Louis D

17 hours ago, HollyHound said:

Where they do differ is immersion... with the Delos, the field edge is visible, but with the Ethos, it isn’t of course and so it really does feel like you’re “floating in space”

You have to watch out for that floating in space feeling.  It can be unnerving 🤣:

42 minutes ago, John said:

You can unscrew the rubber top section of the BST Starguiders which leaves a more suitable shape for mounting the phone adapter

Actually, the rubber cup part just pulls straight off with very little effort compared to the Aspheric's eyecup.  The twist up/down part stays resident on the eyepiece.  It is possible to unscrew the bottom barrel(s) and eventually get the twist up/down part off by sliding it downward from what I've read on CN, but I can't replicate it with my copies.  The upper colored barrel won't budge on mine.  I think they're thread-locked.

30 minutes ago, Thalestris24 said:

PS I should mention that my microscope only cost £216 new so it's not worth spending a lot of time, trouble and money on it. I am, though, a compulsive tinkerer

I've updated the comparison image with the addition of a 25mm Abbe ortho eyepiece.  Notice the lower distortion and narrower field of view.

Your best bet would be a 32mm Tak Abbe orthoscopic costing £189.00 (with 20% VAT) new for pristine images edge to edge.  This would nearly double the cost of your microscope, though.

13 minutes ago, Thalestris24 said:

The microscope does have 23.2mm eyepieces but I've made adapters to fit on the outside of the 23.2 recetacles.

Clever woman you are. 👍

I tried a similar thing by slipping my 27mm Panoptic's 2" barrel over the 1.25" diagonal's eyepiece holder with the securing screw removed.  It worked great with little to no vignetting.  No wonder that it works so well.  With a 30.5mm field stop over a 31.75mm opening, there shouldn't be any issues.

1 hour ago, Thalestris24 said:

I'll wait and see what that Svbony aspheric is like

Remember to pull off the rubber eyecup if you need extra eye relief.  It's on a plastic barrel, so no worries about scratching your eyeglasses on the bare top.

The rubber eyecup can be quite difficult to work off the first time.  You may need to get a butter knife in the gap under it and work it around to pry it upward.

32 minutes ago, Don Pensack said:

Though I notice that, par for the course, all have a fair amount of rectilinear distortion in the form of pincushion.  You could tolerate that in a microscope, but it wouldn't be preferable to an eyepiece with lass or none.

For the binoviewer side of things, this won't matter as it is visual only.  Louise is looking for two, low cost, yet decently performing eyepieces for this purpose.

For the trinocular imaging side, she can probably stick with the BST she already has for now.  It seems to have the least distortion of the three in my image.  I don't think she's going to find a well corrected, budget, low distortion eyepiece that isn't a used Abbe orthoscopic.

I just remembered I had an image for a 25mm Abbe ortho for comparison sake, so I added it to the image, replacing the original image.  It does have lower distortion, but at the cost of a very narrow field of view.  Louis would need about a 33mm ortho to maximize the true field of view in a 23.2mm barrel.  The 32mm Tak Abbe orthoscopic costs £189.00 (with 20% VAT) new for reference.

There are many Chinese made, long eye relief, wide field (50 degrees in microscopy parlance) eyepieces on ebay for cheap that you might want to try.  They come in either 23mm or 30mm barrels already.

4 minutes ago, Highburymark said:

For what it’s worth, not once did false colour spoil the views through my Equinox 80ED.

Agreed.  It's really only at very high powers on bright objects that false color can intrude.  False color suppression also helps discern low contrast details on planets and nebula.  For everyday viewing, though, it isn't much of an issue.

I still remember being unimpressed with the views of Jupiter through a 4" Astro Physics APO at a Texas star party as compared to the view through a 15" Obsession Dob.  It was simply no contest.  Both cost the same with the mount included.  That sealed the deal for me to get a Dob instead of an APO.

I think there's also the point that Chinese manufacturers seem to put more effort into getting a near perfect figure on their ED and APO scopes as compared to their achro scopes.  My ST80 has bucket loads of spherical aberration in addition to chromatic aberration.  So much so, that it makes it nearly useless for terrestrial or astro viewing.  I'm sure an ST80 could be made with as fine a figure as my FPL-51 and FPL-53 based scopes, but at what cost?  Would people be willing to pay a premium for a premium figured fast achromat?

10 hours ago, Thalestris24 said:

On the face of it, it does look quite impressive for not very much! Can I ask exactly how you took the above image? 

Thanks

Louise

1. I taped some rulers and yardsticks together on their back sides with packing tape and then wedged them under the edge of one of my kitchen cabinets, but hanging off to the side.  I always align the 17 inch yardstick mark with the edge of the door for consistency.  I turn on every light in the kitchen/dinette area and open all the blinds to maximize the available light.
2. I put my AstroTech 72ED telescope on its leveled alt-az mount at the other end of our rather open plan house, about 35 feet away and close to level with the yardstick and close to perpendicular with it.
3. I put a 2" GSO dielectric diagonal in the focuser with a TSFLAT2 field flattener spaced 15mm in front of the diagonal body on the scope side.  This pretty effectively flattens the otherwise severely curved focal plane of the scope.  Luckily, I don't need to add any extension tubes to reach focus, unlike when I try this with a 127mm Synta Mak.
4. I put each eyepiece in the diagonal and focus the image with my eyeglasses on so the afocal image is focused close to infinity for the camera.  This allows the field stop to be at its sharpest if it was correctly positioned during assembly and allows the camera to focus at infinity.
5. I then center the yardstick in the field of view and lock the altitude clutch.  Next, I nudge the mount left/right to put the edge of the ruler at the edge of the visible field stop, or at least the edge of the field for those without field stops.  This can be a judgement call for eyepieces that use the barrel for the field edge as the edge will fuzz out.  Also, you can move your eye off center and see more of the field with them by peeking "around the corner" of them, so to speak.  That's why some don't show the edge when the camera is centered.  I'll sometimes take an image with the camera way off to the edge looking at the other edge at an angle to get a clearer image of this effect, just like your eye would be doing in this situation.
6. I use the high resolution, normal wide angle rear facing cell phone camera for most of my images.  In my case, a Galaxy S7.  I cup my thumb and forefinger around the top of the eyepiece to make a landing pad for the phone.  I start well away from the eyepiece and move the camera in toward the afocal image using the screen to guide my movements.  It's important to keep the camera level and centered.  That's where your thumb and forefinger come into play.  With practice, you can get it down to a fraction of a millimeter.  You can roll your fingers get fine height adjustment.  I've tried using adjustable height eye cups on eyepieces that have them to do this, but I couldn't get them to work very well.
7. Now, you have to move the camera phone in and out until the edge of field or field stop just pops into view.  You're at the correct exit pupil distance for that camera at that point.  Any further out, and you miss some of the field.  Any closer, and you start to get blackouts.  If there is spherical aberration of the exit pupil (SAEP or kidney-beaning), you're going to be fighting a dark shadow all around the field.  If you are perfectly centered, you will get a dark circle with a bright center and a bright edge ring.  This cannot be helped as it a defect of the eyepiece and not the camera or scope.  In this situation, you may need to turn down exposure to -1.5 to -2 to avoid the autoexposure circuit trying to make the shadow 18% gray while blowing out the bright areas.
8. Make sure to use the camera's diagonal to get the widest image possible for super wide angle and wider eyepieces.  You'll have to rotate the image in image processing software later.
9. I then proceed to take a series of 3 to 5 images to later pick out the best of the bunch on a large computer screen.  I've found that it's impossible to critically judge these images on the phone's screen.
10. I then take an angled image of the edge for super wide angle or wider eyepieces since the edge of field of even the best camera lenses is not as well corrected as the center.  It may also be cropped off for ultra wide field and wider eyepieces, so this is a necessity for them.
11. If your phone has an ultra wide angle camera, use it to take all-at-once images of ultra wide field and wider eyepieces.  I bought a second hand LG G5 phone for $25 off ebay just for its ultra wide angle camera since my S7 doesn't have one.  That's what I use to take my "full view" images.  I scale them up to match the scale in the center 20% of the S7 images.  Differences in angular magnification across each camera's field of view accounts for the slight width difference in the final images when using the same eyepiece.  Unfortunately, the G5's a 5 megapixel camera compared to the 12 megapixel S7 camera.  When combined with the smaller image scale, these "full view" images are pretty low resolution by comparison.  I'd love to acquire a 24 megapixel or higher ultra wide camera for this purpose.  Anyone know of used ones that sell for cheap on ebay?
12. In post-processing, I do not do any exposure adjustments or sharpening.  I just rotate and flip them to be more readable.  I also crop and composite them for comparison images.

First, does your scope use 23mm or 30mm barrelled eyepieces?  This is what ultimately limits the true field of view of any eyepiece in a microscope.  You can't see what's beyond the barrel.

Second, microscopes operate at high f-ratios (f/13+ as I understand it).  As such, they're not very demanding on eyepieces at the edges.

Third, I love my 23mm Aspherics in my binoviewer when barlowed to f/12 in my Dob or natively at f/12 in my 127 Mak.  It's super easy to get my nose between them and sink them deep into my eye sockets thanks to their diminutive size.  Being super light also makes scope balancing easy; although this isn't an issue for microscopy.

I also remembered I have unprocessed/unreleased 127 f/12 Mak images for several of the eyepieces I've mentioned here.  I whipped together a new composite image below for four of them. (Edit: I just added the Edscorp 25mm Abbe Ortho for reference).

As you can see, all perform really well at f/12 which is comparable to a microscope as compared to with the f/6 flattened ED refractor in the earlier image.

4 hours ago, Pixies said:

Ah right. I guess that's because the secondary isn't flat?

Actually, you guessed more or less correctly.  An SCT primary is about an f/2 and the f/5 secondary effectively slows it down to f/10 due to its curve.  Varying the distance between them varies the resulting magnification.  If the secondary were a simple flat, the focal length wouldn't change any more than it would on a Newtonian by moving the primary forward in its cell to get more back focus.

9 hours ago, John said:

I guess you need a decent triplet to get rid of false colour more or less completely at F/6.3 ?

 

It also depends on the aperture as I pointed out with that achromat chart by f-ratio and aperture in one of these achro/ED/APO threads.  Similar to achromats, an f/6.3 50mm with an FPL-53 doublet would probably be easily as color free as an f/6.3 FPL-53 triplet at 100mm, perhaps better.  The next question becomes, at what aperture would an FPL-51 doublet look as good as a much larger FPL-53 triplet.  I know of no equivalent chart for comparing FPL-51/53 doublets/triplets to each other across various combinations of f-ratio and aperture.

I think it is informative that no premium frac companies make 150mm f/5 scopes, despite Skywatcher/Celestron making an achromat in this size/f-ratio.  I'd like to know how good an FPL-53 triplet at this size/f-ratio could be.  750mm is a very manageable focal length on an alt-az mount.

8 hours ago, Cosmic Geoff said:

I don't have a 2" diagonal. I could never see the point.  You also need a 2" visual back, and it adds up to a significant expense just so you can get a somewhat wider FOV with a low-power eyepiece.   You can go to about 32mm focal length in the 1.25" format.

 

If you already have a bunch of 2" eyepieces for Dobs and fracs, and 2" diagonals for fracs, it just seems natural to invest in a 2" visual back for your Cats.  They're not very expensive at all.

1 hour ago, Thalestris24 said:

Hi

I've seen that on CN but I didn't, and still don't, know what I'm looking at! Anyway, apart from the Starguider, they mostly look too bulky and out of my price range. I need a bright, flat field, with large field stop that I can image through afocally without too much distortion. - preferably for less than about £50-£60

Thanks

Louise

Try the $15 Aspheric first, assuming you're not using it in a f/4 to f/5 system.  At 1.4 ounces and tiny in size, it's worth a shot.  If you don't like it, use it as a focuser plug.

If the 25mm GSO Plossl is similar to my Taiwan made (GSO most likely) 26mm Orion Sirius Plossl, then it has only 11mm of measured, usable eye relief.  This is much too short to take in the entire, measured 53° AFOV with eyeglasses for me.  The problem is the highly recessed, 23mm eye lens.  Due to edge distortion, it works out to have only a 49° eAFOV.  It has a 22.3mm measured field stop.

The 25mm Starguider BST (Paradigm) has a true 60° AFOV, a 61° eAFOV, 17mm of usable eye relief, and a 26.7mm field stop.  It does fall off quite noticeably in sharpness in the outer 25% of the field at f/6.

The now discontinued 25mm Meade HD-60 actually has a 58° AFOV, a 57° eAFOV, 18mm of usable eye relief, and a 24.9mm field stop.  It remains sharper to the edge, but the FOV and FS values are less than the BST.

The real winner at this focal length for long eye relief and maximum true field of view is the 24mm APM UFF, which is now available in a variety of other brands as well.  It has a 63° AFOV, a 66° eAFOV, 17mm of usable eye relief, and a 27.5mm field stop.  It does get a bit fuzzy right near the edge due to some vignetting.

If you want to stay really cheap, the 23mm 62° Vite/Svbony Aspheric, available on ebay for about $15 direct from China, is really pretty decent.  It has a 63° AFOV, a 65° eAFOV, 18mm of usable eye relief with the rubber eye guard removed (it just pulls off), and a 26.2mm field stop.  It gets a bit fuzzy toward the edge, but it's not that much worse than the 25mm BST.

All of the AFOV, eye relief, and FS numbers were measured by me using a variety of direct and photographic techniques.

Below is a comparison image of my 23mm-28mm eyepieces taken through a field flattened AT 72ED.  It's pretty clear how well corrected the 27mm Panoptic is compared to the others.  I would imagine the 24mm is similarly well corrected.

3 hours ago, JeremyS said:

Interesting you mention the XL 28. Not often cited on here. Is it a favourite of yours?

Never tried it, but I love my 14mm and 5.2mm XLs.  It's just that the 28mm XL is generally regarded as the best 1.25" eyepiece in that focal length range ever made.

Here's a review of the Smart Astronomy 27mm EF Eyepiece which I believe is the same eyepiece.  Not quite as good as the 28mm Pentax XL, but not bad at all.

4 minutes ago, dweller25 said:

Personally I quite the look of diffraction spikes on stars. Of course that will not be an issue with DSO’s.

Bright stars themselves are slightly bloated as well in Newts.  This is the difference I immediately noticed between the Dob and the fracs.  It makes splitting close, non-equal brightness doubles easier in the fracs.

10 minutes ago, Mark at Beaufort said:

Thanks for that. I have never noticed that before. It does not help being badly colour blind.

No problem.  My future son-in-law is also color blind.

3 minutes ago, dweller25 said:

6” Newtonian will do all the things you want

Except provide diffraction free, pinpoint stars.  For wide field viewing, the difference is noticeable.

4 minutes ago, Mark at Beaufort said:

However I never really understood the black etched lines. If you look at a 90mms frac at f/5 it shows the black lines.Go to f/6 it's filterable. Go to f/7 you have black lines again.

Notice the hashed lines go in opposite directions and signify different levels of correction.

18 minutes ago, Carl Au said:

Interesting, thank you, I am not alone in thinking this, In what way are they different?

The oldest of the designs is the APM UW 30mm 80° which is a Markus Ludes commissioned clone of the 30m KK Wide Scan II (or III, I can't remember) which was a Japanese designed and built UWA of the 90s.  It has 5 elements, massive field curvature, excellent central sharpness, long eye relief, and very good edge correction once field flattened.

The TMB Paragon came next about 15 years ago.  The 30mm and 40mm versions were designed by the late Thomas M. Back.  The 35mm version was inspired by his designs.  They have very good correction for their size and weight, 6 elements, shorter eye relief, and flat fields.  The design was cloned by multiple Chinese optical shops (as were TMB's 58° Planetary line).

The APM UFF came last about 4 years ago.  They were designed by Mark Ackermann for APM.  Since then, they've been sold by KUO to multiple vendors.  The designs are shown below:

The better question is, can you tell the difference between an ED and an APO visually.  In my experience at low powers, you can't.  It's only at high planetary powers that you can see the better color correction of the APO.

Really long f-ratio achromats can be very good indeed, but mounting them is a pain.  If you enjoy wide field views, this means short f-ratios.  You'll want at least an ED to avoid excessive violet fringing.

The ED you're looking at is made by Sharpstar Optics.  They're probably the highest quality Chinese made refractors.  I have the TS-Optics 90mm FPL-53 triplet and love the optical and mechanical quality.

All Skywatcher products are made by Synta.  Optically, they're very good.  However, mechanically, they are very basic to hit an introductory price point.  Swapping focusers (to MoonLite generally) on the SW 120mm APO doublet is pretty common because of this.

Here's a good chart showing the differences in CA across various apertures and f-ratios of achromats:

Notice the trend that as you increase aperture, you need longer f-ratios to maintain constant CA correction.  This is also true of ED and APO scopes of a constant lens design.  If you want good color correction at f/5 and 100mm, you will need an FPL-53 triplet or equivalent.  However, if you can live with f/8, an ED or APO doublet will suffice.

1 hour ago, Carl Au said:

They have got to be the same eyepice, the clones are everywhere I tell ya, everywhere!

The 30mm APM UFF is an entirely different design from the 30mm Aero ED (TMB Paragon originally).  And these are not to be confused with the APM UW 30mm 80° which is yet another entirely different design.

I think part of the confusion is all are (most likely) made (but not designed) by KUO in China, so they have very similar build qualities.