.91, gives 50 x mag, for the 4mm.

Your method would be 24/5.91 for 4.06 (as near the same as makes no difference).

Funny how ratios work out like that isn't it?

The methods are equivalent (a little algebra will show this). I prefer the former, because it allows me to select an appropriate EP set for a scope simply based on its speed, without knowing the aperture. If you need to know the longest focal length EP, you simply multiply the F-ratio by 5 or 7 as is appropriate, and there you are. You then work out the resulting magnification (if desired).

In the other method, you work out the minimum magnification, by dividing the aperture in mm by 5 or 7, giving you the required magnification, and then you work out the focal length of the EP by dividing the focal length of the telescope by the minimum magnification. More work, same result.