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72ED Diagonal


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I've just bought a Skywatcher 72ED to use as a grab and go, but having tested it in the day it looks like I'm going to run out of in focus at night using the 2" Baader Clicklock diagonal I already own. If you own(ed) a 72ED which diagonals have you found that allow you to reach focus? Based on a few measurements I'd guess that I'm looking for something with a light path in the region of 90mm. I'd like the option of 2" if possible but I'm envisioning that a 24 Pan would usually be my low power option so a 1.25" diagonal would also work. 

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Thanks, Victor. How does the focus position of that eyepiece compare to others? If you add a 2"-1.25" reducer does that prevent your 1.25" eyepieces from focusing? 

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Makes me glad I bought an Astro Tech 72ED.  It has plenty of in focus left with a GSO 2" dielectric diagonal.

Why would SW sell a scope with a 2" focuser than doesn't have enough in travel to accommodate a 2" mirror diagonal?

You might try a 2" prism diagonal to save a few millimeters of optical path length:

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    Approximate direct measure of the diagonal light path (millimeters)

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Just looking at your target light path of c90mm and your most used eyepiece -  the Pan 24mm..

The Baader Zeiss T2 BBHS prism is quoted as having 34mm clear aperture, and the Pan 24mm a field stop of 27mm. This means this diagonal prism will easily illuminate fully your Pan 24mm with a 1.25" eyepiece holder.

Regarding light path.

The above prism comes as a prism body only, so you need to add a 2" -T2 nosepiece and ideally a 2" Baader Clicklock eyepiece holder to allow you to use 2" eyepieces as well as 1.25" ones..

..the prism itself has a stated optical path of 37.5mm. The 2"-T2 Clicklock eyepiece holder has a stated optical path of 37mm, so a total of c75mm. The 2" -T2 nosepiece should use up a tiny amount of optical path as it slides almost completely into your focuser, but let's say it takes 5mm from your remaining 15mm to stay within a total of 90mm..

If you also use a 2"-1.25" adapter or 1.25" clicklock inside the 2" Clicklock, the additional length of the smaller clicklock could take up most, or all, of the remaining 10mm..an alternative might be to source a low profile 2" to 1.25" adapter instead, which might take up 5mm or less of your remaining 10mm..

It all sounds a bit complicated but the attached link to Baaders' excellent prism and diagonal manual may help. I also attach a photo of my own setup of Baader 2" - T2 nosepiece, Baader T2 Zeiss BBHS prism body and Baader 2" - T2 Clicklock eyepiece holder..this setup allows me to use heavy 2" eyepieces, and illuminates field stops up to c 35mm fully. Even my 31mm Axiom LX, with a 44mm  field stop shows little noticeable vignetting in the field.

At the end of the day you want to reach focus, and if you achieve it with only 1 or 2mm to spare..you have still reached focus!! - I have this exact situation with my Tak FS128.

Another alternative is to look for an aftermarket new 2" focuser which takes up less optical length than the factory original..

Good luck👍

Dave

 

IMG_20220114_142600741_HDR.jpg

baader_star_diagonals_t_2_and_2 (2).pdf

Edited by F15Rules
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6 hours ago, Louis D said:

Why would SW sell a scope with a 2" focuser than doesn't have enough in travel to accommodate a 2" mirror diagonal?

 

I think the problem stems from it only having 38mm of travel (!!). They must have designed it to work with the flattener + camera and perhaps their diagonal with a shorter light path. Thanks for the link/graph, I was hoping someone might have taken some measurements.

3 hours ago, F15Rules said:

The Baader Zeiss T2 BBHS prism is quoted as having 34mm clear aperture, and the Pan 24mm a field stop of 27mm. This means this diagonal prism will easily illuminate fully your Pan 24mm with a 1.25" eyepiece holder.

 

I was looking at the Baader T2 prisms and coming to the same conclusions. I've got some Baader T2 extension rings somewhere so I think I could make up a diagonal with a 2" nose and the 1.25" Clicklock that would work nicely and hopefully be a bit lighter too. It would be a shame to lose the ability to use 2" eyepieces and max out with a 5° field but I don't think a 21E is an eyepiece that fits in a pocket anyway. 

3 hours ago, F15Rules said:

Another alternative is to look for an aftermarket new 2" focuser which takes up less optical length than the factory original..

Given the cost of the scope I'm not sure that it's worth it. It would probably be more cost effective to send the 72ED back and buy something else , or to shorten the tube if it is easy enough to remove the focuser and re-fit.

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8 hours ago, Ricochet said:

Thanks, Victor. How does the focus position of that eyepiece compare to others? If you add a 2"-1.25" reducer does that prevent your 1.25" eyepieces from focusing? 

Your welcome!

I can still reach focus with my 1.25" eyepieces in that same diagonal with a 2" to 1.25" adapter. However, not a lot to spare!! Some 1.25" eyepieces might not reach focus with the same diagonal.

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Good news! It was clear last night and I discovered the telescope actually comes to focus with significantly more room to spare than I was expecting. Using the 2" Clicklock diagonal and 1.25-2" clicklock reducer the 24 Pan and Delites focused with about 10mm infocus to spare and XWs  and 21E (without reducer) a few mm further in. The Baader Hyperion Zoom was right on the limit at about 1-2mm depending on the zoom setting. A smaller diagonal is probably on the cards long term to save weight if I find that I do tend to go out with just the 24 Pan and BHZ.

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On 30/04/2022 at 15:30, Ricochet said:

A smaller diagonal is probably on the cards long term

As an update, "long term" was less than a week. 

DSC_3036.thumb.JPG.0d3c652a99b56b26c1f71de42d6db82d.JPG

It's a Baader BBHS prism ( courtesy of @Deadlake) with a Clicklock reducer, 2" nose, 15 and 7.5mm T2 extensions. It focuses near the end of travel with enough in focus for the BHZ and/or my 2x focal extender. It weighs about 330g compared to 530g for the 2" diagonal and reducer combination. It doesn't sound like a lot, but with the scope only weighing 2kg it has greatly improved the balance issues I was having. I was a bit wary of issues using a prism with a reasonably fast scope but I've not noticed anything untoward yet and if anything it is sharper than the mirror diagonal. 

I also upgraded the 2" draw tube clamp to FLO's compression band version, which is a great improvement over the stock clamp. Somehow, despite the two clamps looking like they are the same length, the new clamp prevents the BHZ from reaching focus when used with the 2" diagonal and reducer but I may be able to solve that with the spotting scope modification ring or just by using the BHZ in 2" mode on nights where I want really wide fields. 

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