EP help please

[AlistairHowie]

Hello - I'm on the verge of buying an Altair Astro 115 F7 805mm apo, and am struggling a little on EP choice. It will be used for observing and imaging, so I'm trying to cover my bases with a high power EP for the planets and low power for wide field viewing. I'm trying to limit myself to just 2 EPs to start with.

I'm looking at Baader Hyperion's and have been playing about with telescope simulator and am thinking of the following:

36mm aspheric, 72 degrees field, giving 22x and an exit pupil of 5.1mm, AND EITHER

3.5mm, 68 degrees field, 230x, 0.5mm exit pupil - lets me see Jupiter and Saturn and close detail of the moon (The scope specs says 230x is the highest useful magnification), but I'm not sure how often I could use it in typical UK seeing, and also not sure about the implications of the exit pupil

OR

5mm, 68 degrees, 160x and 0.7mm exit pupil - not convinced on this one, doesn't allow full framing of the moon, Jupiter small and Saturn very small, although exit pupil larger. Seems like a poor compromise between the 3.5mm and the one below ...

OR

8mm, 68 degrees field, 101x, 1.1mm exit pupi - frames the moon nicely, but of littlle help with the other planets

So I'm minded to go with the 36mm as my "standard" lens for viewing all but the planets, then moving to the 3.5mm for the moon and other planets. I'm pretty sure on the 36mm, but a bit stuck on what high power one I should buy and whether 9 times out of 10 in UK seeing I could use a 3.5mm on an apo for the moon ... and whether the eye relief is workable.

I might be able to stretch to 3 EPs - if that's the case - which ones would you suggest?

Thanks

[Tinker1947]

I use a 36mm Aspheric and am really pleased with it, the DSO's i have seem right on the edge of FOV that would have been missed on a smaller EP, i also use a Pentax 8.5mm that very nice, worth spending my pocket money on...

[Atlas]

Have you considered a good zoom lense eyepiece? 7.5MM-22.5MM

[AlistairHowie]

Thanks for the suggestions. I was avoiding a zoom because it would give lots of mid range focal lengths when in fact I expect most observing to be more towards the extremes.

Does anyone have thoughts on my specific questions about whether 230x in a refractor is regulaly possible with typical UK seeing, and also the implications of the associated small exit pupils I mentioned above?

Thanks again.

[glowjet]

Alistair, Brantuck has just recently acquired the same scope and I believe is absolutely delighted with it as it is from all accounts a very nice one indeed, Olly. P is also very pleased with its performance. Might I suggest you send a PM to Brantuck or Olly and inquire what they considers to be the best in the low power range, I am sure they will be pleased to help, having had firsthand experience of the scopes capabilities from two different Countries

John.

[AlistairHowie]

Good idea - thanks!!

[John]

I have a Skywatcher ED120 F/7.5 so the spec is similar to your intended scope. My eyepieces range from 31mm to 3.5mm.

The 3.5mm gives 257x which is on the high side for a 120mm aperture scope however I've found it useful on Mars, the Moon, Saturn and double stars when the seeing conditions have been good. Under more average conditions I've found 180x (5mm) or 225x (4mm) more useful, the lower figure in particular on Jupiter which seems not to respond to very high magnification as well as Mars and Saturn.

My eyepiece set is 31mm, 20mm, 13mm, 8mm, 6mm, 5mm , 4mm and 3.5mm. I do also have an excellent 2" 1.6x barlow lens but out of convenience tend to use just an eyepiece on it's own more often.

At the other end of the scale, the 31mm, having an 82 degree FoV, shows 2.83 degrees of sky which is great for extended deep sky objects and star fields.

In answer to your specific question, I think 230x is realistic with a well corrected 115mm refractor on nights when the seeing and the objects under scrutiny, allow if to be usefully used. It's good to have other high power eyepieces to fall back on when this is not the case though so you have access to the best performance than can be obtained at each occasion.

A high power zoom lens such as the Nagler 3-6mm is imensely useful with fastish refractors for the above reasons of course - you can "fine tune" the magnification really easily with those to the best that can be used at a specific moment. The Nagler zooms don't seem to compromose their performance in any way despite the zoom design - but then, at £300+ a throw, they shouldn't

[AlistairHowie]

Thanks John, great information.

[brantuk]

PM'd you Alastair