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Guide / Finderscope focus


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Hi all, 

I've been using a raci finderscope, which I like, and had a plan to upgrade it in a way that I can repurpose the finderscope as a guide scope down the line. 

I bought this guider/finder 

https://www.svbony.com/sv198-mini-50mm-guiding-scope/

But I can't get it to focus with a correcting diagonal - I run out of inward travel. I've tried with a star diagonal  and all's fine there, but the left/right flipsy kinda undermines this spending! I'd rather stick with the (smaller) raci finder. 

Is it the case that to get focus with everything corrected I'd need more in than this guidescope can offer (the outward distance I'm not using is huge so presumably cameras would be just fine), or is there something else that I can try? 

Cheers, 

P

 

Edited by Penumbrella
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I can't see any diagonal in the images of the scope.  Maybe you need to remove the diagonal and connect the camera directly to the focuser tube as per the image shown under "guide scope"

spacer.png

 

There is no requirement to use diagonals when using as a guide scope.  In fact where astronomy is concerned it doesn't matter about correcting an inverted image.

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So you want to use the new scope for visual and the SVBony diagonal comes to focus but is not corrected.

If that's the case then you'd need a diagonal that can fit to the end of the focuser such as this expensive Baader diagonal one I would think.  Though, as it's pricey please note, I can't confirm either way that it would work in your case with this scope. 

Better idea is to contact SVBony and see if they have a suggestion. There may be correcting diagonals they know will work.

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For low power finderscope usage, draw tube slip focusing is usually good enough, so go ahead and remove the focuser.  You adjust it once, lock it down, and never bother with it again.  It's how I use a Celestron 90° Amici prism diagonal and eyepiece with an old Russell Optics 60mm finder scope made from a 60mm binocular objective.

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Posted (edited)

Hi all, 

Thanks for the responses, I have been trying it as you've suggested without the helical focuser - i could get it focus with it exactly as shown here (with that very svbony diagonal in fact). It really is tight though, almost no inward travel left once its focused. 

image.png.cce3f98051aab695ef5a481de4cb40c1.png

The issue is that this diagonal is a star diagonal and neither of the two correcting diagonals I've tried so far (a Williams optics one I borrowed and the celestron one I mentioned above) both seem to need to be further 'in' than possible. 

Made me wonder if it's a general rule that correcting diagonals need to be further in, and so there wouldn't be one out there I could find that'd work how I'd like. Guess I'll keep looking, though @StevieDvd that's a good idea to contact Svbony - I'll do that and see what they say.

 

Cheers all 

Edited by Penumbrella
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23 minutes ago, Penumbrella said:

Hi all, 

Thanks for the responses, I have been trying it as you've suggested without the helical focuser - i could get it focus with it exactly as shown here (with that very svbony diagonal in fact). It really is tight though, almost no inward travel left once its focused. 

image.png.cce3f98051aab695ef5a481de4cb40c1.png

The issue is that this diagonal is a star diagonal and neither of the two correcting diagonals I've tried so far (a Williams optics one I borrowed and the celestron one I mentioned above) both seem to need to be further 'in' than possible. 

Made me wonder if it's a general rule that correcting diagonals need to be further in, and so there wouldn't be one out there I could find that'd work how I'd like. Guess I'll keep looking, though @StevieDvd that's a good idea to contact Svbony - I'll do that and see what they say.

 

Cheers all 

Definitely see what Svbony say

However your difficulty lies in the optical length of the diagonal, a mirror based diagonal is optically shorter than a prism diagonal as it's only reflecting from one surface not many.

It may be possible to add a barlow to the diagonal to get it to work, similar to how some people have to use binoviewers on their scopes.

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1 hour ago, doublevodka said:

However your difficulty lies in the optical length of the diagonal, a mirror based diagonal is optically shorter than a prism diagonal as it's only reflecting from one surface not many.

Thanks @doublevodka, that resolves the fuzz of logic at the back of my mind I couldn't quite bring into focus (geddit?). I'll pop svbony an email, but I think I'll be chalking this one up to experience and returning to the perfectly adequate solution I'd have stuck with if only I hadn't been trapped indoors by the clouds with only Internet shopping to while away the time. 🤣

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Generally, prism diagonals have a shorter optical path length (OPL) than mirror diagonals of the same clear aperture.  IIRC, A 90° Amici prism diagonal should have the same OPL as a plain 90° diagonal.  A 45° prism diagonal may have a longer OPL because they use a Schmidt prism, IIRC.  Which type(s) of correct image diagonals were your using during your testing?

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I'm missing something here... why do you need to correct  (erect) the star image when used as a guide scope ?  - By nature astronomical scopes produce an inverted image, so if the guide scope has a corrected image then any software used to handle the guiding will provide incorrect movements as it will see the guide star moves in the opposite direction to  what the main imaging camera see's through the mains scope.

The SVBony website shows how it needs to be used as a guide scope, with no diagonal fitted... so why are you so fixated on using a diagonal with a guide camera, which would also make the rig harder to balance as your shifting the CofG away form the centre axis of the guidescope.

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7 minutes ago, malc-c said:

I'm missing something here... why do you need to correct  (erect) the star image when used as a guide scope ?  -

You don't,
I was a bit confused also but I think currently the OP wants to still use the Finder/Guidescope as a finder for visual use and the guidescope is just a future plan.
That's how I read it anyway after a 2nd read through, I guess @Penumbrella will explain when on line again 🙂 

Steve

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I believe that @Penumbrella wants this new scope as a RACI finder, his old finder is planned for guiding usage later (explained in his first sentence).  Also it is mentioned the SVBony would be fine for cameras so I concluded it's usage was intended for visual as a RACI.

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Posted (edited)

Hi, 

Yes, sorry if I wasn't clear @malc-c and others. I want to use this svbony guidescope/Finderscope to replace my existing raci for visual use only. I bought it with the intention that it'd be an upgrade for the finder (my current one is smaller) and I'd use it as a guidescope later on. It's advertised as both so I figured this would be a simpler plan! 

I'm able to use it for visual now using a star diagonal, but what I want is a corrected image like the raci. My problem is just finding a correcting diagonal (or any other solution) that will focus so I can use it like my existing finder. I do realise that a guide camera cares not one jot about a flipped image like I do 🙂

Edited by Penumbrella
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While these guidescopes will also work as an optical finder, they are intended to have a camera directly connected, and a diagonal will excessively extend the path length. If you really want a RACI finder that takes 1.25" eyepieces, you may have to buy again.  And note that to make the Svbony work properly as a finder, you will need an eyepiece with crosshairs.

Edited by Cosmic Geoff
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10 hours ago, Penumbrella said:

Hi, 

Yes, sorry if I wasn't clear @malc-c and others. I want to use this svbony guidescope/Finderscope to replace my existing raci for visual use only. I bought it with the intention that it'd be an upgrade for the finder (my current one is smaller) and I'd use it as a guidescope later on. It's advertised as both so I figured this would be a simpler plan! 

I'm able to use it for visual now using a star diagonal, but what I want is a corrected image like the raci. My problem is just finding a correcting diagonal (or any other solution) that will focus so I can use it like my existing finder. I do realise that a guide camera cares not one jot about a flipped image like I do 🙂

Right, I was getting confused as you mentioned the use as a guide scope and I didn't pick up on the visual aspect.  As others have said, I suspect that as this scope was primary designed as a guide scope you may find what you are hoping to do is beyond its purpose and given the money you are throwing at this might be better off looking for an alternative, and one that is primary designed as a spotting scope rather than a guide scope ?

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  • 3 weeks later...
Posted (edited)

Hi all, 

Just to tie this off in case anyone in future tried to plough this particular furrow of daftness. @Louis D was exactly right and I've been able to reach focus with a 2nd hand correcting prism 90° diagonal. I'm particularly pleased with this, as it's also made the 198 into a rather nice spotting scope for the local wildlife - obviously there's been total cloud cover since I got it sorted! 

Cheers all,

 

P

Edited by Penumbrella
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It is definitely better to look through than my previous raci - that was a 6x30, so not that surprising. But I would say its also a better view than a friend's 9x50 finder which I had a look through recently, if that's what you meant. With the 20mm eyepiece I'm using the svbony works out as a 10x50, so at least part of that improvement is going to be that aspect of it, and I'm in no way expert, but I'd say yes it is 'optically better'.

As for it being worth the money... 🤔😬

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It seems like if you're going down this route, why not start with the 60mm version?  I can see a massive improvement in number of stars visible using a 60mm finder over a 50mm finder.  However, the weight of these custom finders relegates them to positions on the scope where they won't throw off balance in my experience.

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