Using a finderscope as a guider...and how to mount it?

[Dezerker]

Has anyone had any success in adapting a 50mm finderscope for use as a guide scope?  I have a stock 50mm finder that came with a recently acquired Skywatcher ED80 and I'm thinking of using it as a guide scope.

A couple of follow-up questions on this...

I've seen the various adaptors that enable the finder to take a guide cam - presumably these are sturdy enough to do the job and will hold a guide cam securely?

And how would you go about mounting it?  I've read a couple of reports that say the stock finder-scope holder (that has the two screws and one spring-loaded pin) isn't good for the job, though mine seems to be pretty secure.  I'm not sure how I'd balance the scope though - using the stock finder's horseshoe mount would make it heavy at the back I think.

Any tips based on your experiences would be greatly appreciated!

[Davey-T]

Depends, nothing off the shelf I don't think, if you're into DIY and not worried about it looking flash you can knock something up with a bit of aluminium bar and a couple of Munsen rings designed for pipe clamps, available in stainless steel, fixed to the top of existing rings

Dave

[Budgie1]

I started off using the 9x50 finder as a guide camera and wasn't that bad. It was on it's standard mount with an adaptor so I could screw a ZWO ASI 120MM Mini camera on to it.

You can get the adaptor HERE and the camera only weighs 60g so there's no issues with balancing the scope. You may see some signs of flex in the guider mount, but I didn't notice any.

[Dezerker]

36 minutes ago, Davey-T said:

Depends, nothing off the shelf I don't think, if you're into DIY and not worried about it looking flash you can knock something up with a bit of aluminium bar and a couple of Munsen rings designed for pipe clamps, available in stainless steel, fixed to the top of existing rings

Dave

Thanks Dave - definitely not worried about how it looks!  DIY is not my strength sadly, but I had wondered if it was possible to cobble something together and lash it to the main scope somehow, so I'll look into this - thank you!

[Dezerker]

7 minutes ago, Budgie1 said:

I started off using the 9x50 finder as a guide camera and wasn't that bad. It was on it's standard mount with an adaptor so I could screw a ZWO ASI 120MM Mini camera on to it.

You can get the adaptor HERE and the camera only weighs 60g so there's no issues with balancing the scope. You may see some signs of flex in the guider mount, but I didn't notice any.

Thanks Budgie - that's really helpful to know.  I wonder if there's a way to eliminate the possible areas of flex?  I could wedge something into the mount that holds the finder / guider, so that it doesn't have any wiggle room, perhaps.  I'll have a play around!

Thanks again!

[TerryMcK]

Yes. I use one on my WOZS73. It is a Skywatcher through finder 9x50 that came with one of my old scopes. I attach a ZWO ASI120MM to it with this from FLO https://www.firstlightoptics.com/adapters/astro-essentials-sky-watcher-9x50-finder-to-c-adapter.html

The only thing I had to do is remove the locking ring from the focuser to enable it to focus better. I use plumbers PTFE tape wrapped around the threads to lock the focuser. Once set you do not need to refocus.

[TerryMcK]

I threw the standard SW finderscope mount away and use the WO guider rings to mount the guide scope on mine BTW.

[Dezerker]

2 minutes ago, TerryMcK said:

Yes. I use one on my WOZS73. It is a Skywatcher through finder 9x50 that came with one of my old scopes. I attach a ZWO ASI120MM to it with this from FLO https://www.firstlightoptics.com/adapters/astro-essentials-sky-watcher-9x50-finder-to-c-adapter.html

The only thing I had to do is remove the locking ring from the focuser to enable it to focus better. I use plumbers PTFE tape wrapped around the threads to lock the focuser. Once set you do not need to refocus.

Thanks for that Terry.  Does your ASI 120mm screw directly onto that adaptor? 

[TerryMcK]

1 minute ago, Dezerker said:

Thanks for that Terry.  Does your ASI 120mm screw directly onto that adaptor? 

Yes. The ASI120MM comes with everything you need and the C3 adapter is in the box.

[rickwayne]

Differential flexure is pretty easy to  diagnose: If you're getting great guiding but the stars are still bloated or egg-shaped (and you know it's not the optics), you've got it.

Since the pieces from the back of the finder to the camera will stay the same, I'd say go ahead and try the finder mount if you already have the bits in hand. Chances are adding guiding to your routine will improve matters regardless; if you feel the need to push further, you could buy a pair of rings (e.g. these Stellarvue ones), assuming your main scope's rings have the necessary threaded mount holes on top.

[Dezerker]

On 29/11/2020 at 17:42, rickwayne said:

Differential flexure is pretty easy to  diagnose: If you're getting great guiding but the stars are still bloated or egg-shaped (and you know it's not the optics), you've got it.

Since the pieces from the back of the finder to the camera will stay the same, I'd say go ahead and try the finder mount if you already have the bits in hand. Chances are adding guiding to your routine will improve matters regardless; if you feel the need to push further, you could buy a pair of rings (e.g. these Stellarvue ones), assuming your main scope's rings have the necessary threaded mount holes on top.

Thanks Rickwayne, I think you're right, it's a case of giving it a go and seeing how it works out!  Those rings look good and sturdy so it's good to know there's a 'plan b' if the finderscope holder proves to flex too much - thanks for the suggestion!

[rickwayne]

In my experience the biggest problem with finderscope holders is that they privilege adjustability over rigidity, so they frequently have something like an O-ring as one point of contact, with 3 screws as the other. So the whole shebang only as rigid as the rubber. Heck, since a guidescope needn't be perfectly aligned with the imaging scope anyway, you could try sticking in some rigid shims as a first fix if you do find flexure.

[theropod]

40 minutes ago, rickwayne said:

In my experience the biggest problem with finderscope holders is that they privilege adjustability over rigidity, so they frequently have something like an O-ring as one point of contact, with 3 screws as the other. So the whole shebang only as rigid as the rubber. Heck, since a guidescope needn't be perfectly aligned with the imaging scope anyway, you could try sticking in some rigid shims as a first fix if you do find flexure.

I drilled and tapped the front ring on my C-90’s finder to eliminate that stupid O ring, and it was super easy. More importantly, to me, the finder now centers exactly where the OTA is looking/pointing, and does not wiggle at all. I did this to make my 2/3 star alignment easier. Even with a double crosshair eyepiece it sure helps to be able to get close using the finder. As issued by Celestron the finder on the C-90 is an absolute joke, and the penny pinchers in corporate surely don’t give a ****. One would think producing the best product possible would take precedent over saving $.50. I seriously doubt that installing those 3 little screws would add even that measily $.50 to the overall cost of production, and I would have paid several dollars more to have a finder that actually works. With modern machining technology, and tooling, adding those 3 little screws would be a snap. It only took me 10 minutes to do the job by hand, and a CNC could make that happen in far far less time.

[rickwayne]

<slaps forehead> Well of COURSE! I would imagine that for guiding, the finder shoe is still less rigid than rings mounted along the scope, but I bet that nails the biggest source of flexure right there. Good one!

[gaz81]

I have my 9x50 finder mounted on the finder shoe but with rings. 
 

The were SVBONY rings off eBay. 

[rnobleeddy]

I also use the SVBONY rings and can't recommend them enough, especially given they're less than half the price of other brands.

I attached mine to the top of my scope with a spare dovetail and use a SW 9x50 anbd ZWO camera with the commercial adapter. Seems rigid enough. Obviously have no idea how to apportion the guiding error I see, but I'm pretty sure it's not the finderscope flexing!

 

[fozzybear]

Have you thought of one of these, by the time you have bought the adapter for the old finder then some sort of tube rings to mount it to have a gander at this from Svbony you can mount it on your existing finder bracket or attach it directly to a dovetail ontop of the mount . Just a thought

https://www.amazon.co.uk/Svbony-Telescopes-Finderscope-Multilayer-compatible/dp/B08927JRBJ/ref=sr_1_1?dchild=1&keywords=sv165&qid=1606939223&sr=8-1

[Dezerker]

On 02/12/2020 at 19:37, gaz81 said:

I have my 9x50 finder mounted on the finder shoe but with rings. 
 

The were SVBONY rings off eBay. 

 

Ah fab, thanks for that Gaz, that's helpful to see.  How do you find it?

[Dezerker]

On 02/12/2020 at 19:45, rnobleeddy said:

I also use the SVBONY rings and can't recommend them enough, especially given they're less than half the price of other brands.

I attached mine to the top of my scope with a spare dovetail and use a SW 9x50 anbd ZWO camera with the commercial adapter. Seems rigid enough. Obviously have no idea how to apportion the guiding error I see, but I'm pretty sure it's not the finderscope flexing!

 

Thanks rnobleeddy, that's really helpful to know - I've been testing out the 50mm finder as a guider in the little mount it comes in and it seems reasonably sturdy, but the guiding accuracy isn't great, so I'm pondering something like this as a way of being sure to eliminate possible flexure. 

Of course at the moment I can't decide whether the difficulties I'm having are related to how I'm setting the kit up, my polar alignment, the wild lack of understanding I have about PHD, or possible flexure from the finder guider, but I guess that's all part of the learning curve!!

[Dezerker]

On 02/12/2020 at 20:03, fozzybear said:

Have you thought of one of these, by the time you have bought the adapter for the old finder then some sort of tube rings to mount it to have a gander at this from Svbony you can mount it on your existing finder bracket or attach it directly to a dovetail ontop of the mount . Just a thought

https://www.amazon.co.uk/Svbony-Telescopes-Finderscope-Multilayer-compatible/dp/B08927JRBJ/ref=sr_1_1?dchild=1&keywords=sv165&qid=1606939223&sr=8-1

The Svnony kit is tempting, I must admit, and at that price as you say, it's barely more than what I'd pay to rig up my finderscope set up.  I only went down that route because I have a 50mm finderscope sat in a box not doing anything.  Something like that would certainly see time saved, I think.  I've attempted to employ the 50mm finder that I had and have been doing some tests on it, but it's helpful to know what plan 'b' is if that doesn't work out - thank you! 

[Dezerker]

On 01/12/2020 at 17:46, rickwayne said:

In my experience the biggest problem with finderscope holders is that they privilege adjustability over rigidity, so they frequently have something like an O-ring as one point of contact, with 3 screws as the other. So the whole shebang only as rigid as the rubber. Heck, since a guidescope needn't be perfectly aligned with the imaging scope anyway, you could try sticking in some rigid shims as a first fix if you do find flexure.

 Hey Rickwayne, yes that makes sense, and as you and Theropod suggest, I'm definitely tinkering with ways of tightening everything up - thanks again!

[gaz81]

2 hours ago, Dezerker said:

Ah fab, thanks for that Gaz, that's helpful to see.  How do you find it?

Here you go

https://www.ebay.co.uk/itm/SV116-Guiding-Scope-Ring-Kits-or-Telescope-Tube-Diameter-or-Finders-43mm-to-70mm-/283932069061?_trksid=p2349624.m46890.l49292

[Quetzalcoatl72]

On 29/11/2020 at 15:45, TerryMcK said:

The only thing I had to do is remove the locking ring from the focuser to enable it to focus better. I use plumbers PTFE tape wrapped around the threads to lock the focuser. Once set you do not need to refocus.

Been looking for a solution to keep that locking ring, the ring I use to attach my asi120 is different from the one provided at FLO would there be a better focus if the camera was moved further or is that the opposite?, mine is very thin and the thread isn't great, takes many attempt to screw the guider since I have to keep swapping it with both my scopes back and forth.

[rnobleeddy]

+1 for the SVBony rings (e.g. https://www.amazon.co.uk/Guiding-Telescope-Diameter-Finders-astrophotography/dp/B07RQT5CHV)

I wouldn't worry about flex. My experience is guiding isn't particularly impacted by the quality of the guide scope, and even works with less than perfect focus.

I setup two 50mm guidescopes on my 2 OTAs and I haven't had to touch them for over a  year.