Maximum useable magnification

[Star Gazer]

I've got a Megrez 72 with a FL = 432 mm

I bought it for imaging, but these past few nights I have been tempted into viewing Jupiter since it is so easy to see. I have a 4 mm eye piece that hints at the rings of Jupiter but the problem is the image is so tiny.

Could I use a 2x Barlow to help me here or would I go well past the useable magnification of the Megrez 72

I understand the the max usable magnification is said to be 50-60x aperture in inches. This would equal

3x60=120 (well almost 3")

The magnification with my 4 mm eyepiece would equal

432/4=108

So the 4 mm sneaks in under the maximum, but what happens when a Barlow is added into the mix?

I promise to buy a good quality one! Scouts honour!

[John]

The useful magnification limit issue applies barlow or no barlow - no "magic bullet" there I'm afraid. I would have thought you could use 140x - 150x with the Megrez 72 on really good nights - if things seem a bit mushy then just back off the power. I do feel 216x (ie: 4mm eyepiece plus 2x barlow) would be OTT though but you may get away with it on binary stars.

[Moonshane]

I tend to work on 1.5x aperture in mm so 108x would be about the max I'd say. that said, APOs of decent quality can sometimes exceed this.

not the same kind of scope but I have a 6" f11 newtonian and this often gives sharp images of the moon and doubles at 400x (2.7x aperture in mm) and sometimes even 500x (3.3x) but planets take less at around 150-200x max. (1-1.3x).

personally, I'd try to borrow a decent barlow to see if it's worth it. I suspect not, at least with the 4mm.

[ScubaMike]

3x60=120 (well almost 3")

or 3 x 60 = 180

I have read elsewhere on here that 60x per inch is pushing the envelope a little.

50x is more reasonable, and then only under good conditions.

Better a small well formed image, that a large fuzzy & unclear one.

barlows are useful in that they have effectively multiplied my eyepiece collection. but there are a lot of "duplicates" when the barlow is taken into account.

[Andrew*]

I would say with the Megrez 72mm you will be fine with down to a 2.5mm eyepiece (173x). With my old ED80 I used to regularly use 200x, which it was quite comfortable with on good nights.

[LeeB]

if the nose of your barlow unscrews you could use that to get 1.5x approx

[Andrew*]

That's a good idea, and with the 4mm it would be a great high-power eyepiece. I think the Tele Vue 2x barlow does that, and at £85, it's one of the best available for the money...

Andrew

[Star Gazer]

or 3 x 60 = 180

Oops, it was late and I was in a hurry to get to sleep

I also have a 16 mm and 7 mm eyepieces, I was actually considering a 2" 2.5x TV Powermate ( that's nearly as expensive as the Megrez )

My thinking here is it's an investment for the future and besides it's use for visual work I believe it will be good for use with my DSLR and imaging

Do you think it's a bit over kill for use with the a small doublet as the Megrez certainly is?

[brantuk]

I'd suggest a 5mm Radian with Antares 1.6x 2" barlow - that would give you 138x. My manual says 140x is the highest useful magnification so you'd need quality glass and very very dark and clear night with A1 seeing

Really though - the Meg72 is a fast appo, wide field, imaging scope - planets ain't generally going to be great with it at high magnifications.

[Star Gazer]

Thanks everyone for your help.

I hopeful I could push the Megrez, but I think you were all confirming what I already new.

I've decided to save my cash and get back to imaging..

[michael.h.f.wilkinson]

Note that the answer to your question depends on your visual acuity. Mine is high (1.60, I.e 60% sharper than average), which means the image seems mushy at lower magnification than most. I do not push the scope to its limit. A colleague with more average eyesight can push it further with ease.