Not enough inward focus travel with prism diagonal.

[Naughty Neal]

With my Scopetech f12.5 I have run out of inward focus for my EP' s,  I'm ok with a straight thru viewing approach and the back  travel of the focus is fine . The issue is with using a 1/25" StelllaMira 90 deg erecting  prism diagonal, I'm short of approx . 5 or 10mm of inward travel.

The answer I suspect is to shorten the OTA by up to 2cm but was wondering if a different diagonal might solve the problem (one that has a shorter light path , if such an item exists) to save cutting the tube ?

The other  option which I think may help is an observers chair which I have the plans for, I have yet to source the materials to cut and build one but believe  the multi postion seating height will make straight thru viewing much more comfortble position wise.

Edited May 29, 2023 by Naughty Neal

[Franklin]

Don't cut the tube, get a normal diagonal. Plenty of quality dielectric mirror diagonals about but if you want a prism and I suspect you do considering the scope you're using, then the Baader 32mm T2 prism is the bees knees. TSOptics do a similar item for a bit less money, I've got both and they're ace. Takahashi do a very nice 1.25" prism as well and the Baader Zeiss jobs are top of the range. Top money as well.

Your diagonal is an erecting diagonal and it's design uses more of the light path and that is why you can't get focus. In a shorter focal length scope it would work ok.

[Naughty Neal]

Thnkyou for replying Franklin, I shall have a look  at your suggestions.   Cutting the OTA is not my preferred method so only mentioned it in passing really.

[mikeDnight]

I'd suggest a Tak prism as they have a short light path and are very good optical quality and with a strong body. Not too badly priced either considering they are Takahashi. I've used Tak prisms for years and love them. The 1.25" Baader/Zeiss BBHS prism is another diagonal I use. It isn't cheap but it is superb!

Edited May 29, 2023 by mikeDnight

[Naughty Neal]

I replaced the original EP holder as it only had a single screw to hold the erecting prism in place, with that in place I can get focus with approx. 6mm back focus spare ( this was on the sun) .  I replaced the existing EP holder with the Baader 36.4mm/t2 adapter & the t2 EP compression clamp  , this has increased the light path using the erecting prism.

 

I will have to see if I can find the difference in the lightpath between the SL erecting prism and the baader t2 di mirror diagonal before I spend another 100+  £££'s.

Edited May 29, 2023 by Naughty Neal

[Naughty Neal]

For now I have acquired the extra in focus needed.  I removed the baader adapter/compression clamp  and refitted the supllied fitting that came with the Scopetech,  with the prism fitted I have about 12mm spare focus travel. 

I don't like the single screw used to hold the prism in place  so have decided to tap two more m4 holes  (when the tap arrives) ,  I can then  clamp the prism with three thumb screws . 

In the end a cheaper and more sensible solution to purchasing another diagonal.

Edited May 31, 2023 by Naughty Neal

[AstroKeith]

14 minutes ago, Naughty Neal said:

For now I have acquired the extra in focus needed.  I removed the baader adapter/compression clamp  and refitted the supllied fitting that came with the Scopetech,  with the prism fitted I have about 12mm spare focus travel. 

I don't like the single screw used to hold the prism in place  so have decided to tap two more m4 holes  (when the tap arrives) ,  I can then  clamp the prism with three thumb screws . 

In the end a cheaper and more sensible solution to purchasing another diagonal.

It may not be obvious, but three screws can be bad for collimation.

In theory, one screw is used to push the barrel of the eyepiece or diagonal, sideways against the female tube. This tends to to force the two components into alignment.

Three screws can leave the barrel floating and you will lose that hard alignment. Its a bit like those finders that have 3 adjustment screws at the back and a rubber ring at the front.

A compromise, often used (eg in my £1000 feathertocuh focuser!) is to use two screws at 90 degrees from each other. This still pushes the barrel hard up against the tube for alignment, jet gives twice the grip.

[Naughty Neal]

Thanks for your observations Keith.

I have checked, looked and unlike the the small sight scopes there isn't the lateral 3 way play to worry about.  The prism's male fitting in  to the female adapter into the focus tube is a snug fit with no slop, so a tri thimb screw set up will work on this one.

 

[bosun21]

What threads are on your focuser draw tube as Baader makes a few low profile eyepiece holders. I would definitely change to a standard mirror/prism diagonal regardless of your focusing problem.

[Louis D]

Anyone know why this scope was designed to barely work with a mirror 1.25" diagonal?  Most of my refractors come to focus with the focus tube halfway out using a 2" diagonal, and even further out with a 1.25" diagonal.  The lone exception is my 23 year old ST80 which doesn't have enough in-travel to accommodate a 1.25" Herschel wedge.  I'm betting the OP's scope wouldn't, either.  With such a gently sloping cone of light at f/12.5, I can't imagine it was done to avoid vignetting.

[Naughty Neal]

Japanese and they prefer the old straight thru sieeing rather then using diagonal's, so simply don't take in to account the in focus. Another aspect is I suspect they didn't think it would  export well esp an foreigners are now using fast ED doublets and APO triplets.

[Naughty Neal]

On 01/06/2023 at 12:04, bosun21 said:

What threads are on your focuser draw tube as Baader makes a few low profile eyepiece holders. I would definitely change to a standard mirror/prism diagonal regardless of your focusing problem.

The thread is the vixen 36.4mm size , I have a baader 36.4 mm/T2 adapter and the baader T 2/  1 -1/4" compression EP holder. 

I think it is the lowest profile one can get max 19mm is added to the focuser tube.

It seems that the mirror diagonals provide a shorter light path and will give more focus travel to play with on the in-focus , but if changing diagonal  will the seeing be any better then the prism diagonal ?

Optically with White light on the Sol today I found little to no difference between the views/seeing  with either diagonal.

Edited June 8, 2023 by Naughty Neal

[bosun21]

21 minutes ago, Naughty Neal said:

but if changing diagonal  will the seeing be any better then the prism diagonal ?

It will be better than your erecting prism diagonal you are presently using.

[Naughty Neal]

I ended up ordering the SVBony sv188p 99% dialectric mirror diagonal, direct from SVBony themselves via the Ali Express store for £43.  It arrived today exactly 7 days after ordering.

It didn't help much with inward travel or shortening the light path , I gained approx. 2mm of backward travel .

I had to revert to the supplied  36.4mm shorter  EP holder which only had the one thumb screw ( I have since bored two more 4mm tapped holes at equi disatnce a part so now have a tri - thumb screw set up)  The Baader threaded 36.4mm/T2 adapter and the Baader threaded  T2/1-1/4" EP adapter proving to taking up just too much focus travel.

With the original EP holder and the SVBony I have aaprox. 9mm - 13mm of inward travel spare on the focuser and approx. 2mm less with the Stellamira erecting prism using 90% of my current EP's.  The only EP I can't get  focus on due to lack of inward travel is the Stellalira 20mm superview it is just on the cusp of nearly focusing , no such issues with the Japan  & China meade 4k's or the two BCO's.  Even Barlowing some of the others isn't an issue.

 

I  think it was  the l ate John in Derby who cut some 20mm or so off his STL80A F15 achromat to acheive focus.

It seems with the STL80A whether F12.5 or F15 version star diagonals /prisms weren't in the thinking of the design /manufacture so travel on the focuser inwards looks like it was always going to be a bit of a struggle with some EP's.

I was pondering on some new Ep's, greater then 50 AFOV but now seeing the issue I have with the SM superview with 68 dges  now wonder if others with greater AFOV will give me the same out of focus issues.

 

Edited June 8, 2023 by Naughty Neal

[The Lapwing]

I would think carefully before cutting it. Is it really the scope for your needs? You might have difficulty selling it again if you change your mind about it.

 I hope it works out for you. I would be obsessing endlessly about it 

[Stu]

3 hours ago, Naughty Neal said:

I ended up ordering the SVBony sv188p 99% dialectric mirror diagonal, direct from SVBony themselves via the Ali Express store for £43.  It arrived today exactly 7 days after ordering.

It didn't help much with inward travel or shortening the light path , I gained approx. 2mm of backward travel .

I had to revert to the supplied  36.4mm shorter  EP holder which only had the one thumb screw ( I have since bored two more 4mm tapped holes at equi disatnce a part so now have a tri - thumb screw set up)  The Baader threaded 36.4mm/T2 adapter and the Baader threaded  T2/1-1/4" EP adapter proving to taking up just too much focus travel.

With the original EP holder and the SVBony I have aaprox. 9mm - 13mm of inward travel spare on the focuser and approx. 2mm less with the Stellamira erecting prism using 90% of my current EP's.  The only EP I can't get  focus on due to lack of inward travel is the Stellalira 20mm superview it is just on the cusp of nearly focusing , no such issues with the Japan  & China meade 4k's or the two BCO's.  Even Barlowing some of the others isn't an issue.

 

I  think it was  the l ate John in Derby who cut some 20mm or so off his STL80A F15 achromat to acheive focus.

It seems with the STL80A whether F12.5 or F15 version star diagonals /prisms weren't in the thinking of the design /manufacture so travel on the focuser inwards looks like it was always going to be a bit of a struggle with some EP's.

I was pondering on some new Ep's, greater then 50 AFOV but now seeing the issue I have with the SM superview with 68 dges  now wonder if others with greater AFOV will give me the same out of focus issues.

 

Check out this post I made when comparing the f15 and f12. I successfully used both the Tak Prism and a T2 Baader prism with room to spare. That was using a low profile Baader fitting to replace the original single screw job. Can’t recall the part number but it will be around somewhere.

EDIT looks like I had to remove the Baader clamp with my BGOs which need more inwards focus.

 

[Naughty Neal]

No I won't be cutting down the OTA not even by 5mm,  I have seeing with it  with 90% of my EP's so not unhappy.

For me it is a keeper esp as it harks back to the original style of the longer F achromats, I get nice seeing with my meade S4k SP's  china 32mm and 26mm Japan EP, the 15mm China one I also have is also very good and gives better seeing then my 12.4mm Japan EP.  

BCO 18 & 10mm are great as well.