DSLR and finderscope on EQ3-2: how to mount the finderscope?

[KevinPSJ]

For a while now I've been using my Nikon D3200 at prime focus of my skywatcher 150P. I have a skywatcher EQ3-2 mount with RA drive but have struggled to get beyond 30s without trailing stars  despite very careful polar alignment. Recently I switched to imaging directly with the DSLR riding piggyback on the skywatcher 150P and using the 18-55mm kit lens instead of the main OTA. That allowed me to get wide field images with 2 min subs and pretty sharp stars. I think I can get more out of the EQ3-2 mount if remove the reflector and fit the DSLR directly to the dovetail bar so the payload weight is reduced.

To this end, I borrowed a Nikon 55-200mm telephoto zoom and using a ball head I've attached it as shown below. I tried that out last night but locating and framing targets was a nightmare! The problem is I can't figure out how to attach the finderscope to the dovetail bar. I have a standard 6x30 skywatcher finderscope - like this one - and I've already worked out that the mounting point for the finderscope that's screwed to the main tube doesn't attach to the same large mounting plate I'm attaching my DSLR to.

To be honest I'm struggling a little with the terminology which doesn't help with the googling  Is the black plate my camera is attached to in the pic below called a dovetail bar? Or a vixen plate? Or something else? 

From what I can work out I think I need something like this but I'm not certain and am trying to keep this as cheap as possible since it's all experimental at this stage. I don't mind spending under £10 quid on something speculative but over £30 pounds and I'd really like to be sure it's the right thing  

All help appreciated!

Kevin

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Skywatcher 150P on EQ3-2. RA drive.

Celestron 25x70 binoculars

Nikon D3200 with kit lens 

[KevinPSJ]

Put simply - how do I attach this finderscope to the plate? Ideally fixed to the plate so I can mount DSLR and finderscope side by side.

 

 

Edited April 14, 2021 by KevinPSJ

[malc-c]

File off the block off the finder mount, then drill and tap a hole into the base of the finder mount.  Place the mount at 90 degrees to the dovetail bar, feed a bolt through the hole in the end and bolt down the mount.  If you don't have the toold I'm sure a local small machine shop would do that for a drink 😉

[ollypenrice]

As Malc says above, that's what I'd do (and have done.)

Your black bar is indeed a dovetail, in this case a Vixen one which I'd call a Vixen rail. The part of the mount which holds it in place is called the saddle plate. There is another common dovetail standard called the Losmandy, which is wider and better for large setups.

I assume you don't have 'Go To' on this mount? If you do you can just point at a bright star, take a snap, centre the bright star by trial end error and then synch the mount on that.

Olly

Edited April 15, 2021 by ollypenrice
Typo

[BCN_Sean]

The other route to do it, would be to buy a "finderscope hot shoe adapter", Geopik make one and there are clones on e-bay, fasten the finder in to that and then slot it in to the flash shoe on the top of the camera.  Personally, though I'd go with malc-c's method, as whilst an adaptor doesn't need mods when it's all mounted up it soon becomes a bulky contraption that could rip the flash shoe off with a whack in the wrong direction.

[PeterCPC]

If you don't want to do the filing down, you could mount a finder shoe to the rail and then mount the finder bracket into that.

One of these: https://www.firstlightoptics.com/finders/finder-mounting-shoe.html

[KevinPSJ]

Thanks everyone for the helpful suggestions. Since this is an experiment to see if I can get longer exposures on my mount using a borrowed lens I think I won't modify my finderscope - I'll be putting it back to it's intended use soon (bolted to the body of my 150P). In the meantime I will try the hot-shoe piggyback approach for now and just be extra careful not to snag the finderscope ... 

Longer term (based on my results so far) I will probably invest in a star tracker for my DSLR - at that point I will still want a finderscope side by side with the camera. I may come back to to the modification route at that point. 

Just using polarscope alignment I can currently easily get 1 min subs with DSLR and the telephoto lens at 200mm on my EQ3-2 unguided. I'm going to try some drift alignment next to see if I can improve that.

[happy-kat]

I use a flash shoe mounted red dot finder on the DSLR, I got the mount for the RDF on astroboot but I've seen them cheaply advertised elsewhere.