Dual Imaging Rig

[ian_bird]

Hi

I have a WO FLT 98 with the Riccardi Reduced/Flattener (0.75), and a WO 72 Megrez, also with a flattener.

Camera wise, I have an Atik 4000 and an ASI1600MM Pro.

For filters I have 3nm Ha and OIII, plus an LRGB set, with two filter wheels.

I am going to be using them as a dual imaging rig, one shooting either Ha or Lum, the other shooting OIII and/or RGB.

My current thinking is to use the WO FLT 98 and ASI1600MM for Ha / Lum, and the WO 72 Megrez and Atik 4000 for OIII / RGB.

Any thoughts or suggestions? Would you use a different combination?

 

NB - I am happy to refocus per filter when using RGB.

Cheers

Ian

[AKB]

I guess the acid test is what FOV and angular resolution you get with each possible setup?

Or am I missing some other key parameter?

[ian_bird]

These are the figures:

WO 98 FLT / Atik 4000
FOV: 1.87° x 1.87°
Resolution: 3.29"/pixel
Area: 3.51 sq°

72 Megrez / ASI1600
FOV: 2.35° x 1.77°
Resolution: 1.81"/pixel
Area: 4.16 sq°

Or:

WO FLT / ASI1600
FOV: 2.19° x 1.65°
Resolution: 1.69"/pixel
Area: 3.62 sq°

72 Megrez / Atik 4000
FOV: 2.01° x 2.01°
Resolution: 3.53"/pixel
Area: 4.04 sq°

 

But I was thinking more about Doublet v Triplet for RGB to be honest.

Cheers

Ian

[Davey-T]

A few FOV and arc/secs/pixel calculators around to help decide which is the best combo.

Are you mounting them piggy backed or side by side ?

Dave 

[ian_bird]

These are the FOVs.

WO 98 / ASI - WO 72 / Atik

 

WO98 / Atik - WO72 ASI

 

 

[ian_bird]

Hi Dave

I will be mounting them Piggy Backed. They already are - I was just using the 72 Megrez as a Guide Scope. Then I thought "Hang on....." 

Cheers

Ian

[steppenwolf]

I have used a very similar system to you (telescopes, not cameras) and it worked well.

I used the OSC camera in the triplet. However, if I had two mono cameras, I would want to capture Luminance in the triplet (so may as well load up R, G and B in the same wheel) and Ha/OIII/SII in the doublet.

[ian_bird]

Thanks Steve. Glad to see it has been done before.

Interested in your suggested combinations.

My normal mode is either Ha and OIII, or LRGB. Hence my thinking to spit Ha and OIII, and to spilt L and RGB.

So I can shoot Ha and OIII at the same time (I don't do SII). Or I can shoot L and RGB at the same time.

I'm just wondering if RGB on the Megrez is a good idea. Or would it be better to shoot RGB on the 98 FLT.

I have shot Ha and OIII (3nm) on both OTAs (with the Atik) and both give silky results.

Cheers

Ian

[steppenwolf]

I think that narrowband in the Megrez makes most sense as although it is a lovely telescope (I still have mine), the Megrez does display a small amount of CA so using much reduced bandwidth filters obviates any sign of this. Luminance in the FLT98 makes most sense as this is a superb instrument on all levels so using it for your most important data would make most sense to me. None of your sampling rates are frightening and I have imaged successfully outside these rates.

I see and agree with the splitting up of the L from the RGB and the Ha from the OIII from the point of view of maximising a narrowband or LRGB but I do wonder if the blue channel might suffer with the Megrez?

Perhaps give this a try with the Luminance firmly lodged in the FLT98 and the RGB tested in both. If you do split the bands up, consider using Registar as your alignment engine of choice.

 

 

[ian_bird]

Steve

Thank you for your thoughtful reply - most helpful and encouraging!

Yes - they are both lovely scopes. Like you I have the Starlight Instruments focuser for my WO98. Plus the Ricarrdi Reducer / Flattener - which is a work of art in it's own right!

I have Registar (I am one of Ollie's alumni ).

I will give your suggestion a go.

Thanks again.

Regards

Ian

[swag72]

As good as you plan it, you may find that a doubling up of a filter helps....... so for example I have luminance, Ha and OIII in my second camera, and a complete set of LRGB and NB in the other...... I think I'd also like a colour filter in the second scope, perhaps a red, again that would be a duplicate, but I'm not buying a complete set of RGB filters for just one!!

See how you go and be open to changing things around a little and duplicating if needed  

[ian_bird]

Hi Sara

Not sure I could afford a second 3nm Ha filter either!  Just the one nearly broke the bank!

Curious as to why you would consider doubling up on the Red? Is that based on your experiences (of course it is - why am I asking?). 

My limiting factor is 5 slots in each of my filter wheels.

5 minutes ago, swag72 said:

See how you go and be open to changing things around a little and duplicating if needed

Good advice! Thanks.

Regards

Ian

[swag72]

Actually it could be any colour..... but I've found that I could get data from 2 colours in one and luminance and the other colour in the other..... I'd never actually thought of duplicating a colour filter until I was using the rig.

[ian_bird]

9 minutes ago, swag72 said:

I could get data from 2 colours in one and luminance and the other colour in the other

Ah! Now I understand. Sorry - I was being a bit dim.

Thanks for explaining.

Regards

Ian

[ian_bird]

Hmmm. Interesting Sara.

Steve was worried about Blue in the Megrez.

So I could shoot RG in the Megrez, and LB (Luminance and Blue) in the WO98.

You are right. There are lots of possibilities to play with.

Thanks again.

Regards

Ian

[steppenwolf]

You have lots of options here, Ian you are going to have some fun trying them all out but then, that is part of the game!

[ian_bird]

Steve

Between you and Sara - I have enough food for thought to last me a good long while! 

Thank you both.

Regards

Ian

[ian_bird]

Steve

BTW - Love your latest book! My copy arrived recently.

Regards

Ian

[steppenwolf]

54 minutes ago, ian_bird said:

Steve

BTW - Love your latest book! My copy arrived recently.

That's good to hear, Ian, thanks!

[ollypenrice]

Hi Ian, the thread hasn't discussed aligning the two scopes. Don't expect the alignment to be right without fine tuning. In our early dual rig experiments they never were and attempts to shim them into position proved too exasperating to persue!  At the time the only option was the Cassady T-GAD which promptly went out of production as soon as we needed it. We obtained a second hand one (for an eye waterng 600 euros) but now FLO have commissioned a similar device which I believe Sara uses successfully. The price is positively trivial by comparison...

https://www.firstlightoptics.com/misc/jtd-dual-rig-telescope-alignment-saddle.html

In our dual rig...

... we run Ha LRGB and OIII LRGB respectively but the scopes are identical and well corrected for colour. I agree with the advice already given on how to play to the strengths of the triplet. It makes sense for us to scroll RGB on one side and BGR on the other so that we have a complete dataset if cut off in our prime. Blue is likely to be the weakest link in your doublet but that needn't prevent you from shooting red (and green?)  in it to be sure to get a set.

The dual rig is bliss. It is nearly as fast as a fast astrograph but unlike any fast astrograph I've even encountered it is reliable, untemperamental, consistent and gives good stellar images. Going out for four hours and coming back with eight hours of data still makes me want to find a private corner in which to dance a little jig!

Have fun.

Olly

[ian_bird]

Hi Olly

Thanks for the feedback. I can just see you dancing your jig! 

I'm hoping that I might get away without an adjuster. I will be using a piggyback setup, and the FOVs are not identical, with overlaps vertically and horizontally. But the proof will be in the doing - and then I suspect I will be buying one of FLOs Alignment Saddles as you suggested! 

I could save some time and just buy one now, but it will cost nothing to try without it first.

Do you use two computers to run your dual rig? Or have you managed to run it with just the one?

I can see me saving up for a second 3nm Ha filter at some point. But a second LRGB set would probably be wasted around here (too much LP).

Cheers

Ian

[ollypenrice]

15 minutes ago, ian_bird said:

Hi Olly

Thanks for the feedback. I can just see you dancing your jig! 

I'm hoping that I might get away without an adjuster. I will be using a piggyback setup, and the FOVs are not identical, with overlaps vertically and horizontally. But the proof will be in the doing - and then I suspect I will be buying one of FLOs Alignment Saddles as you suggested! 

I could save some time and just buy one now, but it will cost nothing to try without it first.

Do you use two computers to run your dual rig? Or have you managed to run it with just the one?

I can see me saving up for a second 3nm Ha filter at some point. But a second LRGB set would probably be wasted around here (too much LP).

Cheers

Ian

You know what I'm like with computers, Ian, so I run the tandem setup in two for the sake of simplicity. It's quite nice not to have to flip screens so much to keep an eye on things as well. However, in theory I could do it all in one. This would require a USB hub and my experiences with these don't fill me with confidence.

You're right to try without an adjuster at first. If you were very close you might be able to shim one scope as well.

Olly

[carastro]

I have the same distribution as Sara for the each filter wheel = full 7 set in one FW and Lum, Ha and Oiii in the 2nd FW. 

From a FOV point of view I would choose the camera/scope combi where the FOV are the closest or you'll be chopping lots off one of the frames.  

I have larger FOV camera in smallest FOV scope + Smallest FOV camera in the largest FOV scope and the images from each are pretty close in size.  Then you just need to choose which camera/scope combi to put the relevant filter wheel on.

I find this arrangement works well for me rather than matching scopes and cameras as it allows me to swap around should I need to do a large target, or conversely a small target on a single scope and camera. 

Carole 

 

[steppenwolf]

Olly is quite right about the considerations for alignment but I was lucky with my own version when I used it as I had a large OSC sensor on the longer focal length telescope and a smaller mono sensor on the Megrez so there was a generous field overlap. Shimming can indeed be done (I know from bitter experience) but it is time consuming and only mildly more satisfying than watching water disappear down a plug hole .......

[swag72]

Overlap between the two scopes is the key to avoiding the need for precise alignment. In mine and Olly's case we are running identical rigs on both sides and so we really need to have precise alignment between the two scopes..... With an overlap you may find that you are OK. 

If you do need the alignment saddle, I can thoroughly recommend the one from FLO. It held my second scope precisely for all the time I needed it. It's a great tool.

I should add that I have always used one computer to run the two rigs...... it seems OK. I ran a powered hub for a number of months last year when i was using it without problems and now I have a PC with sufficient USB ports for all that I need.