Max magnification & EP's

[Aquavit]

My Mak has an aperture of 102mm and focal length of 1300 with a maximum magnification of 204.

I have a 10mm and a 20mm basic EP of indeterminate make supplied standard with the scope, giving me either 60x or 130x magnification. I have just acquired a 2x Barlow so I'm thinking of getting a better quality EP to pair with this. It would appear that a 13mm (if such a size exists) along with the Barlow would give me the full 200x, is it wise to max out the scope or should a little margin be left?

[Zermelo]

"Maximum magnification" is often described as if it were as fundamental to the scope as its weight or colour, but it's not really.

The value you quote is from the commonly-used rule of thumb - double the aperture size in millimeters. It's OK as a rule of thumb, but it usually applies when observing conditions are ideal. I don't know what your situation is in Hungary, but in the UK our conditions are often far from ideal. In particular, if the seeing is poor (air column is unsteady) then there's little point in blowing up your image to the point where it's dancing around. My 150mm in theory gets me to 300x, but even at 200 I've found it rather unsteady on most nights.

So I wouldn't worry too much about how close you can get to the theoretical limit. Of more use is to build a set of eyepieces/barlows that give you good coverage and will get used enough to justify your spend. In particular, think carefully before buying a short eyepiece at the limit of your usable range, it's better to buy a longer one that you could barlow when suitable - this is the way you seem to be thinking above. If you do that, then you won't mind if you the barlowed combination pushes your scope beyond the observing limits, because you've not lost much. The other option to consider is a zoom eyepiece than can cover a range of focal lengths, which could (perhaps when barlowed) straddle the extreme end of the range.

[Tiny Clanger]

12mm is a common EP size, so you will have plenty of choice. I have a BST starguider 8mm, which in my 1500 f. length 127mak gives 187.5x ,comfortably below the rule of thumb 254x theoretical limit (which I've read is quite a rough figure) . It works well, but adding the 2x barlow is pushing things too far ,  Mars scuds out of the field of view too fast to keep up with, and the 'seeing' here has just not been good enough to make the extra magnification useful.

I had originally decided to buy the 12mm starguider , it gets the best reviews, and it promised what I thought would be the most useful combination of barlow/no barlow magnifications , but it was out of stock everywhere. I managed to track down an in stock 8mm from Alan at The Sky's the Limit and snapped it up. I think the 12mm would have been better from the point of view of useful with barlow range , so suggest you seriously consider a 15mm , which will fill the gap between your two existing EPs , and offer you more magnification than the 10mm if you get the conditions where it is possible to use it with the barlow.

As I typed that, Zermelo's post popped up ! Reassuring to see I'm in good company .😀

Heather

 

 

Edited December 7, 2020 by Tiny Clanger
omitted 'with the barlow' DOH !

[dweller25]

As Zermelo suggests, a decent 8-24 Zoom - like the Baader may be a good option (x163 to x54).

[vlaiv]

That rule x2 aperture in mm (or x5 aperture in inches) is very loose rule and it really depends on how good your eyesight is.

Not all people see equally sharp and there is a measure of visual acuity that determines how sharp is your vision. Sharper your eyesight - less magnification you'll need.

Theoretical limit for 20/20 vision is somewhere around x115 for 102mm aperture. At this magnification you'll see all you can see at threshold of your visual acuity if you have 20/20 vision. Image will look the sharpest without loss of detail.

Sometimes it is easier to see if you bump up magnification so we prefer slightly higher powers that theoretical, but there will be no additional detail to be seen after we pass above threshold.

Back to original question - I have Mak102 and have 6.7mm eyepiece to go with it, giving about x194. I think I would not mind having something like 8mm as my highest power eyepiece for that scope - I would not loose much over 6.7mm. In fact - image would be sharper with such eyepiece.

But that is my vision and my perception of the image. You might find that you prefer higher powers (I don't usually find them as useful - I like smaller sharper looking image).

 

[Aquavit]

Thank you for the replies, quite a few variables in this it seems, with no obvious answer.

I'm in a Bortle 4, on the edge of a small village with a small hill five minutes away possibly giving me Bortle 3 skies. So, potentially good viewing conditions. I'm a varifocal wearer but don't use them to observe.

I did think about a zoom lens, but if I have a Barlow do I need a zoom as it would probably take me outside the useable range at both ends of the scale? If it's a fixed EP something in between 10 and 20mm seems obvious, but is it? Sky-Watcher do this one for £99 https://www.astroshop.eu/eyepieces/skywatcher-hyperflex-7e-high-performance-zoom-7-2mm-21-5mm-eyepiece/p,22263#tab_bar_1_select is it going to be an improvement on my standard lenses, can I get better for similar money? It quotes eye relief of 15mm, how do I know that is right for me?

Or would this fixed 15mm EP be better https://www.firstlightoptics.com/bst-starguider-eyepieces/bst-starguider-60-15mm-ed-eyepiece.html

My main criteria would be quality of image rather than maximum magnification.

Edited December 7, 2020 by Aquavit

[Zermelo]

I wouldn't expect to see much difference between that zoom and the BST, in terms of sharpness, possibly the BST has a slight edge, I don't have the 15mm to compare myself. They are both good performers in the sub-£100 bracket. Though the BST will give you a wider field of view (60 degrees AFOV) than the zoom.
Yes, you could certainly start off with just the fixed eyepieces and barlow, there's no right answer to that. But once I got my zoom, I found that I used it more than I had expected. They are very handy, for example in splitting double stars. I've also found that it's useful to magnify a view to the point where the background sky darkens, and the contrast with the target object increases (I'm also in Bortle 4 skies). This is more convenient to see in a zoom, though you could do the same thing by swapping fixed eyepieces. There's no reason you can't combine the zoom with a barlow either. Yes, you may go beyond the limit of the scope if you push it to far, but you'll still have a reasonable range to play with. I barlow my Hyperflex quite frequently. (by the way, that Skywatcher version looks pricey - there are several "badges" out there on what is essentially the same eyepiece. FLO may not be convenient for you, but their OVL version is only £69+PP. )

[Merlin]

For spltting close double stars we can go quite some way beyond the theoretical limit of magnification, as stars are only point sources.

[Louis D]

5 hours ago, vlaiv said:

That rule x2 aperture in mm (or x5 aperture in inches)

Make that x50 for inches.  I'm sure just a typo on your end.

In my personal experience, I find x30/inch more realistic for most nights and objects.  The moon's terminator and double stars are the notable exceptions for going higher (high contrast subjects) and planets for going lower (low contrast subjects).

As for the OP's original question, I like to look at exit pupils.  I tend to favor 0.7mm at the high end, and 7mm at the low end.  Given the f/12.7 focal ratio, that would equate to 9mm and 90mm.  You can forget about the 90mm end of things.  It is simply unrealistic to get a large exit pupil in an f/12.7 Mak.  Let's stick with a 32mm Plossl then for a 2.5mm exit pupil and budget for a fast ED refractor or Newtonian for wide field views down the road.  At 9mm, the 8mm BST comes close with a still usable 0.6mm exit pupil.  Your 20mm yields a 1.6mm exit pupil while your 10mm yields a 0.8mm exit pupil.  That 10mm, if it were a decent eyepiece, might indeed be your maximum magnification eyepiece on most nights.  A 15mm BST would indeed split exit pupil sizes at 1.2mm between the 10mm and 20mm.

The more I think about it, the more I think you might be better off with an 8-24mm zoom plus a 32mm Plossl to get you started.  The Celestron/Meade/SW/Astromania/etc. would be a good choice for the zoom and the SW/Astro Essentials/Revelation 32mm Plossl at the long end.  Once you have a better idea of the magnifications and exit pupils that work well for your observing style, you can pick up some wide and better corrected fixed focal length eyepieces.

[Zermelo]

... and if you were interested in a zoom, this OVL Hyperflex has just appeared in the FLO discount section for a bargain £62.

 

[Aquavit]

I finally made a decision on this and ordered the  Hyperflex 7.2mm - 21.5mm zoom and an Astra Essentials Super Plossl 32mm from FLO. This, I think, should cover most bases for the time being and hopefully give me an optical improvement on the standard lenses.