Cheap 70mm Amazon Scope as a finder.

[marcopolo]

Has anyone tired buying one of the sub-£50 amazon 70mm refractors and using/mounting that as a finder? They come in 300-400mm F.L flavours ao I was going to plonk in a 40mm generic plossl I have to give an aprox 8-10 x 70. Obviously that makes the exit pupil a bit large but they generally come with a 25mm kelner anyway. There's going ot be considerable Chromatic aberation and i'd imagine a fair bit of pruple fringing but as a finder it could be OK.  Finderscopes in the 70-80mm arent exactly cheap or widely avaialable.

[PeterCPC]

The problem you might have is how you are going to mount it. Ideally finders have to be able to be tweeked in direction so that they correspond with the main scope. So can you find rings that will allow you to do that? If so, why not.

[marcopolo]

Hi Peter, no problem for me . I'm an ATM'er, built many scopes, mechanical engineer, have CAD, 3D printers and mahining access etc...It's just an optical consdieration.. FYI i'll put it on a slider rail so I can also use it to tune OTA balance.

Edited February 7, 2023 by marcopolo
typo

[PeterCPC]

Way to go.🙂

[marcopolo]

One of ther reasons i'm asking if anyone tried it was because the optics could be horrifc!!

[Louis D]

I bought one of the 70x300mm scopes back at the beginning of the pandemic for $25 off of ebay for this purpose.  The scope has a 0.965" focuser, diagonal, and eyepieces.  I will say that the focus tube is very long and narrow which in and of itself probably would cause vignetting of the objective.  However, they stuck an aperture stop partway down the focuser tube to choke down the aperture and improve performance.  On top of that, they put about a 30mm diameter aperture stop directly behind the objective for the same purpose.  So, as shipped, it is not very useful as a 70mm finder scope.  You'd be better off with a GSO 8x50mm RACI.

Undaunted by the mechanical surprises, here's the steps I went through to convert it to a 2" eyepiece capable 70mm finder scope.

1. I got to work and disassembled the whole thing.  It's just held together with screws.
2. The focuser went back in the box.  Maybe I'll figure out something to do with it someday.
3. I pushed out the aperture stop behind the objective (it's pressure fit) which scratched up the tube blackening.  So, I sprayed flat black paint in there and let it dry a few days (the paint really stinks).
4. I mocked up a 2" focuser from 2" PVC plumbing parts, thumb screws, and more flat black paint.  I'll have to work out something better someday.  I'm not into 3D printing yet.
5. I screwed the PVC plumbing parts inside the original tube which is close to 70mm in diameter.  I still need to work on getting the centering better with spacers.
6. I put a cheap Amazon 2" diagonal in the PVC focuser.
7. I put a 40mm Pentax XW in the diagonal and slipped the diagonal forward and back until it came to focus and then tightened the thumbscrews.
8. I attached a Vixen rail to the photo block on the tube so it could be mounted on the other side of my alt-az mount.
9. It's not perfectly aligned, but at 7.5x and 9 degrees, it doesn't matter all that much.  You just make a mental note about how far off-center and in what direction something is when centered in the main scope.

As far as image quality, the 70mm achromat is actually quite good for wide angle views.  They really didn't need to stop it down for wide angle usage.  It is pretty bad at mid to high powers due to extreme false color, but not because of its figure, polish, or coatings.  The achromatic doublet objective lens cell comes with a nice dew shield.  The original main tube is also good quality metal with a nice paint job.  Mine came in a sparkly sky blue that my wife commented was really nice looking.  That's a high compliment.  She doesn't think anything else in my astro collection is attractive looking. 😃

I need to flock the whole thing.  Reflections off of everything in there are pretty bad on bright objects.

It does sound kind of stupid to hang over $400 of eyepiece and diagonal off of a $25 scope, but I generally have them sitting around unused.  I'll have to try the 40mm Lacerta ED in it which I didn't have back in early 2020 when I went through with this project.  I think I may have tried my 24mm APM UFF in it with similar success.  I don't care that the exit pupil is 9mm with a 40mm eyepiece.  I was just going for maximum possible true field.

I haven't been using it much, if at all, since.  It was more of pandemic project for me.  I'll have to revisit it someday.

Edited February 7, 2023 by Louis D

[marcopolo]

Great info Loius, exactly the kind I was looking for.

[Louis D]

I just remembered I had taken ruler test images through a few different eyepieces in the scope once I finished it.  I just now composited them.

Edit: I just remembered that due to the extreme field curvature of a 300mm refractor, I added a TSFLAT2 to the nose of my 2" diagonal for these images.

First, I used a Meade Series 5000 SWA 40mm for the widest field and best possible correction.

Second, I used an OVL Aero ED 35mm for near widest field and decent correction at a much lighter weight.

Third, I used a Pentax XL 5.2mm for near highest possible power just to see how it would perform.

I took images at both full aperture and reduced aperture.  Unfortunately, I don't remember what reduced aperture meant.  I might have used the aperture stop that came with the scope for this purpose.  If so, the 30mm effective aperture increased the scope's f-ratio from f/4.3 to f/10.

As you can see, at widest field of view, it does a pretty good job with a highly corrected eyepiece and slightly less well with a somewhat less well corrected eyepiece.

At high power, it is pretty soft until stopped down, and then it's not too bad.

Overall, I was pleased with the objective's quality for $25.

Edited February 8, 2023 by Louis D

[vlaiv]

8 hours ago, marcopolo said:

Hi Peter, no problem for me . I'm an ATM'er, built many scopes, mechanical engineer, have CAD, 3D printers and mahining access etc...It's just an optical consdieration.. FYI i'll put it on a slider rail so I can also use it to tune OTA balance.

Then it might be better option to just go for 60 or 70mm achromatic doublet in wanted focal length from AliExpress and DIY your finder?

I purchased 80mm F/7.5 doublet that is waiting to be turned into nice 3d printed scope (well, not all parts will be 3d printed - main tube and focuser draw tube will be aluminum tubing).

[marcopolo]

Vlaiv, that’s actually a good idea. The only thing that would annoy me would be the fact the tube would not be a nice shiny metal finish. I’d still like it looking “manufactured” rather than printed or blue petered.

[vlaiv]

16 minutes ago, marcopolo said:

Vlaiv, that’s actually a good idea. The only thing that would annoy me would be the fact the tube would not be a nice shiny metal finish. I’d still like it looking “manufactured” rather than printed or blue petered.

I have access to some fairly cheap aluminum tubing - they cut it to size and charge by kg. Maybe you have too?

I plan on using that. Some primer paint for aluminum and then mat black for inside and shiny black for outside . Few holes so you can attach lens cell and 3d printed focuser.

[marcopolo]

Guys, I succumed to the dreaded "iv'e just no spare time for this syndrome". I got this second hand unwanted present on gumtree for £30 basically unsued, unopened. It appears to have no fieldstops/aperture stops internally. It gives nice crisp images right out to the edge of the field of view which is is surprise (even with the generic kelner 25mm included). So i'm just goign to 3D print some simple mounting rings. Will report back once it's done and tested! Here is the scope: (the 400f.l./70mm version)

 

https://www.amazon.co.uk/Telescope-Astronomical-Magnification-Erecting-Eyepiece/dp/B07FJVWLKM/ref=sr_1_2_sspa?crid=1AW7HZBKKQNXK&keywords=70mm%2Btelescope&qid=1677517490&sprefix=70mm%2Btelescope%2Caps%2C80&sr=8-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1