Suggestions for a short light path diagonal.

[focaldepth]

Suggestions for a short light path diagonal.

Hi all,

does anyone have any suggestions for a 1.25” 90 degree star diagonal or prism with a short optical light path?

I have calculated that I need less than 70mm (including eye piece holder) but the shorter the better, to give some focus head room.

All my current diagonals, including WO and SW 45 degree prisms are far too long.

I am already using a zero length 1.25” adapter in the draw tube.

Google is not finding the information I need.

Thanks in advance.

[MARS1960]

40 minutes ago, focaldepth said:

Suggestions for a short light path diagonal.

Hi all,

does anyone have any suggestions for a 1.25” 90 degree star diagonal or prism with a short optical light path?

I have calculated that I need less than 70mm (including eye piece holder) but the shorter the better, to give some focus head room.

All my current diagonals, including WO and SW 45 degree prisms are far too long.

I am already using a zero length 1.25” adapter in the draw tube.

Google is not finding the information I need.

Thanks in advance.

What scope are you using? what eyepieces are you using?

I assume you are running out of focus with some of your eyepieces? 

If that's the case i would think any mirror diagonal would work, it' seems it's the prism ones that are causing you problems. 

[focaldepth]

I actually posted a complete description but it was so long that I got no replies, so I wrote this short version it.

Summary SW ST80 + TS crayford, zero length 2-> 1.25 adaptor + Quark, SW Plossl + extension focuses great.

TS Crayford is same length as original focuser.

ST80 + any diagonal + quark + plossl can't focus.

I have measured and calculated that less than 70mm length diagonal is required.

The consensus on the web seams to be that prisms use less back focus than mirror diagonals. I don't have any 1.25" mirror diagonals, only 2" which are way too big. I will try and borrow one though.

I wanted to know that a particular diagonal was correct before buying one.

I have identified Vixen Prism, Celestron Prism and Baader T2 Prism as possible but want to know the light path length including the eye piece holder. first.

For full story see

 

 

[focaldepth]

I for got t say ...

Thanks Mars1960.

[MARS1960]

37 minutes ago, focaldepth said:

I actually posted a complete description but it was so long that I got no replies, so I wrote this short version it.

Summary SW ST80 + TS crayford, zero length 2-> 1.25 adaptor + Quark, SW Plossl + extension focuses great.

TS Crayford is same length as original focuser.

ST80 + any diagonal + quark + plossl can't focus.

I have measured and calculated that less than 70mm length diagonal is required.

The consensus on the web seams to be that prisms use less back focus than mirror diagonals. I don't have any 1.25" mirror diagonals, only 2" which are way too big. I will try and borrow one though.

I wanted to know that a particular diagonal was correct before buying one.

I have identified Vixen Prism, Celestron Prism and Baader T2 Prism as possible but want to know the light path length including the eye piece holder. first.

For full story see

 

 

Mmmm , a bit over and above my current understanding, you could always enquire at FLO if you don't get advice from someone far more experienced than me.

[Stu]

Some info on path lengths here:

https://www.cloudynights.com/topic/362946-mirror-vs-prism-diagonal/

http://www.baader-planetarium.de/news/mirror-Prism-Dielectric-Diagonal-Comparison_2014-03-06_v2.pdf

http://www.baader-planetarium.com/en/downloads/dl/file/id/154/product/1265/instruction_manual_for_baader_maxbright_binocular_viewer.pdf

[ronin]

Two 1.25" diagonals I have are both 25mm (end of focuser to mirror) and 55mm (mirror to top of eyepiece tube) to the path so 80mm in total. They are both cheap ones and there may be a diagonal with a slightly shorter length of the tube that holds the eyepiece, the 55mm measurement on mine.

Cannot shorten the eye piece tube as at least one has a threaded to hold it to the diagonal body, so shortening either end is not feasible.

Better say that the measurements are approximate but when made they seemed about right.

[Alien 13]

Time to get the Dremil out and chop the tube me thinks..

Alan

[focaldepth]

Stu, thanks, I already had some of that data but found some new data for me to consider. It is great to have some real numbers

ronin, Another data point, thanks.

Alien, DREMEL! Sould be accurate enough to keep the rays in sync to 1/12 lamba. I'd thought about a lathe but my one is too small to even think about it. I wonder how other people do it.

 

I have a feeling that this diagonal will cost a little more that my lovely lady had budgeted for.

 

 

[Louis D]

I'm thinking you may want to consider the Baader T-2 prism diagonal.  Both ends are fully configurable to reduce mating distances to a minimum.  I know of folks on CN using them with binoviewers to minimize their focal path length.

[focaldepth]

Thanks all,

I bought a Baader T2 with adjustable holder kit. I can now focus the Quark but with only 1-2mm focuser travel left. I have also ordered a low profile holder.

So Averil and I have now had first light through a thin veil of cloud. Looks promising, can't wait for some clear views.

[focaldepth]

For future reference here is the solution I found to my problem. Thanks for the above suggestions.

Problem: SW ST80 with TS Crayford, 1.25 diagonal, Quark Chromosphere, SW plossl eyepiece could not achieve focus.

Solution:

1) Replace 2" to 1.25" adaptor with Starlite low profile one (zero length)

2) Replace diagonal with Baader T2 diagonal, plus Baader low profile eyepiece holder.

3) Swap the very long Quark 1.25" nose piece with one from a plossl eyepiece.

So we now have 12mm spare back focus. (the focuser tube is out by 12mm at focus)

 

[Piero]

id have a look at the Baader T2 options. The Zeiss prism ones are reported to be excellent particularly at f7+.

[Redbird48oo]

Same problem I have with my ST80, need more inward focal travel with 2" diagonal and 2" eyepieces. I am thinking about shorting up the tube 1/2" to 3/4". Can always use an extension with 1.25 diagonal and eyepiece. [ Orion ST80, ScopeStuff 2 speed crayford focuser, HIgh Point Sci. Dia. Electric diagonal]

[Davey-T]

30 minutes ago, Redbird48oo said:

Same problem I have with my ST80, need more inward focal travel with 2" diagonal and 2" eyepieces. I am thinking about shorting up the tube 1/2" to 3/4". Can always use an extension with 1.25 diagonal and eyepiece. [ Orion ST80, ScopeStuff 2 speed crayford focuser, HIgh Point Sci. Dia. Electric diagonal]

Be careful shortening the tube as it's that length for a reason, too short may cut into the light path, no idea if it will in your case, maybe a bit can be removed from the end of the actual focuser tube as well.

Dave

[Louis D]

29 minutes ago, Davey-T said:

Be careful shortening the tube as it's that length for a reason, too short may cut into the light path, no idea if it will in your case, maybe a bit can be removed from the end of the actual focuser tube as well.

Dave

Shorten the tube and replace the focuser with a two inch drawtube version to limit vignetting in the new configuration.

[Redbird48oo]

crayford focuser is a 2", so should not have any problems I would think with vignetting. Going to find out, will keep you posted

[Louis D]

On 10/31/2017 at 12:14, Redbird48oo said:

Orion ST80, ScopeStuff 2 speed crayford focuser,

Missed that you are already running with a 2" focuser.  Go for it!