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How 'long' is a diagonal?


AndyH

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When I say, how long, I mean how long optically, or how much focus does a diagonal use up.

Those of you that have a moving mirror SCT or Mak will have no such worries, but anyone with a fixed mirror Mak(or in my case, Klevtsov), will understand the importance of this exercise. Perhaps those with refractors that are a bit 'tight', when a 2" diagonal etc is put on the back, might be interested too.

I used the following diagonals. All 90 degrees.

1/ cheap, all plastic bodied 1.25" mirror diagonal

2/ cheap, but much better quality than the above. Has metal nosepiece and a noticable better mirror.

3/ Tal 1.25" mirror diagonal. Klevtsov/100R version, with 10mm flange.

4/ Takahashi 1.25" prism diagonal

5/ Burgess 2" CED mirror diagonal

6/ Takahashi 34/41mm prism diagonal. Clear aperture of 41mm on nosepiece end, 34mm on 2 "eyepiece adapter end.

7/ Astro-Tech(US) 1.25" dielectric diagonal. I've sold this, but kept calculations made last year.

After doing some calculations with a Meade 20mm RG widefield eyepiece(quite wonderful 'piece) I worked out the following.

Cheapest diagonal 1 uses up 80mm of focus.

Cheap diagonal 2 uses up ... 76mm of focus.

Tal diagonal 3 uses up ....... 71mm of focus.

Tak diagonal 4 uses up ...... 64mm of focus.

Burgess diagonal 5 uses up 108mm of focus.

Tak diagonal 6 uses up ...... 81mm of focus.

A-T diagonal 7 uses up .. 85/86mm of focus.

A few extra comments.

The Burgess 2" diag has a low profile 1.25" adapter which has a height of 4mm, so when using this diagonal in 1.25" mode, the focus used is 112mm.

The 1.25" Tak prism is a smashing item, let down ever so slightly, by the plastic(though good quality) body. Everything else is superb, including the high quality prism inside. It has about 29mm clear aperture on both scope and eyepiece side. The compression eyepiece holder has to be the best design on the market today. Far, far superior to the brass ring compression types. Optically as good as any mirror diag I've used.

The Tal diagonal I have, which I mentioned, was designed to be used on two ranges of their scopes, has a 10mm flange, which would come into play if used on a focuser without the recess to hold it. So those 10mm's would need to be added to the above measurements. The diag's as provide with the current refractors and Klevs have the std nosepiece, so the above measurements will be valid.

The big Tak prism came from the US many moons ago and is of superb quality. My plan of action with this is to get a special machined nosepiece to let it fit my old r&p focuser on the 200K. This low profile nosepiece will let me achieve focus with my wonderful 2" Tal 24mm UWA lens. Marketed as 80 degrees afov, this is so far off the mark it's laughable. I reckon closer to 95 degrees. This could give me close to a 0.91 degree fov on my 200K, which has to about the limit with these old F10's. I am barrel limited by the r&p focuser, but having done some tests, I see no noticable image breakdown. It has a T2 thread on the front and there are 1.25" and 2" adapters readily available. Will probably get a 2" one in time.

Whilst the big Tak prism is long gone out of circulation, Telescope Service have Baader/Zeiss units which may be useful. Not cheap though, especially if you go for the one with the biggest clear aperture.

Hope this has been of interest, though probably not ;)

Cheers,

Andy.

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  • 1 month later...

Very useful information Andy, which can be vital in some cases where focus travel is at a premium. When I was researching the Chromacor I came across this on the William Optics 2" diagonal which might be useful to someone:

post-12764-133877625193_thumb.jpg

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Great. :hello2: If you want a short (unmeasured!) optical path, the Baader T2 diagonal is (as claimed) useful. Moreover the 2" eyepiece holder fits the (rather curious) stepped nose of Hyperion eyepieces. It is a truism that most people don't use the FULL width of 2" diagonals, so the 35mm aperture suffices for most purposes too... :D

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  • 2 months later...

An addition to the above diagonals....

Tal 2" mirror diagonal = quoted on the NPZ forums as 'using up' 92mm of focus.

A massive saving of 16mm, compared to my Burgess unit.

As always with 2" diagonals, add the height of the 1.25" adapter you're using to the quoted measurements.

A few extra comments.

The Burgess 2" diag has a low profile 1.25" adapter which has a height of 4mm. A lot of 2" diag's adapters can be 10mm in depth, so that could add 6mm to the above measurements.

Not sure if it's possible, but could a Mod remove the above from the original post, as it's wrong?

And replace with....

A few extra comments.

The Burgess 2" diag has a low profile 1.25" adapter which has a height of 4mm, so when using this diagonal in 1.25" mode, the focus used is 112mm.

Cheers,

Andy

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