Celestron Finder Scope not good enough

[Jane C]

Hi:

I am losing patience with the inability of my Celestron 50mm finder scope to maintain its alignment during a meridional flip.  I know all about the secret nut to tighten the finder once aligned, but this does not fix the problem.  Therefore I want something more dependable.  

I'm thinking about a red dot finder, and would be interested to hear other users' experiences.

My scope is a C11 XLT Schmidt Cassegrain.

Thank you in advance,

Jane.

[johninderby]

Not Celestron but I replaced the stock finder bracket with one of these which would work with the Celestron finder.You would need to replace the finder shoe also.

https://www.amazon.co.uk/Guiding-Telescope-Diameter-Finders-astrophotography/dp/B07RQT5CHV/ref=sr_1_22?dchild=1&keywords=guide+scope+bracket&qid=1630827159&sr=8-22

This finder shoe should fit the C11.
https://www.firstlightoptics.com/finders/baader-universal-quick-release-finder-shoe-base.html

Edited September 5, 2021 by johninderby

[RobertI]

John’s suggestion above should solve the problem I think. 

I guess it’s also possible that you are also experiencing some “mirror flop” which will require some realignment of the finder after a meridian flip?

For your info I use both a red dot finder and a normal finder. The Rigel quickfinder is great when I am  observing DSOs at low power and just need to get to the general area, but when I am trying to find fainter objects at higher power I also need the 10x60 RACI finder (so I have both attached to my scope). I use the Altair Astro RACI which is really lightweight. 
 

 

Edited September 5, 2021 by RobertI

[Cosmic Geoff]

I have a Celestron 50mm finder on a 8" Celestron SCT, and I doubt that this is the problem, as the construction of the bracket is substantial. As others have stated, there is the possibility of mirror flop.  Devices such as an atmosphere dispersion corrector also have a significant effect on the telescope's aim.

[Jane C]

Interesting.  Mirror flop is believable as I also had to refocus.  Perhaps the answer is two finders, one for each side of the meridian?

[Cosmic Geoff]

What exactly are you doing with the telescope? Suggested solutions may vary depending on whether you are using it for visual, planetary imaging or deep sky imaging.

10 minutes ago, Jane C said:

Perhaps the answer is two finders, one for each side of the meridian?

I have never heard of anyone doing that.

[Jane C]

Ultimately, I made a movie last night of Jupiter and Io.  Eventually I propose to plot the position of Io against time.  Local seeing was good, transparency was not so good.  I eventually upped stumps because of cloud.

Thinking about it, I am beginning to doubt the mirror movement hypothesis.  First I focused and checked collimation on Artcurus, with scope east, counterweights west.  I also then realigned the finder.  Then I slewed to Albireo to get an image of the double to calibrate my photos for size in arcmin (the two parts of Albireo are about 35.3 arcmin apart).  Here the scope was west, with counterweights east.  The I went to Jupiter.  The GOTO put the scope to the west and the counterweights to the east.  Throughout all this the finder worked fine.  Then I realised that I was going to hit the limits as Jupiter "moved" west, so I did a meridional flip.  That's when the finder went out.  Not slightly out.  Miles out.   By the time I had found Jupiter in my main scope, and checked the finder scope, Jupiter was right at the edge of the finder's field of view.  So I think it's the finder.

I also do some galaxy photography from time to time.  I don't even try to find those in the finder.  Instead I use the Celestron mount control software CPWI, and use a variant of star hopping.  The software improves its "model" of the sky every time you tell it you have found your target.  So I start with bright nearby stars and star-hop to less bright ones until I have calibrated the software on stars all around the galaxy.  Then I slew to the galaxy.  That usually works.

 

[RobertI]

Things are so much easier on an Alt-Az mount, no flopping, flipping, flapping and the finders and eyepieces are always perfectly placed. But you probably already know this! Good luck with your endeavours. 🙂

[Cosmic Geoff]

1 hour ago, Jane C said:

 By the time I had found Jupiter in my main scope, and checked the finder scope, Jupiter was right at the edge of the finder's field of view.  So I think it's the finder.

Very odd.  I still don't see why the finder should move that much. That amount of play should be obvious if you handle it.  If you attach a second finder (so two finders are attached) even temporarily, you should find out where the problem lies.

 

1 hour ago, Jane C said:

 By the time I had found Jupiter in my main scope,

It wasn't in the field of a low power eyepiece after the meridian flip? Was the finder cross hair on Jupiter after the meridian flip?

[RobertI]

Also, is it a straight through finder or a RACI? RACI’s have more internal parts which could potentially move. 

[Jane C]

It’s a straight-through finder. 

[Jane C]

Mystery solved.  All I had to do was look at my finder.  🙃  The O-ring holding the finder at the objective end has snapped.   It's a 60mm x 50mm x 5 size.   I have ordered a pack of five from Amazon Prime for £3.99.   They arrive on Tuesday.

I'm still a bit tempted to get a Telrad.  I had one on my old 8" Newtonian.  That finder and I got along just fine.

 

[Jane C]

Thank you all for your suggestions.

[Jane C]

My new O-rings have arrived.  When I examined the old one, I found several straight cracks in it, all perpendicular to the circumferential direction.  The straightness of the cracks is a classic sign of fatigue damage, due to cyclic loading.  In principle this could be worsened by ozone damage, though I don't think there'd be a source of confined ozone in my draughty observatory.  (I don't fix the draughts to stop mould forming on the plywood walls.)

I suspect that the cause is daily thermal cycling.  I never imagined that there would be a lot of thermal cycling but evidently there is.

The other place where I am critically reliant on stretched rubber belts is in the transmission from the motors to the RA and Dec drives on my CGX mount.

Perhaps I should also lay in spare rubber drive belts for those.  🤔

[Jane C]

Well, I've fitted the new O-ring.  This afternoon in the sweltering heat I aligned the finder on a mountain-top antenna two miles away.  Just now I fine-tuned the alignment on Arcturus.  Let's hope that's the end of that little saga.