What is the best way to align two OTAs held by losmandy side by side bars?

[g__day]

Folks just curious as to determining the best way to align a C9.25 and a Williams Optics 110 FLT that are held by Losmandy D plate attachments on a Losmandy side-by-side dovetail bars?

My WO refractor is held by two mounting rings which have effectively no wiggle room or adjustments and they mount direclty onto the Losmandy plate bolted in to precision drilled holes. The Refractor needs to be raised about 0.5mm at the front and shifted about the same amount to the left.

example http://www.skypoint.it/12577-home_default/5-rings-for-130-starfire-type-2.jpg

For the time being I just folded tin foil about 20 times to judge the required thickness I had to use to elevate the front bar. The process I used was loosen the front two bolts holding the WO refractor to the Losmandy bar whilst the mount was elevated at about 35 degrees looking at Canopus. I very carefully raised the losmandy mounting attachment and then slipped the tin foil underneath it then clamped the Allen bolts down tight again. Using two cameras running BYE in video mode on each OTA I checked how centred each star was in both framed cross hairs.

So the elevation is now about spot on - but the WO needs to move about the same distance to the right. I am pondering how best to shift the OTA about 0.5mm to the right?

The mounting shell looks way too snug a fit to simply insert the same thickness of tinfoil and hope this shifts the OTA enough (but I haven't tried this simple experiment yet) - otherwise it looks like I will have to maybe just re-drill the holes in the Losmandy bar say bigger by about 1mm so I can shift the two bolts up to 0.5mm left or right?

Many thanks for any ideas,

Matthew

[rally]

Shims are probably the best option - so what you have done is good so long as its not going to slip out

Bearing in mind that you might be inducing some flexure into the OTAs themselves - if they are pairs of rings.

Is there any option to rotate the OTAs a little ? - you might just find that variations in the optical alignment and the physical tubes etc is enough to get you what you need, or even swapping OTAs from left to right might provide a positive correction - but it might do the opposite too !

Otherwise loosening off the screws and realignining while slightly loose and then retightening may allow you a small amount of wiggle room on both scopes.

Failing that, your final drilling option is possibly all thats left.

How accurately and repeatably are your cameras attaching ?
The cameras themselves may or may not have their chips centred on the optical or physical centres of your systems.

Is there any movement in the focussing systems ?

[g__day]

The Cameras are as locked down as is humanly possible given the level of the gear involved. The WO uses a high quality Moonlite focuser - so its rigidity is better than good. The SCT has a lot of gear hanging off a Meade electronic focuser which I would say isn't of the same quality level as the Moonlite.

But regardless the optical miss alignment of the centres of the two cameras seems rather consistent over the sky - so it too me looks like genuine fractional offset of the axis somewhere.

[doppler]

You should only have to make the holes oval shaped at one end of the bar, drill the holes as little as possible to keep everything tight.

[g__day]

Rick,

That is completely correct. I was only going to drill the holes at the end nearest the front of the OTA if the shim doesn't work.

Knowing the tube is 0.5mm out over a 300 mm distance means an insert of that thickness night just work - I can try it and see first before I go drilling metal!

[jenchris]

Can you neck down the bolt a bit?
Might give it a bit of play in the hole.
Like filing off the threads where they penetrate the plate

[g__day]

Jennifer,

Wow that is a clever idea that hadn't occurred to me!

[jenchris]

I'm a disciple of Edward deBono :)

[Amaranthus]

I used washers on the bolts to change the pointing angle slightly when I did this with an AZ-EQ6 - worked a treat, and ultra-simple.

[PRejto]

The expensive way to do this is with something like the Optec Libra. It carries my TEC140 but I had to modify it to eliminate a little flex. I've found that just rotating my cameras is enough to move the centering enough so that I'm not happy with the alignment of the two FOVs. Consequently, I'm happy to have the Libra because a simple "bolt and forget" solution wouldn't be accurate enough.

http://www.optecinc.com/astronomy/libra_dovetail.htm''

http://www.pbase.com/prejto/image/160559467

Peter

[g__day]

No a bad solution either Peter - looks a bit like a half price version of the beautiful Robin Cassidy mounting plates.

http://www.robincasady.com/Astro/TGAD.html

[g__day]

Hey Barry,

Your post gave me an idea that worked a real treat. I simply moved the shim from running all the way under the front mounting ring, to only having it insert maybe 10mm on the side closest to the SCT. This slightly tilts the front ring up and to the left - which moved the second OTA's optical path much closer to the first OTA's optical centre. After making this adjustment to the position of the shim then I simply separated the two OTAs by pushing them apart gently by placing my left fist between the OTAs and clenching it; whilst under this slight load I then tightened the front mounting ring's bolts. I had to adjust the shim thickness three times, but using this technique the mis-alignment is now only about 1/15th of the original delta. I have gone from around a 3 arc minute misalignment to about 10 arc second.

With a bit more thickening of the shim I reckon I could get it under 5 arc seconds (say one or two star diameters) so it's really much improved!

In BYE when I take an image with the Refractor whilst the SCT is dead centre on a star the has gone from being a bit outside the outer framing circle to now well within the much smaller inner framing circle.

You folks really helped - many thanks all!

[Amaranthus]

I re-read my post and realise it might have been a little vague - I meant the bolts on the dovetail plate, of course.

Anyway, glad you've worked it out Matthew, sounds like a good solution!