Help/advice please aligning a dual/twin-scope setup

[Adreneline]

Hi Everyone.

I've recently treated myself to a WO71 to sit alongside my existing ED80DS on an NEQ6 Pro mount. I've purchased an ATM twin parallel vixen mount bar on which to mount the two scopes. Everything has gone to plan and I have the setup nicely balanced. The question I have is should I be able to achieve a perfect alignment of the two scopes, i.e. if I centre an object in the ED80 should I expect it to be centred in the WO71? If I should then I failed at the first hurdle! The alignment is pretty good (checked on a distant pylon as no stars are currently visible in this part of the northern hemisphere!) but it is not perfect. I feel like I need a vernier adjustment on one of the vixen saddle mounts. If I get it close then every time I tighten the screws on the saddles it shifts an imperceptible amount and the objects diverge again. This is the best I've achieved.

WO71+428 - ED80+414 - ED80 overlaid on WO71

This is the mount setup:

If it should be possible to get perfect alignment then any advice on how to go about it would be much appreciated. I've spent several hours on it so far and if perfect alignment is possible then there is clearly something wrong with my approach - or I've given up too easily

Thank you.

[swag72]

A 1mm difference at the saddle will make a big and noticeable difference in alignment when you are looking at the sky. To that end, the way I have found to ensure perfect alignment is the JTD saddle from FLO. https://www.firstlightoptics.com/misc/jtd-dual-rig-telescope-alignment-saddle.html It really is VERY good

[Peter Drew]

It should be possible to adjust them to a satisfactory alignment, the procedure is pretty much the same as aligning a finder. Success, as Sara mentions, relies a great deal on the mechanical quality of the saddle and its adjustment facility. It might be better to attempt the alignment on Polaris using a crosshair in both telescopes, this would remove any error due to parallax.   

[Waldemar]

What you are noticing might be tordation of the relatively thin Vixen dovetail.

Mounting your scopes closer together may make a difference. If this is indeed due to flexure or tordation of the dovetail, be prepared for another kind of misalignment when your scopes point in another direction...  probably a Losmandy dovetail would do a better job.

[Adreneline]

Thank you everyone for your advice and comments.

I guess I naively thought that spending close to £200 on a purpose built dual mounting bar would guarantee a very high degree of precision, especially when precision is the name of the game. At the first opportunity I will check out the alignment using a star as a target object but I anticipate a less than perfect result. It would be good to get a perfect alignment but an acceptable result (for now) would be getting the whole of the FoV of the ED80/414 combination within the FoV of the WO71/428 combination - it's nearly there but not quite. Tordation is a term I've not come across but I presume has something to do with the mechanics/properties of the dovetail design.

I think I may need to get creative with some shims - I must have some dating back to the days of setting contact breaker and spark plug gaps!

Many thanks again for taking the time to help me.

[Waldemar]

8 hours ago, Adreneline said:

Tordation is a term I've not come across but I presume has something to do with the mechanics/properties of the dovetail design.

Tordation is the effect of torsion in the length of the (in this case) dovetail.
So in fact it is uncontrolled bending of the material in the radial direction. There will be lateral flexion  as well.
the only way to deal with this is to use a stronger bar.

[RayD]

I think very good, relatively rigid, alignment of 2 OTA's is actually harder than it seems to achieve.  This is mostly for the reason noted by Sara, which is the tiniest of movements here, means HUGE movements there.

Personally in your case, particularly with the long extension on your WO etc, I would be inclined to start with basics and replace the vixen with a decent losmandy plate (your saddle has the facility).  From there you have a good base to add adjusters if necessary (certainly the easiest way to get good alignment), rough align on your pylons and then fine tune on Polaris as noted above also.

[carastro]

I tried initially with my side by side mounting bar but found the tightening bolts turned each scope fractionally in the opposite direction, so that didn't work for me.  Sara's looks to tighten both in the same direction. 

My magic solution was to use the Skywatcher guidescope mount which adjusts in both altitude and azimuth and can get my scopes aligned exactly.  I remove the top scope from the Guidescope mount without touching the alt/az and it's back in exactly the same position next time I set up (I have to set up and down each time).

Also I found the side by side set up caused pier/tripod clash much sooner than the piggy back arrangement.

HTH

[Adreneline]

Hi Everyone,

Thank you very much to everyone for their comments, advice and explanations.

I did wonder whether I would be better taking the piggyback option. I am reluctant to spend a significant amount more on the JTD saddle but Carole's suggestion of a Skywatcher guide scope mount might be a option I could/should consider even if I stick with the side-by-side arrangement. I quite like my existing guidescope mounting arrangement especially as I have changed the standard SW focussing unit to a Baader Diamond Track. I might try moving the WO inboard a little more though to try to reduce some of the flexure.

There's always something else to think about and another challenge to be overcome!

Looking at the photo from @carastro prompts another unrelated question. Is it better to place extension tubes between the filter holder and the camera or between the focus drawtube and the filter holder? I notice you have the 314 attached directly to the filter wheel whereas I have extnsion tubes between the 428 and the filter wheel. Or doesn't it matter?

Many thanks again to everyone for taking the time to respond - it's much appreciated.

Adrian

[carastro]

Glad my post gave you some food for thought.  

Quote

Looking at the photo from @carastro prompts another unrelated question. Is it better to place extension tubes between the filter holder and the camera or between the focus drawtube and the filter holder? I notice you have the 314 attached directly to the filter wheel whereas I have extnsion tubes between the 428 and the filter wheel. Or doesn't it matter?

It was Atik themselves who told me to attach the EFW directly to the camera, and indeed the thread is exactly right for this purpose.  I don't have any extension tubes, only the focal reducer and any spacers needed on the opposite side of the EFW.  

Carole 

[Waldemar]

It is best to have the filters as close as possible to the sensor, to prevent vignetting.
Sometimes, when they are too close, halo's around bright stars may occur. The difference could only be 1 or 2 mm.
if using unframed filters, blackening of the edges with schoolboard black may help, too. See: http://www.baader-planetarium.de/sektion/gemeinsame_dateien/filterproblems-E.pdf

Like you say:  ap is a never ending endeavour, new possibilities or problems constantly arise. The involved techniques progress really fast.
For me that is the most interesting part, I love the challenge of solving problems and getting things to work perfect.

[AngryDonkey]

I use the Avalon X-Guider to align my two scopes which works very well for me.

https://www.unitronitalia.com/prodotti/AV-XGuider-tech-sheet-eng.pdf

It is rated to 6kg so it really depends on how heavy your OTA plus imaging train is. I did consider the JTD saddle that Sara mentioned above but needed something smaller and lighter (the X-Guider is only 0.5kg). It's not cheap though and there is no mechanism to lock the azimuth adjustment. This is not as bad as it sounds as the spring that keeps it in place is very strong and doesn't move at all - even during a flip. I just have to remember to leave the adjustment knob alone...

[Adreneline]

Thank you again Carole and Waldemar for the feedback and advice. Looks like I need to make a few changes!

Again, not sure I've made the right decision, but I've opted to use the 414osc on the ED80 with reducer/flattener and variable spacer to get the correct spacing of 55mm. I am using the 428 on the WO-ZS71 at present without reducer - hence the need for extension tubes to achieve focus without withdrawing the focuser to full extent. I will get a reducer/flattener for the ZS71 in the fullness of time (which equates to wallet recovery time) so I can use my Canon 70D as well for a slightly wider FoV. I'm trying to keep my options open and give myself some flexibility with how I can use the two scopes.

Challenges are just opportunities - the trouble is the frustration that arises from the lack of opportunity to conquer the challenges!

Hold on! The BBC Weather app says clear skies tonight. What's all that about?

Thank you again for all your help.

Adrian

 

[Adreneline]

15 minutes ago, AngryDonkey said:

I use the Avalon X-Guider to align my two scopes which works very well for me.

https://www.unitronitalia.com/prodotti/AV-XGuider-tech-sheet-eng.pdf

It is rated to 6kg so it really depends on how heavy your OTA plus imaging train is.

Thanks Mike. The ZS71 plus camera/filter/flattener comes in at more than 6kg. The SW unit suggested by Carole looks interesting and is half the price of the JTD.

All food for thought though.

Thanks again.

Adrian

 

[ollypenrice]

The alignment device of choice is the Cassady T-Gad but the JTD one to which Sara links is similar and works for her, so should be good. The Cassady is out of production, very exensive on the used market and very heavy. It can hold large scopes, though, larger than you need. The smaller and cheaper ones than the JTD may or may not be stiff enough for imaging scopes. I'd be sure to find someone who has tried them with the scopes you intend to use. Buying twice is always expensive!

Olly

[Davey-T]

Trying to set up a triple imaging rig I've made my own adjusters only for Star 71s so not heavy although the CCD cameras weigh a fair bit.

Made them as light as possible while still being rigid enough, hopefully I can adjust the centre scope orthoganal with the mount then align the other 2 scopes with it, only done 2 so far.

Cost about £40.00 each in materials.

Dave

 

[Adreneline]

Thanks for the advice Olly and Dave. I've been giving some thought to a means of making one mount adjustable and have been thinking along similar lines.

I tried the dual setup last night and obtained better results than I anticipated. Guiding was very good indeed so I feel I got the balancing pretty much spot on.  I imaged M81 and M82 for four hours with 300s exposures on the 414 and 600s on the 428 with the Ha filter. The alignment wasn't perfect but there was 100% overlap of the 414 on the 428 and over the four hours there is no perceptible change so I'm assuming there was no/little relative movement between the two scopes.

Only had a quick look at the resulting images because the priority today was the thirteen month old granddaughter. Who cares about celestial stars when you have a real one down on earth - who is every bit as difficult to images because she's constantly on the go! Joy personified ?

I'll post some images once I've had time to process them - of M81 and 82, not the granddaughter ☺️

Many thanks again for the help and advice.

Adrian

 

 

[Adreneline]

Hi Everyone,

SO here is a somewhat rough and ready combination of the images from the ED80/414 and the WO71/428 combination. Processing is minimal (ABE+HT) because with a full harvest moon in the sky there was little point in wasting time trying to get a good image of M81/M82 - this was all about checking the alignment of the two scopes.

So this is 42x300s osc + 21x600s Ha:

There is clearly a rotational problem which is easily fixed and a small but significant lateral misalignment, but at least there is overlap - it certainly didn't prove a problem for Registar or PI to align the images. I can now give some thought as to whether to go for a homebrew solution of buy off-the-shelf.

Onward and upward.

Thanks to all for your comments and interest.

Adrian

[RichLD]

4 hours ago, Adreneline said:

Hi Everyone,

SO here is a somewhat rough and ready combination of the images from the ED80/414 and the WO71/428 combination. Processing is minimal (ABE+HT) because with a full harvest moon in the sky there was little point in wasting time trying to get a good image of M81/M82 - this was all about checking the alignment of the two scopes.

So this is 42x300s osc + 21x600s Ha:

There is clearly a rotational problem which is easily fixed and a small but significant lateral misalignment, but at least there is overlap - it certainly didn't prove a problem for Registar or PI to align the images. I can now give some thought as to whether to go for a homebrew solution of buy off-the-shelf.

Onward and upward.

Thanks to all for your comments and interest.

Adrian

That looks pretty good to me - I reckon if you got the cameras aligned/rotated to RA and DEC you'll be nearly there 

[Adreneline]

On 05/10/2017 at 10:07, carastro said:

My magic solution was to use the Skywatcher guidescope mount which adjusts in both altitude and azimuth and can get my scopes aligned exactly.

Hi Carole.

I followed your advice and purchased the SW mount; it seems very good and more than up to the task of holding my ZS71+428ex. I just need some clear skies now so I can align it properly.

Thank you again.

Adrian

[carastro]

Pleased to hear it was some help, I find it invaluable and it’s not hugely expensive.    Once you have adjusted the scopes once you don’t really need to adjust them again provided you don’t move the  alt/az on the SW guidescope adapter,  then the only job then is to get the cameras orientated the same.  I try to put the cameras on so the chip is horizontal  so there is a fighting chance.  Not so easy when the camera is cylindrical.  Lol

Carole