Eyepieces & Associated Optics

[Nomad Z]

Got some questions on eyepieces and the associated bits like diagonals and Barlows. At the moment, I'm just thinking ahead to what I might consider getting assuming I'm less than happy with the EPs and diagonal that came with my Mak 127. I'm also trying to keep in mind that a wider field scope might be acquired one day, and that I might want to use the same add-on optics with that as well (or at least have some crossover where practical).

If I use a shorter focal length EP, the image brightness reduces. If I use a 2x Barlow with a longer EP instead, is the amount of dimming the same?

For a given quality level of optics, will the Barlow+EP combination result in lower image quality compared to using a shorter focal length EP?

I've read that scopes with a large f/ratio (like my Mak 127 at f/11.8) are more forgiving of lower quality EPs. What optical differences would better quality EPs make? By the same token, what shortcomings would become more apparent if cheap EPs were used on a faster scope?

With star diagonals, what sort of differences can be expected between the cheapies and the better ones? Is mirror oxidisation an issue?

On the face of it, an erecting prism is appealing, but does it introduce optical compromises? (Chromatic aberration? Reduced contrast? Dimmer?) If so, is there a price point for a prism where it's on a par optically with a decent star diagonal (presumably one that might be an upgrade of the supplied piece).

 

 

 

 

 

[Ricochet]

11 hours ago, Nomad Z said:

If I use a shorter focal length EP, the image brightness reduces. If I use a 2x Barlow with a longer EP instead, is the amount of dimming the same?

The dimming/brightness will be the same in both cases. For an extended object brightness is proportional to the square of the exit pupil, which can be calculated by eyepiece focal length / telescope focal ratio. If you used a 5mm eyepiece in an f5 telescope you get an exit pupil of 5/5=1mm. If you then change to a 10mm eyepiece and 2x barlow the barlow  doubles the focal length of your telescope so that you have 10/(5x2)=1mm.

11 hours ago, Nomad Z said:

For a given quality level of optics, will the Barlow+EP combination result in lower image quality compared to using a shorter focal length EP?

In general, adding extra lenses (i.e. those in the barlow) must result in extra light losses. However, the excellent coatings on top end optics will reduce these to the point where the human eye wont detect them. On the other hand, lower quality eyepieces will struggle with fast f-ratios. Adding a barlow will turn the telescope into a slow f-ratio instrument from the perspective of the eyepiece, which may lead to better performance from the eyepiece. In addition, in a scaled design such as a Plossl, eye relief (the distance your eye must be from the lens) is proportional to the eyepiece focal length. This means that short focal length eyepieces may be uncomfortable to use so a barlow and longer focal length eyepiece can be more comfortable to use.

So at the top end of the market using a barlow won't make a difference to the view, but at the bottom end whether or not the view is improved or degraded will depend on the combination of equipment that you use.

11 hours ago, Nomad Z said:

I've read that scopes with a large f/ratio (like my Mak 127 at f/11.8) are more forgiving of lower quality EPs. What optical differences would better quality EPs make? By the same token, what shortcomings would become more apparent if cheap EPs were used on a faster scope?

For a slow f-ratio like with your Mak I would suspect the biggest difference will be in the transmission coatings on the eyepiece, with more expensive eyepieces showing a slightly brighter or clearer image, perhaps with better colour rendition. For a fast f-ratio scope the field of view edges of a lower quality eyepiece will be much more prone to aberrations and excessive distortions. Stars and planets may be pulled into a whole variety of shapes depending on the eyepiece involved. Fast f-ratio telescopes also introduce their own distortions so a high quality eyepiece may allow you to see those distortions while with a low quality eyepiece the eyepiece aberrations will swamp those of the telescope.

I don't use star diagonals with my current scope (I've only got storage space for one!) so I'll leave discussion of those to the more experienced, but the general consensus I've picked up is that cheap mirror diagonals are pretty decent but cheap prism diagonals are not.

[Louis D]

4 hours ago, Ricochet said:

I don't use star diagonals with my current scope (I've only got storage space for one!) so I'll leave discussion of those to the more experienced, but the general consensus I've picked up is that cheap mirror diagonals are pretty decent but cheap prism diagonals are not.

I prefer the dielectric coated mirrors not just because they don't tarnish (though that is vastly overstated as a problem), but because the housing is much more robust and less prone to flexure.  Also, cheap diagonals use a single, tiny set screw to hold the eyepiece rather than a compression ring.  I've also had the nose piece try to come unscrewed from the diagonal housing, allowing the eyepiece end to flip upside down and almost dump the eyepiece on the ground.  Saving a few bucks on the diagonal only to destroy a costly eyepiece doesn't seem like a very good trade-off to me.

Image erecting diagonals can introduce a spike when bright objects are in the field of view due to the particulars of the prism construction.  Otherwise, they're fine.  Non-erecting prism diagonals at the high end (Zeiss) are actually better than any mirror diagonal because they have basically no surface roughness, so they reduce contrast the least.  This is good for low contrast objects like Jupiter.

4 hours ago, Ricochet said:

I've read that scopes with a large f/ratio (like my Mak 127 at f/11.8) are more forgiving of lower quality EPs.

Yes, they look better, but they are still poor at the edges.  A Mak won't make an Erfle into a Delos.  I've compared them in both fast and slow scopes, and the improvement in slow scopes is much less than I'd been led to believe.

[Nomad Z]

Thanks very much for the responses, folks (and for the Barlow & focal lengths calculation - no free lunch there, then).

Aside from perhaps image quality towards the edge, it would seem that the main advantage of upgrading the eyepieces is more to do with ergonomics and maybe a general feeling of being better made. I'd have to say that a scaled design doesn't appeal. When I used the supplied 25 and 10mm EPs, I felt the eye relief on the 10mm was too short (I don't wear glasses, but it just seemed more fiddly to get settled into a position where I could observe without bumping the eyecup and causing vibration).

There was something else that I noticed (when looking at the moon). With the 25mm, the image was nice and crisp, but when I changed to the 10mm, the crispness seemed to be lost - everything was bigger, but I wasn't really able to refocus and get the sharpness back. This was in the middle of the field for both. The seeing seemed to be pretty good in the sense that the image was stable. Could this be the 10mm eyepiece? Something related to the increased magnification? My immediate impression was that the EP was amplifying the limitations of the scope, but 150x is still comfortably within the scope's suggested limit of around 250x.

While a top end Barlow and EP might perform better in terms of transmission and optical quality, that level of kit is outwith my budget. I've been thinking of pieces in the £50-70 range. I was interested in the Sky Panorama ones with the very wide AFoV, but found some discussion of them here, where the overall run of opinion was that they aren't so good optically (in other words, too much emphasis on wide angle, with other aspects perhaps compromised too much). Then I found the BST Starguider ones, which seem to be better regarded - they're spoken well of by several people in the New Eyepieces thread, and I note that that thread is related to a fast scope (f/5), which would bode well for a possible future purchase of something for wider field viewing. Would I be right in thinking that, if these are a good step up from supplied EPs for a fast scope, they'll be very good with my Mak 127?

I've been looking at the focal lengths, and note that the addition of the Barlow tends to end up with magnifications that are very close to what another EP on its own would provide (25mm & Barlow against 12mm, 18mm or 15mm & Barlow against 8mm, etc), so it would seem that the Barlow is of limited use unless the number of EPs overall is limited, or where the EPs are longer focal lengths, and the Barlow is used to get the shorter ones. The Starguiders do look tempting, though - I get the impression that they're at a sweet spot in terms of AFoV, optical performance, ergonomics and price. I need to think more about focal lengths, and whether or not to include the Barlow (suggestions welcome).

Points taken on the diagonals. I suspect a good erecting prism (be there such) would be too pricey if the cheaper ones tend to have optical artefacts (which I'd rather avoid). I don't mind a mirror diagonal - having up and down correct is fine if having both correct becomes expensive. Also noted that the cheaper mirrors can be okay. Again, it seems that it's the non-optical things that can be a bit more of a concern, like the use of screws to hold the EP rather than a compression ring. I haven't looked into these in any depth yet, and FLO seem to have a limited choice for 1.25", while Rother Valley seem to have several, so I need to spend some more time looking at them. Suggestions would again be welcome. One thing I noticed was that a few of them have some sort of helical focusser - is this for astrophotography?

For what it's worth, the budget is fairly flexible up to around £300, to cover EPs, possible Barlow, and a better diagonal that has a compression ring. At the moment, I have a scope that suits planetary and some DSOs, and which would be used for some terrestrial spotting as well. There is a fair chance that I'd add a scope in the future with a wider view for larger DSOs. Ideally, I'd like a range of eyepieces that would suit both. So far as I can work out, the grey area is at the shorter focal lengths - with the Mak 127, scope magnification can become a limiting factor, while short EPs would extend the usefulness of a shorter scope, at least for solar/lunar viewing. That said, I'm not averse to trying something short in the Mak 127 to see what happens when the magnification is pushed, provided it will be useful in a shorter scope later even if it turns out that it's not practical with the Mak.

At £300, if I bought it all from FLO, that would be £92 for the William Optics dielectric diagonal and (with discount) up to five Starguider pieces. With that as a starting point, and the above envisaged requirements, what would the panel recommend?

 

[domstar]

I'm not one of the experts here, but just from a practical point of view, I find I'm too lazy to use a barlow. It's too easy for me to put my thumb print on the glass, it's too pointy, too fiddly, and my fingers are too cold between September and April. (Just typing this has made me think a zoom might be for me. I've never considered one before.) 

[Ricochet]

3 hours ago, Nomad Z said:

Could this be the 10mm eyepiece? Something related to the increased magnification? My immediate impression was that the EP was amplifying the limitations of the scope, but 150x is still comfortably within the scope's suggested limit of around 250x.

1. The 10mm Skywatcher supply with scopes is generally not well regarded and will be degrading the view.
2. High magnification requires a well cooled scope. I can't remember the rule of thumb for a mak but it might be in the ballpark of 20 minutes per inch (so 100 minutes for your 5 inch scope).
3. The 2x aperture in millimetres is optimistic/only for double stars. For planetary detail a 10mm might be the limit of your optimum range.

The Starguiders are good but you'll probably only want 12-18-25 from the range.

[Cosmic Geoff]

Some notes here that may help. You don't want a Barlow with a 127mm Mak unless you are observing with spectacles on and have trouble with eye relief.  Unless you are trying to split close double stars, or similar, the highest powered eyepiece that is of practical use is around 7-8mm - no need for a Barlow. A good quality astro prism diagonal would be a sensible choice, but don't be surprised if you can't see any improvement on the stock diagonal, which will have adequate surface accuracy.

The 10mm stock eyepiece usually supplied with the Maks is not very good (I could use a ruder word), and on replacing it with decent eyepieces (the 8mm and 10mm in my signature) I saw a marked improvement.

[Nomad Z]

Thanks again, folks. I've been trawling around, and here's what I propose buying...

BST Starguider EPs, 25, 18, 12, 8mm
Baader T2 Prism diagonal with 1.25" nosepiece and 1.25" focussing EP holder
Baader short T2 1.25" EP holder

My reasoning is as follows...

The 8mm might be pushing it a bit, but should still have the potential to be usable in good conditions. Plus, as a noob, I need to try it out anyway, and if turns out to be under-used, it still has a possible future in a wider field scope later.

The Baader prism diagonal seems to be well regarded in optical terms, although I appreciate that prisms might have their own issues. In all the reading I've done, I don't get the impression that there's a lot in it optically. Having said that, the main reason for choosing this with the second EP holder is primarily mechanical - I want to go to compression rings rather than plain setscrews.

The visual back on the scope only has setscrews, and is attached using some weird thread that measures 45.4mm outside diameter (M45.5? 1.8"?). I can't find a replacement visual back for this, and the only adapter ring is one to change to SCT. Aside from that, the visual back is decent quality, and there doesn't seem to be much point in replacing it if it can be avoided. On the plus side, it has a T2 thread at the ocular end, to which the short T2 EP holder can be attached, giving a compression ring for the Baader diagonal's nosepiece. The diagonal's EP fitting also has a compression ring as well as a helical focusser (which can be locked), so that protects the nosepieses on the EPs and gives me the option of finer focus. I realise the T2 diagonal can go straight onto the visual back, but it occurred to me that rotating the diagonal after the scope has slewed would be a bit annoying by slackening the big lock ring on the visual back because the forces involved might be enough to knock the scope off target, while slackening a setscrew on the short EP holder is a more benign force. There are a couple of other advantages to using the short holder - it brings the overall optical length closer to the original (about 2-8mm longer), and it means that the diagonal's nosepiece could have a filter fitted rather than having to move it between EPs.

Can I have a sanity check?