Jump to content

NLCbanner2024.jpg.2478be509670e60c2d6efd04834b8b47.jpg

Shortest possible 2" Diagonal

Rossco72

By Rossco72
in Discussions - Eyepieces

Recommended Posts

Rossco72

Rossco72

Posted
Posted

Hi all,

I am looking for a 2" diagonal which has the shortest possible addition to the optical light path.

I have an issue with in focus and my current diagonals are just a couple of mm too long and wont come to focus.

Any suggestions?

Link to comment
Share on other sites
  • Replies 26
  • Created
  • Last Reply
Rossco72

Rossco72

Posted
  • Author
Posted

I have fitted a low profile 2" crayford to a TAL 200K. The amount of focus is limited and I can't get any of my 2" Naglers and ethoi to reach focus. Looking at the straight through focal position I would say I only need to save about 3-5mm.

Link to comment
Share on other sites
SpookyKatt

SpookyKatt

Posted
Posted

Stuart I'm pretty certain you will have to adjust the backfocus I'm afraid,

as the range of focus on a 200K is extremely limited. I have done

something similar with my 200K but I'm still finding the right adjustment

for what I'm trying to achieve.

I hope a different diaganol resolves the issue but personally I doubt it.

What crayford have you fitted ?

Kathleen

Link to comment
Share on other sites
AndyH

AndyH

Posted
Posted

A Zeiss/Baader prism diagonal will give a potential 25mm saving over a 2" mirror diagonal. The Zeiss/Baader units aint cheap though.

I measured that the following kit would require 10mm more infocus than the standard Tal 1.25" diagonal.

I'm (still!!) in the process of getting my old Takahashi prism adapted so I can use my 2" Tal 24mm UWA on my 200K. There should be threads on the main forum somewhere regarding stuff with my 200K(I think).

Using a prism type diagonal may still not be enough though. I had moved my corrector unit forward to give roughly 10mm more infocus before my Tak idea.

I'm beyond poor at the moment, so everything is on hold for the forseeable future :-(

Link to comment
Share on other sites
m.tweedy

m.tweedy

Posted
Posted

Hi Stuart I have one of the Clicklock diagonals which is excellent However it does not seem that short.

The enclosed link shows the T2 from Baader

T2 MAXBRIGHT Zenitspiegel - beidseitig T2 Anschluss

The shortest 2" diagonal mirror we know...

The T2 MaxBright diagnoal allows 2" adaptations if adapters are used.

This makes sense with telescopes who have a short back focus where normal 2" diagonals can't be used. One example is the Pentax 75 SDHF.

You can use 2" eyepieces up to ca. 40mm field stop diameter. The picture remains steady. You only have small vignetting on the outer rim of the image.

Link to comment
Share on other sites
Alma

Alma

Posted
Posted
Hi Stuart I have one of the Clicklock diagonals which is excellent However it does not seem that short.

.

I would agree, steer clear of the white 2" Baader Click-Lock, it has a longer light path than the WO dielectric and Astro-Physics MaxBright diagonals that I use now.

The others allowed me to achieve focus in my binoviewer-friendly refractor without any optical correctors or a barlow, but not the Click-Lock.

Link to comment
Share on other sites
Rossco72

Rossco72

Posted
  • Author
Posted
Stuart - Any pics of the back of your 200K with the new focuser, you could upload, would be great.

Kathleen - Is it a JMI focusser you're using? Out of interest, what's it's length?

Cheers,

Andy.

post-16557-133877681695_thumb.jpg

post-16557-133877681703_thumb.jpg

It's a Crawmach Low Profile.

Cheers

Link to comment
Share on other sites
nightfisher

nightfisher

Posted
Posted

stuart, did you get the low profile 1.25 adaptor with the crawmach, maybe you would be better of sticking with 1.25 ep`s for use on brighter objects like lunar and planets, by the way the tal looks lovely with the conversion

Link to comment
Share on other sites
astro_al

astro_al

Posted
Posted

I have the Baader/Zeiss prism diagonal that Andy mentioned. It is a nice diagonal and does have a short light path but I only use it with 1.25" EPs. I think its clear aperture is 35mm which will be OK for some 2" EPs but not all.

Link to comment
Share on other sites
  • 2 weeks later...
split_city

split_city

Posted
Posted

First of all, when will we see the social groups, specifically the Tal social groups posts migrated? Rather annoying as I really want to see the 2" focuser mod thread.

Anyways, my dad recently made up a 2" backplate with a SCT thread. The plan is to get a Crawmach ELP. It's a good time for me to purchase given the strong Aussie $. However, it has fallen slightly so I might have to purchase the crawmach soon.

It looks like Rossco72 has Eliot's setup. Nice. My new backplate is different as mine has a SCT thread. I have been informed by Danny that the ELP, with the female SCT thread adapter, has a racked in height of ~47mm. Add another ~3-4mm for the adapter plate. I can't remember the racked in height of Eliot's, now Rossco72 setup? Anyone remember or could you please measure it Rossco72?

I hope to use the tal diagonal. I know it has a flange but I was hoping to make a suitable 2"-1.25" adapter for it. If I do make an adapter, I wonder if I will be able to achieve focus with this setup?

Link to comment
Share on other sites
split_city

split_city

Posted
Posted

Thanks for that Andy. I guess the question remains as to whether I will be able to achieve focus with my setup. Assuming that the focuser will sit flush against the backplate, my racked in focuser height will be around ~50-52mm.

What diagonal did Eliot use? What's the "length" of that diagonal? Did you test this one Andy? The tal one is quite short.

If my plate doesn't work, I can always ditch the SCT adapter and make up something similar to Stuart's.

OT question: Is the two speed focuser worth it? It's another $30.

Link to comment
Share on other sites
AndyH

AndyH

Posted
Posted

I've measured the optical lengths of various diagonals over the years. Not sure if you've seen it....

http://stargazerslounge.com/discussions-eyepieces/141922-how-long-diagonal.html

I need to add another to the list. Stuart gave me a 1.25" revelation dielectric diagonal. It uses up 80mm of focus.

So measure how far out your std Tal R&P focuser and various diagonals(if you have more than one)racks out with a 25mm plossl. Then you can figure out if your 200K will come to focus with a new focuser and backplate.

Cheers,

Andy.

Link to comment
Share on other sites
split_city

split_city

Posted
Posted

Yep, I have seen you thread Andy. I just couldn't remember the focuser Eliot used when going through this modification, and if you tested this one. Eliot said he could use all his 1.25" eyepieces with the revelation diagonal, but only after he made up an adapter to allow the diagonal to fit snug up against the crawmach. Given the shorter length of the tal diagonal, I should be able to achieve focus with my current setup i.e. including the SCT adapter. I probably would still need to make up a special adapter to allow the tal diagonal to go as far in as possible, including the flange. Please correct me if I'm wrong! I will need to go out and try with a 25mm eyepiece to confirm.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.