Losing focus when adding star diagonal ?

[Newforestgimp]

Hi All,

I have a 9x50 svbony sv106 finder scope with slide an helical focuser which works admirably in straight through mode with a bog standard 25 “super” eyepiece.

my question is when I introduce a William optics 45deg image correcting diagonal into the train I can no longer achieve focus. I assume the WO45 is increasing the focal length beyond the limits, is there a “short FL” diagonal option ? I’m not concerned about image corrected 45 or 90 deg, this is purely for comfort.

Any suggestions ?

thanks in advance...

[John]

If the finder was originally intended as a straight through one, it's tube is probably too long to enable a diagonal of any sort to be used. Diagonals use up a fair amount of light path so the finders that are designed to use them have shorter tubes to accommodate this.

 

 

[johninderby]

Thete are a number of straight finders with helical focusers but none will work with a diagonal. I thought about getting one of these finders but changed my mind when I couldn’t find one that would work with a diagonal.

[Newforestgimp]

1 minute ago, John said:

If the finder was originally intended as a straight through one, it's tube is probably too long to enable a diagonal of any sort to be used. Diagonals use up a fair amount of light path so the finders that are designed to use them have shorter tubes to accommodate this.

 

 

This is what i feared, however, to achieve focus 'straight through' the focuser is almost all the way out hence why Im assuming i need more inward travel than is available to accommodate the WO 45deg ? or am i thinking the wrong way here.

[John]

7 minutes ago, Newforestgimp said:

This is what i feared, however, to achieve focus 'straight through' the focuser is almost all the way out hence why Im assuming i need more inward travel than is available to accommodate the WO 45deg ? or am i thinking the wrong way here.

Inward travel is what is needed but quite a lot of it. Probably more than the helical focuser can manage.

Edited January 12, 2021 by John

[johninderby]

Yes more inward travel is needed. Shortening the tube of the finder is the only real way to compensate for the additional light path length of the diagonal. A machine shop could do it but might cost more than replaceing the finder.

Edited January 12, 2021 by johninderby

[Newforestgimp]

[DaveL59]

4 minutes ago, johninderby said:

Yes more inward travel is needed. Shortening the tube of the finder is the only real way to compensate for the additional light path length of the diagonal. A machine shop could do it but might cost more than replaceing the finder.

That'd be the way to do it tho it wouldn't be reversible. Another is to add a camera angle finder if you can adapt it to fit onto the existing eyepiece. It can then be easily removed if you want to use the finder straight through. 

Right Angle finder mod - DIY Astronomer - Stargazers Lounge

 

[Newforestgimp]

4 minutes ago, DaveL59 said:

That'd be the way to do it tho it wouldn't be reversible. Another is to add a camera angle finder if you can adapt it to fit onto the existing eyepiece. It can then be easily removed if you want to use the finder straight through. 

Right Angle finder mod - DIY Astronomer - Stargazers Lounge

 

Ingenious, but a little too hands on for my Lazy brain. 👏

[johninderby]

Wish more manufacturers would fit finders with a drawtube. My Lumicon Superfinder has a drawtube so can be used straight through or with a diagonal. 👍🏻

Edited January 12, 2021 by johninderby

[Newforestgimp]

I Agree (now) although in my defence with this one having slider and helical i assumed there would be enough to use a diagonal, im wondering if one of the really cheap tiny Diagonals is worth a punt ? or a different eyepiece maybe ? dont want  to give up just yet 😂

[johninderby]

Perhaps replace the helical focuser with a regular short eyepiece adaptor or a shorter helical focuser.?

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4591_TS-Optics-short-T2-Helical-Focuser---Adapter---camera-does-not-twist.html

Edited January 12, 2021 by johninderby

[John]

There are low profile diagonal units around. Celestron use one on their illuminated RACI finder and the Meade ETX scopes had a low profile diagonal that screwed straight onto the back of the scope. An eyepiece with the focal plane well below the "shoulder" would help as well eg: the Tele Vue 40mm plossl.

 

Edited January 12, 2021 by John

[Newforestgimp]

3 hours ago, John said:

There are low profile diagonal units around. Celestron use one on their illuminated RACI finder and the Meade ETX scopes had a low profile diagonal that screwed straight onto the back of the scope. An eyepiece with the focal plane well below the "shoulder" would help as well eg: the Tele Vue 40mm plossl.

 

Thanks I'll see if i can hunt one down 🙂