Zoom vs prime eyepieces

[robhatherton]

Not reviving an old argument: I know my prime eyepieces - a couple of 32 and 25mm decent Plössls and 12 and 8mm Starguiders - are better than my Starguider 8-24 zoom but the trouble is I really don’t think I can tell the difference apart from field of view.

Is it my bad eyesight, bad judgment or is my f8.3 achro so forgiving? Does anyone else suffer from the same problem😳.

[Louis D]

Generally, the central 50% may be indistinguishable except on challenging objects like planets, globular clusters right at the verge of resolution, unequal doubles, and close doubles.  It's the outer 50% that differs markedly between pretty much all zooms and premium primes, especially in fast scopes.  Even at f8.3, I would imagine the zoom's outer part at 8mm should be easily bested by the 8mm Starguider.  At 24mm on the zoom, the view is so narrow that there isn't likely all that much outer field degradation.  You're also limited by the chromatic aberrations of the achro to some extent.  No eyepiece is going to correct that.

If you ever move up to a fast APO or really fast reflector and spring for premium widefields like Pentax XWs, Delos, Ethos, etc., you will be astonished at the difference in the views between them and an f8.3 achro with a Starguider 8-24 zoom.  The images are very sharp across the field revealing dimmer stars that are blurred out by lesser optics, the apparent fields of view are satisfyingly wider, and polish/coatings/baffling is so much better leading to darker backgrounds, better stray light control, and less scatter around bright objects.

[robhatherton]

Yes I expect I’m not really comparing the edges of the field as I should. Anyway I’m looking forward to upgrading to an Ed or apo frac next birthday!

Thanks!

[Louis D]

6 hours ago, robhatherton said:

Yes I expect I’m not really comparing the edges of the field as I should. Anyway I’m looking forward to upgrading to an Ed or apo frac next birthday!

Thanks!

That's the way to do it.  Start out with what you can afford and gradually upgrade your equipment while you learn the skies and observing skills.  For me, my backyard trees grew up and blocked most of my views and my town grew from 4000 to 80,000 people in 25 years, so I'm having to look at relocating to continue observing in the future.

[robhatherton]

So just to follow up - and I hope this isn’t too tedious! - but last night I was pleased to find Tegmine for the first time and it split comfortably in my 8mm eyepiece. By comfortably I mean I could roughly fit two other stars the same size between them. I let them drift to the edge of the frame and they stayed clearly separate all the way if not quite as crisp. 
I tried the same with the 8-24 zoom expecting so see a marked deterioration towards the edge but that wasn’t the case. Things got slightly blurry and they had just started to fuse at the very edge. Was this a reasonable test comparison or too easy a target? I reckon the zoom is just as capable of making the split anywhere near the middle of the frame.

Still having difficulties with Izar though! But I reckon the CG4 Mount isn’t quite enough for the xlt 120 at very high magnifications. 

[John]

Do you mean the two closer stars of Tegmine ?

These are quite a tough target. Harder than Izar I think.

This is a sketch by Nick Cox with a 6 inch refractor:

[robhatherton]

No I’m afraid not, John! The two obvious ones!

[Timebandit]

 

 

Hello . Your F/8.3 is quite forgiving on eyepiece's and the Starguider will perform well in these slower ratio scopes. But if you put these in a fast scope then you will start to see the detrimental effect that fast scopes have on views. I know myself I have been surprised that how good a views you can get with cheaper eyepiece's in very slow scopes. But put the same eyepiece's in a fast scope and its a completely different story and the views suffer considerably. 

Well done looking at a ED scope for your birthday. It is a market increase in viewing performance when you can get a good ED doublet. The CA is Sooo controlled and Luna is such a pleasure in a quality ED

 

 

 

[michael.h.f.wilkinson]

The speed of the scope is really important here. I have a MaxVision 24mm 68 deg EP in my travel set-up, and it works just fine in my APM 80 mm F/6. I did once put it in Olly's big F/4.1 Dobson and was rather horrified at the weird star shapes outside the centre of the FOV (granted, they are sold as "good down to F/5", which they are). My 22 mm Nagler was crisp right to the edge of the FOV, but then you pay for that performance.

[Second Time Around]

I'm a great fan of zooms and the 8-24mm Baader zoom (that's better quality but pricier than your Starguider) is easily my most used eyepiece.  I use it with the matching 2.25x Barlow.  That plus a wide field eyepiece as a finder does me for most of my sessions.

I particularly like the ability to increase the magnification to make use of brief moments of good seeing.  It takes more time to swap out an eyepiece, and the opportunity may then be missed.  You can't see anything if you haven't got an eyepiece in the focusser!

Zooms also enable the field of view to be varied to frame an object to get the prettiest view.  For this reason I particularly like them for clusters.

Granted, fixed focal length eyepieces may be slightly better corrected when compared with a zoom at the same magnification.   But that's not always a fair comparison as that magnification may not be the optimum for a given object.  However, one of the many advantages of a zoom is to be able to dial in precisely the best focal length.  For instance, this may be 13mm or even 13.2mm, which may actually show more detail than slightly shorter or longer fixed focal length eyepieces.

Many of those who post here and advocate fixed focal lengths are experienced observers.  It's so easy to forget what it was like as a beginner!  A zoom eyepiece enables beginners to easily learn what difference a change of magnification makes on all the various classes of object.  It also shows them what focal lengths would be most useful to their eyes, their telescope, and their observing conditions.  They then have the option of buying/not buying the most appropriate fixed focal length eyepieces for them.  For these reasons I'd always recommend that beginners buy a zoom as their first eyepiece.

[RT65CB-SWL]

I tend to agree with you @Second Time Around. The Baader zoom is one of the 'better' 8-24mm zoom e/p's that are available. I am contemplating in purchasing the 'complete outfit'.