Zoom eyepiece and barlow advice please?

[stephec]

I've got a SW 150/750 reflector that takes 1.25 EPs. 

I'm currently thinking of getting three or four different BST Starguider EPs for it but might also consider a zoom. 

My question is, coming from a photography background it was always said that a prime lens was better than a zoom for quality, is the same true in astronomy? 

Using a 2x converter would also degrade quality as well, is this also the same when using a barlow, ie. if I use a BST 25mm with a balow would the image quality be the same as using a 12mm BST? 

[wulfrun]

I don't think there's a definitive answer to your questions, mainly because you'll find it hard to compare "apples to apples". A good quality zoom will be very close to a good quality fixed EP, probably too close to tell under most circumstances. Likewise, a good quality Barlow will be almost indistinguishable from using an EP of half (or whatever) the focal length. Modern optics are pretty good and it's more a case of "technically" inferior for a zoom or with a Barlow. Provided you steer clear of the likes of a no-name fiver-off-Ebay Barlow.

One thing with most zooms though, they tend to have a narrower field-of-view and it's narrowest at the longer f/l end of the range (annoyingly). On the other hand, if you have a zoom you can set it to whatever is the perfect f/l for your situation. You can get 4 BSTs for the price of the well-regarded Baader Hyperion zoom (roughly speaking). Which is better is largely personal preference.

Incidentally, photographers tend to shun zooms based on historical performance. Early photographic zooms were noticeably poorer and the reputation has stuck. Nowadays the penalty for a camera zoom lens is more about f-stops than image quality.

[Guest]

I have a Orion (US) 8 to 24 variable.  Its better then the Celestron and is just a bit behind the Baader.  I used to use it a lot.  Now I use it a little.  We buy them because they are convenient and they do a reasonably good job on a little bit of everything.  However a fixed focal length eyepiece will almost always give a better image then a variable as long as you are comparing apples to apples   As to the barlow, well for me this is where i dont mind spending some money.  Eyepieces come and go but a good barlow is forever.  The barlow i use is a telecentric and it really does a nice job.  The Barlow it replaced.... Not so much.  

Edited December 30, 2022 by Mike Q

[Louis D]

Just like camera lenses, high end prime lenses (eyepieces) generally stomp on entry level prime lenses (eyepieces).  High end zooms generally stomp on entry level zooms.  High end teleconverters (Barlows/Telecentrics) generally stomp on entry level teleconverters (Barlows).  A high end zoom might stomp on a series of entry level prime lenses/eyepieces.  It's a matter of comparing apples to apples rather than apples to oranges, just as in photography.

Specific to your question on the Barlowed 25mm Starguider, it might perform similarly to the 12mm.  It definitely would have much longer eye relief.  By itself, the 25mm Starguider is a poor performer in the outer field in sub-f/8 scopes.  The 12mm is quite good in comparison.  Rather than degrade eyepiece performance, high quality Barlows generally improve the performance of marginal eyepieces by slowing down the light cone to within the design range of the eyepiece.  Unlike photography, it is necessary to consider appropriately matching eyepieces to scopes based on the f-ratio of the scope and the design f-ratio of the eyepiece.  Unfortunately, most eyepieces don't list their design f-ratio.  Thus, you need to read up on eyepiece reviews in scopes of varying f-ratios to get an idea of how it might perform in your own scope(s).

I have a GSO version of your scope; and at f/5, it is fairly demanding on eyepieces.  If you move into wide apparent field of view eyepieces someday, you'll want to invest in a coma corrector for it as well to get the most out of your eyepieces.

[Second Time Around]

I'd very much recommend a zoom eyepiece as your workhorse. One zoom eyepiece will cover multiple focal lengths and so is really excellent value for money.

Despite having high quality fixed focal length eyepieces, I use my zooms a lot more often. The zoom plus a Barlow lens and a low power, wide field eyepiece is often all I use the whole evening.

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. This is because 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 13.1mm, which may actually show more detail than shorter or longer fixed focal length eyepieces - even better quality ones.

I particularly like the ability to increase the magnification to make use of brief moments of good seeing (a steady atmosphere). 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 focuser!

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.

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.

Good luck with whatever you decide, and do let us know how you get on

[Second Time Around]

I'm also a great fan of Barlows, as long as they're not the horrible plastic ones packaged with some cheap scopes.

I'd recommend one of the dual power 1.5x/2x Barlows.  These will effectively triple the magnification of each of your eyepieces.

For those of you who aren't aware, a Barlow lens multiplies the magnification of any eyepiece it's used with.  It goes in the focuser before the eyepiece.  The multiplication factor varies but 2x is most common.  Some of these 2x Barlows can also be used at 1.5x, although it's not always mentioned in the blurb. These dual 1.5x/2x Barlows allow the black lens cell to be unscrewed from the body of the Barlow and then screwed into the filter thread at the bottom of an eyepiece to give approx 1.5x. 

Another tip with most zoom eyepieces is to use the lowest amplification Barlow possible.  This is because most zooms have the widest apparent field of view at the shortest focal length.  For instance, if you want the magnification of a 6mm focal length then using a zoom at 9mm with a 1.5x Barlow will give you a wider field of view than using the zoom at 12mm with a 2x Barlow.

[stephec]

43 minutes ago, Second Time Around said:

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.

Good luck with whatever you decide, and do let us know how you get on

I was hoping someone might say this as I did wonder about this at first, but then I've also read about the idea of buying the best you can first time round in fixed lengths. 

 

I think it might be the way to go thanks. 

[stephec]

Thanks everyone for the replies, I wasn't really aware of the idea of different EPs suiting different telescopes, something else to have fun learning along the way. 😂

[Spile]

The zoom, Barlow and wide angle, low power combination works for me. It’s not just the convenience factor, I also appreciate being able to dial in and find the most appropriate magnification without having to guess what that might be.

[Don Pensack]

As counterpoint to the zoom argument:

--zooms have narrow fields of view compared to fixed focal length eyepieces.  Generally, wider is better--more engaging.

--zooms provide too many in-between powers.  If your scope yields 50x, you really don't need a 60x or 70x eyepiece.  When you change the magnification, you want there to be a visible difference.

With the zooms I've used, I pretty much always used them at 4-5 settings and skipped the in between settings.

--no zoom I've tried, and that includes the Leica Aspheric, was the equal of separate eyepieces in edge of field correction, lack of spherical aberration, sharpness of star images, lack of edge of field brightening, or contrast.

(well, the Nagler Zoom was, but I wanted more than a 50° field).

Fixed focal length eyepieces can be better (though not all are, of course, which is why to read reviews) that zooms, and if you have those better fixed focal length eyepieces, returning to zooms is not a viable option.

 

Which is why I have advocated for a starter set of 3 eyepieces to get your feet wet in astronomy:

https://theskysearchers.com/viewtopic.php?t=18374

 

 

[Ratlet]

I got the svbony 8-24mm zoom and use it with an F5 130pds.  Visually it's okay.  No complaints but the FOV at 24mm feels tiny although it does get better as you move towards 8mm.  Its worth pointing out that it isn't parfocal so you need to adjust the focus as you move through the magnification.

The lack of fov is what primarily put me off.

I mostly ended up using it for planetary with a 2x Barlow where the small fov was less of an issue, but the zoom with a Barlow was quite long and heavy.

Ultimately I got more use out of a 32mm plossl, 8mm starguider and an x2 Barlow.  I think if I could go back and pick again I'd get a 12mm instead of the 8mm.  In fact, it was the 12mm I just got for Christmas.

So from my limited experience of one zoom, I'd suggest just getting 2 fixed focal lengths and a barlow and  rather than the svbony 8-24mm.  If strapped for cash like I was go for a long focal length plossl and short focal length starguider and a Barlow.

All that being said, I've ordered the svbony 3-8mm zoom because it's got a decent FOV, is almost parfocal and should be pretty much perfect for sniping planets.

Edited December 31, 2022 by Ratlet

[Franklin]

I use a zoom for solar white light with a Lunt wedge, it means I don't have to faff around swapping the continuum filter over. A zoom is a handy accessory to have, like with a minimalist travel set up for example, but fixed focal length eyepieces are better.