How to accurately align 2 scopes on an SBS plate?

mldee (Mike) · #1 28-03-2010, 11:44 PM

I have an All-ADM Losmandy style Side By Side mount on my EQ6. It normally holds a C8 on the LHS and a refractor (prev. ED100, now ED80) on the RHS.

I have never been able to get both scopes aligned so they are both centred exactly at the same star. I use a DSI/QHY8 CCD on the C8 and a 25mm 2" EP on the refractor(s) while trying to align them. I centre the star on the CCD then, when looking in the refractor EP, it's always visible but normally well off centre. As the 700/1000mm refractors have a much wider field of view than the C8, I would expect to see the star plumb in the middle of the EP.

I've checked all mechanical alignments and tightened all the saddles within an inch of their life, but to no avail.

I am thinking of using brass shim, but it sure seems a clunky approach, so thought I'd ask and see if any other SBS users have had the same problem and how they fixed it.

citivolus (Ric) · #2 29-03-2010, 02:53 AM

Robin Cassady makes a mounting adapter to compensate for this, although it isn't cheap. Greg Bock uses what I think is a home-made mounting plate that achieves the same result, if you ever get a chance to see his setup.

You can shim the mounting plates on the scopes with any of a number of objects, but of course there is a lot of trial and error involved without something that is designed to be both adjustable and rigid.

I just live with the error myself at this point, but some time would like to get an adjustable base plate.

Barrykgerdes · #3 29-03-2010, 07:00 AM

I made a secondary plate that has micro adjustment of x and y. Aligning on a distant object is easy. Of course I have a well equip workshop to build these ideas.

Barry

Terry B · #4 29-03-2010, 09:49 AM

Just wondering, why does it matter?
From a guiding perspective the scopes don't need to be aligned and you can't look through 2 scopes at once.

mldee (Mike) · #5 29-03-2010, 10:56 AM

Quote:

Originally Posted by Terry B

Just wondering, why does it matter?
From a guiding perspective the scopes don't need to be aligned and you can't look through 2 scopes at once.

Because I'm an obsessive compulsive tinkerer

Seriously, I quite often "look" though both scopes at once, using Laptop and eyes. Just dunno what it achieves, apart from minor annoyance that the two images are not quite the same

Not worth spending a lot of time and money on though, (unless you have a well-equipped workshop - I don't).

Thanks all for the inputs.

mldee (Mike) · #6 29-03-2010, 11:07 AM

Quote:

Originally Posted by citivolus

Robin Cassady makes a mounting adapter to compensate for this, although it isn't cheap. Greg Bock uses what I think is a home-made mounting plate that achieves the same result, if you ever get a chance to see his setup.

You can shim the mounting plates on the scopes with any of a number of objects, but of course there is a lot of trial and error involved without something that is designed to be both adjustable and rigid.

I just live with the error myself at this point, but some time would like to get an adjustable base plate.

The Casady stuff sure looks nice and would do the trick, but umm,,talk about overcapitalising

[1ponders] (Paul) · #7 29-03-2010, 12:17 PM

Apart from shimming or developing some form of X/Y adjustment mechanism, have you considered getting a set of guide rings?

jjjnettie (Jeanette) · #8 29-03-2010, 01:10 PM

Yep, guide rings.... that's what I was considering for my set up.

sheeny (Al) · #9 29-03-2010, 01:25 PM

On my ED80, there are 3 screws on the front and back of the dovetail (vixen style dovetail). The centre one fastens the OTA ring to the dovetail, and the one on each side are jacking screws. I used this arrangement to align my ED80 with my C11, so both scope point to the same place.

then I use an Orion XY guider to select a guide star if I can't guide off one in the FOV.

Al.

mldee (Mike) · #10 29-03-2010, 02:24 PM

Quote:

Originally Posted by jjjnettie

Yep, guide rings.... that's what I was considering for my set up.

HI Jen, This is all to do with preps for mounting your Newt (swap with the C8 as needed), you know

.

BTW, C8 hyperstar was ordered last week. EQ8 Pro this week, so if anyone's looking for an as-new QHY8 soon, give me a PM.

I have a 70x500 apo in guide rings on top of the ED80! (For guiding with DSI II). I didn't want to put guide rings on top of guide rings.

The ED80 is WO and has good rings, so I'd like to stay with them if possible.

Al, thanks for the info. Yes, I'd previously thought about that concept a few months ago, but was waiting on the ED80 arrival to proceed further if needed. It is.

Although I have Losmandy-style plates, I might investigate Al's suggestion of the jacking screws approach. I may have to swap the ED80 ADM plate for a vixen, but that's not a big deal.

All good fun......

[1ponders] (Paul) · #11 29-03-2010, 04:21 PM

Mike, after rereading your thread title I realized I didn't answer your question about how accurately to align your 2 scopes.

As I understand it, if you were absolutely perfectly polar aligned then it shouldn't matter how much the two scopes are misaligned. In practice though it does.

The further you are from exact polar alignment and the further you are from guiding on the same point that is central to your image the greater effect it will have on your image, normally in some form of field rotation. Length of exposure has an effect as well on the outcome.

That's how I had it explained to me. Having said all that I can't say I've noticed many problems, but then my polar alignment process is very accurate (arcsec accuracy) and I stick to fairly short exposures, rarely longer than 10 min. I also use guiderings so its rare that I need to guide at a distance from the point I'm imaging.

Don't know if that's helped or not

mldee (Mike) · #12 29-03-2010, 08:05 PM

Quote:

Originally Posted by [1ponders]

Mike, after rereading your thread title I realized I didn't answer your question about how accurately to align your 2 scopes.

As I understand it, if you were absolutely perfectly polar aligned then it shouldn't matter how much the two scopes are misaligned. In practice though it does.

The further you are from exact polar alignment and the further you are from guiding on the same point that is central to your image the greater effect it will have on your image, normally in some form of field rotation. Length of exposure has an effect as well on the outcome.

That's how I had it explained to me. Having said all that I can't say I've noticed many problems, but then my polar alignment process is very accurate (arcsec accuracy) and I stick to fairly short exposures, rarely longer than 10 min. I also use guiderings so its rare that I need to guide at a distance from the point I'm imaging.

Don't know if that's helped or not

Thanks for the input, Paul.

As I'm looking at an "instantaneous" viewing divergence, I don't think polar alignment will have much to do with it, it's just "two scopes looking in different directions".

Just for everyone's info, (I know you're all enthralled), I went out and took a look at the ADM plate under the ED80, and it has two threaded holes where I can put some jacking screws a la Al's suggestion, so will try that as soon as the clouds run away.

I was just ordering a WO 4 FF so threw in a set of 100mm guide rings as well, so hopefully the whole RHS can get tied down well.

My concept is to have a permanent RHS setup of the WO 80 triplet with DSI II guidescope on top, and the LHS swapping between F5 newt + MPCC and various combinations of C8 FR/Hyperstar.

Somewhere in all of that tinkering, I may even find time to take a happy snap of M42