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DRT

Sky-Watcher EvoGuide ED50

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Hello Peter, if you are willing to machine the structure for a binoscope, I think you can consider to simply cut the tubes, remake the M53 thread and solve once for all the problem of not enough backfocus.

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Thanks Rick, from reports by others it seems that a focus can be achieved without cutting if a suitable diagonal is employed. I am proposing to make a straight through unit for use on a binochair or terrestrially with diagonals and a tripod.

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Posted (edited)

Peter, it's up to you (of course). However:

1. If the use is mostrly terrestrial, then there are far better alternatives on the market for straight through units which (i) are rugged enough to be backpacked and light enough to be used hand-held, (ii) provide correct-oriented vision and (iii) are flat-field. Example: APM MS50ED.

2. If the use is mostly astronomical, then 90° degree are far better than straight-through (I can't stress enough the "far better"). In this case it really does not exists ANYTHING in the market comparable to the 50mm ED binoscope you want to build. ANYTHING in term of size and in term of costs (500€ w.r.t 1500€ of APM angled binos).

So, whatever it takes, use 90° diagonals, even at the cost of cutting the tube :)

Edited by Rick_It

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Cheers Rick. I'm well aware of the "ins and outs" of binoculars/binoscopes having made them professionally for 40 years. I also have several models using refracting and reflecting optics from 50mm up to 300mm aperture. What I was looking for in the proposed SW50ED binoscope was one with minimal number of optics for straight through astro viewing for use on my modified zero gravity chair. The terrestrial application was simply because it's possible.  🙂

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Sorry Peter, obviously I was not aware that you built professionally binoscopes :)

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Further update:

1. I could not quite reach focus on infinity using the Baader T2 Prism with the focus ring removed and my TV 32mm Plossl. Sigh.

2. I purchased one of these...

1044574144_ScreenShot2018-06-12at20_52_39.png.e8eb6a7b41aebbd5b827504cf1e0c7b4.png

3. Replacing the Baader ClickLock clamp with the TS 15mm Extension Tube has moved the Plossl 5.5mm closer to the objective. If this doesn't work nothing will.

4. This has been a very expensive experiment :rolleyes:

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If you can laugh yet sigh and hear yourself saying 'oh dear' all at the same time, I think i did just that after reading statement #4 :redface:

 

Fingers crossed !

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Thank you for continuing, I have tried everything I have here. The bit I don't get is that in theory the distance should be about the same as the adaptor plus that 15mm extension when used straight through so what is it with diagonals giving an increased distance greater than the physical distance we already know works when viewing is straight through. Do diagonals bend the light path like say a barlow does?

Did you try any other eyepiece makes?

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Posted (edited)

With a 1.25" mirror diagonal, in theory they should have the same or very similar light paths, BUT, it's the eyepiece holder on top that makes different diagonals vary.. eg: many are between 78-81mm, yet the Tal version is about 71mm, simply because it has a very low profile ep holder and no stop at it's bottom, so the ep can sometimes slip slightly into the diagonal body itself(without interfering with the light path or touching the mirror, unless it's got a very long chrome barrel bit) . Same idea for a std 90 deg prism, except the bigger the prism inside(giving a bigger clear aperture), the longer the light path. My Tak 1.25" prism(27mm clear aperture?) has a light path of 63mm, yet my bigger Tak prism(34mm clear aperture) has a light path of 81mm with it's big 2" ep holder. I could shave a few mm off that with an adpter similar to what Derek has done above(if i was only using 1.25" ep's).

Mmmm. Hell, hope that isn't confusing. Sounded ok in my head ;)

Edited by AndyH
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Posted (edited)
28 minutes ago, happy-kat said:

Thank you for continuing, I have tried everything I have here. The bit I don't get is that in theory the distance should be about the same as the adaptor plus that 15mm extension when used straight through so what is it with diagonals giving an increased distance greater than the physical distance we already know works when viewing is straight through. Do diagonals bend the light path like say a barlow does?

Did you try any other eyepiece makes?

I didn't, I have 3 TV Plossls and just bought a set of Lunt/APM 100 degree monsters which are the same size as the EvoGuide so haven't tried those yet :lol: 

We have to keep reminding ourselves that the Meade Prism that I bought second hand for £20 worked just fine and it looks very similar to the one produced by Celestron for all of their SCTs so there are loads of diagonals around that will work with this thing. Perhaps my mistake is trying to achieve a balance of quality at both ends of the light path :wink: 

Edited by DRT
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Thank you both.

Did you think the cheap diagonal that did work spoilt your view from a practical point of view?

I did this to my diagonal that gave me focus 30+ feet away with it's own barrel holder. I unscrewed it and did the below with the shorter baader helical. The eyepiece basically couldn't be much closer to the mirror and the silver fitment fitted to the hilt the other end. The diagonal when you have it your hand can't measure 60mm even at a push so why it didn't work but gave focus instead to around 100+ feet puzzles me.

15283006215221.jpg

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2 minutes ago, happy-kat said:

Did you think the cheap diagonal that did work spoilt your view from a practical point of view?

 

I didn't spend any time with it to be honest. I had already decided I didn't want my TV eyepieces held and scratched with a screw so just tested it and have since been focused (sorry :lol: ) on the getting the Baader Prism to work.

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Posted (edited)

Here is how the 71mm of the Tal diagonal works out. Please excuse the childlike attempts at straight lines in the pic ;) . Also, ignore the fact that it's a special flanged Tal diag. Same sizes for the std nose ones.

46mm for the diagonal body light path plus the 25mm of the ep holder.

1074957457_200kdiagsizes.jpg.8a1f149f54e25fae857d27f402494f69.jpg

Edited by AndyH
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Thanks. I will measure mine tomorrow and report back.

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Focusing straight through places the eyepiece roughly at the focal point. If a diagonal is then placed between the telescope and the eyepiece then this moves the eyepiece back by the light path distance that the diagonal consumes. The result is that everything has to move closer to the objective and this is the problem under discussion.

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Yup. I've made a few R/A finders out of old/broken binoculars and when modifying my Klevtsov to a bigger focuser, that's when i was awakened to the frustrating world of available backfocus ;) . Every milimeter counts !

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Hello all, I'm new here, I pretty much joined because of this product / thread!

I have tried a standard SkyWatcher 2x barlow screwed onto the front of my diagonals (just the lens group, not the whole thing).  The barlow was giving way more multiplication of the focal length than I wanted and it needed a lot of T2 extension tubing so I don't recommend it!  I then bought a T.S. Optics 1.6x glass path corrector / barlow and used that along with the short T.S. eyepiece clamp (the one mentioned a few posts above) to clamp the diagonal in place (although I possibly could have gotten away without it).  While I would still prefer to reach focus at the scope's native focal length, this arrangement works fairly well and the field of view is only slightly more narrow than my 9x50 SkyWatcher Finderscope when using a 40mm Plossl eyepiece.  Although I would say it's a more comfortable and better view than the 9x50 finder.  It would be nice to get the barlow closer to the eyepiece, the thought of cutting the nose-piece on my diagonal shorter has crossed my mind.

The diagonals I used were the basic 1.25" SkyWatcher mirror diagonal that came with my Mak and the 45 degree erecting prism that came with my ST80.   I am tempted to buy the Williams Optics 90 degree erecting prism, but then the light path would probably grow further still, leading to more barlow effect.  If anyone knows of an even lower power barlow that can be screwed directly onto a diagonal I'd love to hear about it, or an eyepiece longer than 40mm focal length in a 1.25" format.  There is a Baader 1.2x glass path corrector / barlow, but that is meant for a T2 thread.

I would have thought that the Baader T2 prism would have reached focus with the short T.S. eyepiece clamp, does this not work, or is the T.S. clamp on order and yet to be tested?

I think SkyWatcher have a very odd product here; it has ED glass which I doubt would make a big difference for guiding (probably better off using a 60mm non-ED design for the money I would have thought), but everyone seems to want to use it visually, but it's too long to focus with a diagonal!  And for guiding it's odd because it uses a finderscope foot rather than a dovetail bar or clamp.  I took the finderscope foot off the metal plate and drilled, tapped and mounted an ADM dovetail clamp onto the plate.  Way better!

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20 hours ago, happy-kat said:

Thanks. I will measure mine tomorrow and report back.

Right I've measured it, obviously give or take +/- 1 mm

18mm each side of that diagonal then 28mm for the baader helical giving total of 64mm which pushes focus with a 30mm NPL to about 100 feet. When I trimmed 4mm of the diagonal I gained about 70 feet more distance.

So I think looking a sub 60mm light path diagonal at a guess.

And welcome to Death Star Moon

 

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Cheers happy-kat.

I stuck my camera onto it's T adapter (which ought to be 55mm from the front flange to the focal plane) and racked the focuser out until I got focus on some stars.  I worked out that I had moved the focuser back by approximately 7.75mm to reach focus (very rough measurement (I don't have callipers)).  So from the flange on the back of the Evoguide 50ED to the focal plane is about 62.75mm on my model.  For normal non-T2 diagonals there's about 3mm less than that because of the T2 threads being there, so about 59.75mm.

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5 hours ago, Death Star Moon said:

I would have thought that the Baader T2 prism would have reached focus with the short T.S. eyepiece clamp, does this not work, or is the T.S. clamp on order and yet to be tested?

I tested this set-up last night with a TV15mm and the focus point was bang in the middle of the focuser travel :smile: 

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Got a picture of this setup please?

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13 hours ago, DRT said:

I tested this set-up last night with a TV15mm and the focus point was bang in the middle of the focuser travel :smile: 

Nice, I thought it would work  :)

I think my 1.6x barlow set up gives me about 80% of the field of view that going straight through does, the penalty isn't too horrendous.

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So if I cut my eyepiece holder down to 12mm like the T.S short clamp I'll be in business, off to ponder cutting it down as I can't justify the cost of that prism and clamp.

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