Small 3" reflector as finder scope on an 8.5" dob

[furrysocks2]

I've got my DIY 8.5" f/7.6 dob and am thinking of putting together a mount for my 3" f/4.6 reflector to use it as a finder scope. It's a fiendishly difficult thing to search for...

Presumably, I'd want to keep angle of the focusers the same for same rotation of FoV, and provide sufficient adjustment for aligning. For balance, intending to mount more or less centrally over the alt axis, main scope is a little front heavy to begin with as I've a slow-mo alt pole supporting the front end. Also have to keep it clear of obstructions in front, such as red dot or main focuser and high enough off the main OTA given the intended FOV?

Anything I need to consider or does the above make enough sense?

[Chris]

It has been done:

(image not mine)

[Daniel-K]

id imagine a scope that size on a small dob would create some balance woes. 

[furrysocks2]

1 minute ago, Chris Lock said:

It has been done:

I had no doubt...

[Chris]

I can't imagine any problems if you mount it over the bearings, or even lower to off set your top heavy scope...the only problem I see is crouching down to use the finder then having to stand up to get to the eyepiece. Could be good exercise though if you bend from your knees

[furrysocks2]

Just now, Daniel-K said:

id imagine a scope that size on a small dob would create some balance woes. 

Yeah... I'm not too concerned at moderate altitudes as it's already front heavy and propped but certainly towards the zenith it might pull back a bit, so mounting on the center tube of the double truss perhaps best slightly forward to compensate. I don't think it would look good right up top.

[furrysocks2]

4 minutes ago, Chris Lock said:

I can't imagine any problems if you mount it over the bearings, or even lower to off set your top heavy scope...the only problem I see is crouching down to use the finder then having to stand up to get to the eyepiece. Could be good exercise though if you bend from your knees

Finder should be about 3'6" or so from the ground, main EP at zenith about 5'6" - so it's not waaay down. Good for the kids, though.

Think I want to keep it all slightly top heavy otherwise my alt slo-mo doesn't lower itself too well.

[furrysocks2]

27 minutes ago, Chris Lock said:

It has been done:

(image not mine)

Also...

http://www.robertbrunck.com/Bob-Web-Pages/Guide-Scope.html / http://www.robertbrunck.com/Bob-Web-Pages/Astrophotos-02.html

[Chris]

49 minutes ago, furrysocks2 said:

Also...

http://www.robertbrunck.com/Bob-Web-Pages/Guide-Scope.html / http://www.robertbrunck.com/Bob-Web-Pages/Astrophotos-02.html

Ok...soooo...that's a finderscope/guide scope as big as any actual telescope I've ever owned!

 

[John]

In the past, UK companies such as Fullerscopes and Astro Systems made small and fast 3" or 3.5" newtonians for this purpose. They seem to have gone out of fashion a bit. I guess features such as RDF / illuminated reticule finders and GOTO have changed the thinking on finders over time 

 

[PeterW]

A good right angle finder or 80mm jumbo finder might be easier to find. I like the finder having the right way around view, should be able to drip you on target is bless you lack stars due to light and pollution like I do. I am interested in small fast reflectors. How fast can they come??

 

peter

[Peter Drew]

I have a 6.25  F3.8 that's my fastest small reflector. 

[PeterW]

There are various 4-6" f4 commercial cheap scopes out there. I am tempted to try grinding a 6" faster, how hard can it be ;-)

 

peter

[furrysocks2]

My wee reflector came from Aldi or Lidl... ignore the "fork" mount.

 

I was pondering how to mount and align it with the main scope, trying to come up with a way to have simple X/Y adjustment rather than your typical rings - for ease of aligning and also just to see what I could come up with.

This got me thinking on perpendicularity and such things. Now, the play in an M6 or M8 drive rod and a coupling nut or such, and the very small range of motion required, is probably going to be sufficient to ignore keeping to the tangent of the arc, but I drew some diagrams and then came out to the workshop.

This was more an exercise in maths and a way to pass some time and while I think the geometry is sound, the engineering is far from it and I'll be the first to state that it's totally unsuitable for anything other than discussion... but I came up with the following thingy from scrap:

What did I just build? Does this kind of thing have a name? I'm intrigued...

[Marci]

That’s exactly how most RDFs adjust the LED position... so the theory’s sound!

[furrysocks2]

14 hours ago, Marci said:

That’s exactly how most RDFs adjust the LED position... so the theory’s sound!

Interesting, thanks for that. Got a great wee book called "1800 Mechanical Movements, Devices and Appliances" in a box somewhere - I'll have a flick through when I unpack.

 

Cracking on with something more likely, I drew up a design:

 

I don't have any dovetails, so I'm using 3/4" x 1/2" aluminium T-track. A ply base screwed to my plywood rings, alt adjustment behind the primary mirror and az adjustment either in front of the scope or underneath the front end, as dimensions/balance dictates. I'll size the height of the az based on a decent compression of the spring (likely half it's 1/2" travel) and a rough alignment. I should get a range of motion in alt of around 2 degrees,

For az, I can't go through the T-track otherwise I can't slip the scope from the track, so I intend to make a 3/4" ply U-shaped cradle, thread some M6 rod with knobs directly through the ply and countersink some divots on the side wall of the T-track - one either side. A couple of springs to keep some tension when adjusting. Should be ok with single divots as range of motion is not going to be that large and there might be a bit of play/give of the M6 rod through the ply. I'll do a rough alignment of ply base before screwing down to try and have the T-track close to centered between the bolts.

Scope will probably be in a ply cradle or some sort, probably zip-tied on, with just enough clearance underneath to loosen the T-track fittings.

 

Quick mock of the alt, allows enough up-down mis-alignment of the T-track while the spring prevents play:

 

Az tension bolts should prevent longitudinal rotation. I don't see the scope being heavy enough to impact anyway. Top edge of the 3" should be no more than 8" off the main scope, including depth of the main scope rings. I'll start with that.

[Astrobits]

Good back-of-the-envelope design.

I used the same principle about 20 years ago for the mounts of my focussers ( made from binocular objectives ). In place of the spring in your design I used a incompletely separated bar of aluminium as can be seen at the far end of the mounting. This prevent any slop in that arm while providing a bit of flexibility for the vertical movement which is controlled by push-pull screws.

Here's a pic of the one on my current telescope. Although my finders were 50mm aperture, I see no reason why they cannot be scaled up for any sized finder/scope.

Nigel

[furrysocks2]

16 minutes ago, Astrobits said:

Good back-of-the-envelope design.

Ha! Yes.  You can see I've thought more about alt than az...

 

Very similar in principle and not too dissimilar in construction really when it comes down to it - very compact, too.

Does the scope swivel in az on a pin or bolt set into the top face of the spring bar?

 

The spring is leftover from my primary mirror mounts. There are actually a couple of smaller washers the same diameter as the ID of the spring and a slim spring to keep the washers separated... that lot helps to keep it centered - it's a 5/8" spring on a 6mm bolt.

Without tightening the push bolts, the 5/8" springs fare pretty well under the weight of an 8.5" primary provided they're reasonably well compressed, hence little concern about it here though time will tell. Easy enough to add a push bolt and require a quick double-check when locking down alignment, I suppose.

[Astrobits]

16 minutes ago, furrysocks2 said:

Very similar in principle and not too dissimilar in construction really when it comes down to it - very compact, too.

Does the scope swivel in az on a pin or bolt set into the top face of the spring bar?

 

Yes, the horizontal movement is via the bar on top of the vertical moving component. ( I am avoiding using Alt and Az as those terms will only apply if the telescope is horizontal. ) The pivot at the back end is a small bolt with a wavy washer to provide smooth movement which is controlled by the push-push bolts ( one each side ) at the front end.  As the whole unit is so compact, I remove it when necessary with the finder. For this the base bar is located on the telescope tube with two bolts held in place with nuts. The bottom of the base is grooved underneath for clearance of those nuts and held down with wing nuts. The finder is held on the bracket with two nuts & bolts installed from inside the finder tube.

Nigel

[furrysocks2]

It's on, just to make cradles to bolt to the T track.

Despite my best efforts, the 5mm pilot holes for the az controls through the plywood bracket are not coaxial, and the divots in the side of the T track needed fettled, but it's solid enough. After tapping the two holes to M6, I smeared some thick superglue around the threads, a wee spray of accelerator and tapped them again. They're stiff, but the knobs are large enough. I've got a full inch of thread, so would have to be ham fisted to strip.