Now that's a Finderscope!

[Stu]

Just a bit of fun really, had all the various bits lying around and thought I would try this out.

The objective is from an old pair of 20x80 binos off Astroboot. I did repair them, but found the collimation was a bit variable so decided to try this instead.

I had the diagonal and other bits of hardware from previous setups. Ideally I want a 90 degree prism so it is easier to use, but will try this for the moment.

The bit between the binocular and the prism is actually a section of a toothpaste pump dispenser! Perfect for, just needed a bit of a filler inside to take the prism, plus a clip to hold it tight. It bolts to the rings of my 106mm, and can slide along the dovetail to assist with balance on the Giro II.

The ep is rubbish!! Will have to use another, will probably end up putting a 24mm Panoptic in there ;-)

As said, just a bit of fun during cloudy nights but may prove useful, who knows.

Cheers,

Stu

[Naemeth]

Looking good, I think it might give too much magnification for a finderscope, but you can always fit another one beside it (or use a longer FL eyepiece).

[Stu]

Yes, quite possibly, although I think the focal length is around 330mm by a rough calculation.

A 32mm Plossl will give just over x10 mag and around 4.8 degree fov which should be enough. I've got an RDF on the scope anyway to get in the right ball park.

Stu

[AndyH]

Most excellent, Stu !!

 I've been using 80mm finders for years now, along side a 50mm. With the 80mm, you can actually see the target you're going for, rather than the correct area, more often than naught.

With a appropriate adapted 'middle piece', the big finder can fit into std finder rings. On the one below, I got the adapter made so the barrel threads to it for added security and alignment.

With these binos roughly around F4, you'll be able to have a usable magnification. I think I'm getting around 16x, with the illuminated eyepiece I'm using.

A highly recommended DIY job. I wouldn't be without mine now. To my shame, I have another 80mm AND another 2 waiting for adapter pieces.

Andy.

[Stu]

Thanks Andy, yours does look very good, and a lot less cumbersome than mine :-).

Think I'm probably stuck with this arrangement though as the length of 'middle piece' required in mine is much shorter than yours by the looks of it. I am treating this as a zero or very low cost project so don't want to spend cash on it, apart from maybe a 90 degree prism. Cheapest one I've seen is a Seben for abou £13! No idea if it is just trash though?

Cheers,

Stu

[mikehab]

Probably a silly question ... but how do you focus?  Or is it a case that you just get the eye-piece positioned so that it's focused @ infinity and then glue/clamp it in place and it never moves again?

[AndyH]

Hi.

 Yes, the middle adapter is always going to be problematic, when you want to include a diagonal, especially an amci prism one, which uses up more backfocus than a std 90° prism or mirror diagonal.

On my Chinese 80mm bino, I measured that the length of adapter would have to be only about 40mm, for me to reach focus ! Not much.

I decided to get round this and extend the length, by doing this......

Note, that I'd need to have a finder bracket that had rings with an I/D to suit. I do, but it's something to be aware of. I decided on getting the bino and adapter as a thread fit, due to the weight of the bino optic. A tight fit and some form of gluing 'may' just be as good.

My way of doing this, isn't necessarily better, it's just another way. The options are endless, to suit the purpose or wallet. You have the option of using an extension tube, if you want to use it as a straight through type. For 90° viewing, add a diagonal and eyepiece. I've used the cheapest diagonals and a Tal 25mm plossl with Tal screw in reticle. Works brilliantly. You don't need to spend a fortune on a diagonal and illuminated eyepiece. Whatever way, the user will have a lovely 80mm finder for a fraction of an off the shelf one. 80mm finders are very expensive.

Andy.

[AndyH]

Take care with amici prisms, especially at the cheap end, as the internal clear aperture of it, can be very small. This would severely viginete(sp?) the system, so the true field of view would be much less than desired. ie: if the C/A of the diag is say 20mm and you're using an eyepiece with 25mm field stop, you can see, it's not using it's full FOV. You can of course get away then of using a higher power 'piece with a smaller FS.

As for focusing - You can go about it a couple of ways. Set the plastic grub screw(that secures the diagonal in the adapter), so the diagonal can be moved in or out. Or, simply set the thumbscrew on the diagonal to let the eyepiece be moved to suit. If marking of the eyepiece barrel is a problem, replace the steel thumbscrew with an aluminium or plastic one.

Andy.

[Stu]

Probably a silly question ... but how do you focus? Or is it a case that you just get the eye-piece positioned so that it's focused @ infinity and then glue/clamp it in place and it never moves again?

Mike, as Andy says, I have made the optical path length just slightly short of that required, then I just pull the ep away from the prism slightly to achieve focus. It can then be held in place with the locking screw. Seems to work ok. One issue I may find is if other eps focus in a very different place.....might have to buy another toothpaste dispenser and have very good oral hygiene for a week or two to get through it ;-)

Thanks Andy for your input. If this turns out to be useful, I may invest on a 90 degree prism but I really don't want to spend much/any cash on it so may just live with it as it is.

Cheers,

Stu

[AndyH]

Your teeth will be so white, you'll need to purchase red film to cover 'em, should you attend any star parties !

If you do make another, I'd love to see some pics of the process. Your finished piece really looks the part.

Andy.

[Stu]

Just to update this thread with the finished article....

I gave in and bought a 90 degree prism to make this a proper 80mm RACI finder, should be very useful.

I had to extend the middle section to get the focus distance correct for the new diagonal and to use it with the 24mm Panoptic. This should give x13 mag with just over 5 degree fov and a 6mm exit pupil. It does not seem to vignette at all with the new prism or extension piece. I can mount it on the 106mm and it will slide back and forth to help with balance when I change eyepieces. Will also be useful with the Mak, but I do now have another solution for this which I will share separately.

The other benefit is that I can use an Ultrablock filter with it to help find faint nebulae. The max field I can get in the 106mm is 3.68 degrees with a 31mm Nag so the extra field, and RACI, will still be useful for finding 'stuff', and it saves changing ep if working at higher mag.

A few pictures attached, perched a little precariously on the mini giro as I couldn't be bothered to get it's big brother out :-).

Cheers,

Stu

[Stu]

Just given this a quick try out on the mini giro just on its own and I'm really pleased with the results.

Old habits die hard and I'm so used to the view through a refractor/diagonal that I was pushing it the wrong way initially, but the widefield view, correct image is great. The star shapes aren't brilliant, but it certainly does the trick in terms of showing objects and fov.

Sulafat and Sheliak took up under half the fov and the view matched a 5.1 degree circle on Sky Safari (attached) with Sheliak split clearly. DSO's were easily visible, M13, M31, M11 and NGC 457 quite clear so finding new objects will be far easier with this setup. The whole of Kemble's Cascade fitted in easily as shown.

Looking forward to giving it a proper try out now :-).

Might have to find a way of adding cross hairs, maybe just with a reticle ep, possibly illuminated.

Cheers,

Stu

[faulksy]

that's brilliant, must try this. Michael Wilkinson has done similar and loves his, his is on a c8