Finderscope issues

Alasdair · #1 21-08-2015, 10:12 AM

I have a newly acquired SkyWatcher 10" collapsible dobsonian. My main problem is that I can never get the finderscope steady; there's always a wobble to it, both at the mount which attaches to the OTA, and in the finderscope housing itself.

I have no problem with the finderscope optics, and I don't see I need to obtain a telrad, or a laser finder; all I need to do is somehow eliminate the wobble so that when the finderscope is aligned it stays aligned.

Are there any tricks or techniques for this? (Blu-tack? Gaffer tape? Cable ties?)

Many thanks!

Astropic · #2 21-08-2015, 11:03 AM

Hi Alasdair,

I think you are missing the rubber "O ring" for your finderscope. http://www.opticsmart.com/media/cata...inderscope.jpg

Allan_L (Allan) · #3 21-08-2015, 04:14 PM

Hi Alasdair,
Great scope you have there.

I agree with Astropic as first place to check might be the rubber o ring

There are no tips or tricks because there is not supposed to be a wobble.

For the OTA connection wobble, first thought is have you tightened the two nuts and bolts securing the mount base to the OTA?

As to the finder scope itself wobble, the two adjusting thumbscrews ans the third non adjustable spring loaded pin are designed to hold it firm.
Check to ensure it is assembled correctly per the manual.

Good luck with it.

Alasdair · #4 21-08-2015, 04:55 PM

Thanks you both very much! As far as I know the rubber o-ring is in place; but the mounting shoe:

http://www.teleskop-express.de/shop/.../sksu630gs.jpg
seems almost impossible to attach firmly to its bracket. There's only the one screw to hold it in place, which seems to be not quite sufficient. Also, as has been pointed out on other forums, the finderscope is mounted from the bottom, so if it's loose enough, it could fall out - this could be disastrous with an collapsible tube. (Maybe a shroud would help here, or even a bit of plastic wrapped around and taped in place.)

Anyway, I'll double check that the o-ring is properly in place, and maybe bung a cable tie about the mounting shoe so that the finderscope can't fall out.

Ahh... we lives, and we learns...

Thanks again!

Allan_L (Allan) · #5 22-08-2015, 07:37 AM

Yes! I always thought it was ridiculous that the finder bracket could only be loaded from the bottom, and there is only a safety to stop it from falling upwards!

but not from falling out if it came loose.
But the thumb screw tightens up securely, so no issues have arisen.
If your thumb screw does not tighten up (hence your reported wobble) then maybe its not the correct size.
It should tighten firmly and still have more thread left over.

There has to be someone with similar setup living close to you, I suggest you try to compare their setup to determine what is causing your issues.

In the meanwhile, I agree some sort of fail-safe system would be good.

NoTan2 (Paul) · #6 07-11-2016, 08:31 AM

Apologies for reviving an old thread but I'm having the same problem.

The finderscope is nice and snug in the mount but the thumbscrew seems unable to hold the mount securely to the OTA. It has fallen off twice despite my tightening the thumbscrew (carefully) with pliers. I didn't over-tighten it - just a tweak. Fortunately no harm was done but I suppose it's not inconceivable that it could have fallen through the trusses and onto the primary.

It's been discussed here and elsewhere and suggestions range from Dremelling a small hole, using some rubber packing or cable ties but noone seems to have come up with the definitive solution.

Edit:
Allan, after reading your last post more carefully, it seems like you are not having this problem with the same scope as mine (apart from the GoTo mount). Perhaps I just need to apply a bit more torque to the thumbscrew? (And a cable-tie just in case.)

Allan_L (Allan) · #7 07-11-2016, 09:19 AM

Quote:

Originally Posted by NoTan2

Apologies for reviving an old thread but I'm having the same problem.

The finderscope is nice and snug in the mount but the thumbscrew seems unable to hold the mount securely to the OTA. It has fallen off twice despite my tightening the thumbscrew (carefully) with pliers. I didn't over-tighten it - just a tweak. Fortunately no harm was done but I suppose it's not inconceivable that it could have fallen through the trusses and onto the primary.

It's been discussed here and elsewhere and suggestions range from Dremelling a small hole, using some rubber packing or cable ties but noone seems to have come up with the definitive solution.

Edit:
Allan, after reading your last post more carefully, it seems like you are not having this problem with the same scope as mine (apart from the GoTo mount). Perhaps I just need to apply a bit more torque to the thumbscrew? (And a cable-tie just in case.)

Yes, I am not sure what the issue is.
I do not tighten it excessively, and only by hand.
I have had several SW collapsible Dobs and have friends with them as well, and I do not recall ever having the finder scope fall out, (even though it was always a concern due to the design, as discussed earlier).
It seems that some have a looser thread than others?
Maybe a bit of plumbers' tape?

NoTan2 (Paul) · #8 07-11-2016, 09:03 PM

Thanks for the advice, Allan.

I've had a good look at the mount and, in spite of tightening the thumbscrew with pliers, there's not a mark on it (from the pointy end of the thumbscrew). If it can't "bite" into the mount then it's not likely to stay tight.

I'll have a look at the end of the thumbscrew and see if it's sharp or blunt. Maybe drilling a tiny hole with my Dremel is a possible solution. I have some number drill bits somewhere.