Matched vs mismatched dual rig

[Marmo720]

I have been reading up on dual rigs and considering the move myself but one thing I haven't seen an answer for is the difference between having equal/almost equal telescope + camera compared to combining different telescope + camera combinations. For example, I am considering this setup: 

1) 1600mm + Altair Wave 80

2) 183mm + Redcat 51

The combined FoV would look like:

In a dual rig, would it simply be the case I would crop the larger FoV of the Redcat combo and should then have double data for the Altair Wave 80 FoV? Any issues with doing this that I am missing? 

The advantage with this setup is then for a single rig, I have 4 different setups I could use from small to large targets while still being able to dual rig most targets.

Is there a big advantage to having matched telescope + camera that I am not aware of? I notice @ollypenrice and @swag72 use matched/almost matched setup. 

Thanks

[tooth_dr]

I’ve used matched ED80s with KAF-8300 in the past.  Alignment of the scopes is therefore much more critical.

Currently using a 250px with an ED80, and scale the fov of the ED80 to match the 250px. There is a loss of resolution, but it works ok using the smaller Wider field of view scope for colour. 

[vlaiv]

34 minutes ago, Marmo720 said:

In a dual rig, would it simply be the case I would crop the larger FoV of the Redcat combo and should then have double data for the Altair Wave 80 FoV? Any issues with doing this that I am missing?

There are a few issues that you should consider:

1. Mismatch in resolution

2. Mismatch in sampling rate

3. Mismatch in SNR

If your software is capable of dealing with all of those and you are prepared to go with "lowest common denominator", then you should be fine with that combo and cropping away.

Let me just quickly explain what those above are.

1. Mismatch in resolution. Let's assume that both scopes are of good optical quality and close to perfect aperture for our purposes. In same guiding and same seeing 80mm scope will have advantage in resolved detail over 51mm scope. Quite a bit of a difference, almost double, because with small apertures, guiding and seeing have very small impact and most of scope resolving is down to airy disk size. 80mm scope will have almost double Airy disk size compared to 51mm.

If you accept that images will have resolution of smaller scope, then you can combine the data (data from small scope will simply be more blurred and that will impact total stack)

2. Although you are close here in sampling rate there will be some difference. ASI1600 + 480mm fl will give you 1.63"/px while ASI183 + 250mm FL will give you 1.98"/px. These are not matched and your stacking software needs to account for that. Also - you'll have to go with lower sampling rate of smaller setup - 1.98"/px

There is interesting point here - why not put FF/FR on Altair 80mm to have closer to 2"/px - that will also match FOV more closely?

3. Mismatch in SNR - well this one is easy, if you match resolutions above with using FF/FR on 80mm scope - you'll have same sampling rate but one scope will have almost the half of aperture of the other 80mm vs 51mm. In same time - less capture photons by smaller aperture and you end up with considerably different SNR per sub.

Regular stacking works because it assumes that all subs have same SNR (there are algorithms that can sort of compensate for different exposure lengths for same setup). PixInsight has per frame weights. Neither of the two is good enough for seriously mismatched SNR. PixInsight approach can seem to be solving the problem - but it's far from it. There is no Single SNR for image - in fact every pixel in the image has different SNR and therefore we can't adjust things with only single constant / weight per sub.

There is a good algorithm for dealing with this - but no software has yet implemented it.

Btw, look at FOV matching with ASI1600 + 80mm F/6 and x0.8 FF/FR. Sampling rate is also better matched:

Moral of the story - if you want to go with dual rig, best choose same rigs as that will give you the least issues to solve.

[carastro]

Mo, that's exactly what I do, go for it.  You will need to use a software that will re-size and register the images to combine.

Also like you suggest, I have different sized scopes and cameras so depending on which camera goes into which scope I can have different sized FOV as single rigs as well.

My combo is for dual rig:   ED80/Atik460EX which almost matches the FOV of the WOZS71/Atik428EX   

For large FOV targets:  WOZS71/Atik460EX

For small FOV targets:  ED80/Atik428EX 

I don't find any issues with different size samples, indeed I often bin some of the data, I just feel it gives a good sort of dither.   Having said that I am non technical and don't analyse everything I just do what works.  I think my results are proof of the pudding. 

Well perhaps if I was more technical my results could be better lol. 

Carole 

Edited June 15, 2020 by carastro

[ollypenrice]

While I agree with Vlaiv's analysis there are some creative practical solutions to consider which nullify some of the problems.

1) You can shoot luminance at the higher resolution and RGB at the lower, at the same time, and the lower resolution in colour will be of no significance whatever. After all some people bin colour 2X2 and let the luminance restore the resolution. This is what we do with our dual TEC in which one camera has larger pixels than the other.

2) You could shoot Ha at higher resolution and OIII at lower since, on the vast majority of targets, there is less fine structure to be found in OIII. Some notable NB imagers like Sara use Ha as luminance anyway so you'd be restoring almost all the resolution as in the case above.

3) There might be some smaller targets (galaxies and PNs come to mind) in which you might care to image the object of interest at the higher resolution and yet not crop the RGB starfield around it.

So, long story short, you certainly don't have to have identical rigs if you think through the business of what you intend to do with each side.

If you use perfectly matched scopes and cameras you will need a very precise alignment between optical tubes or you'll lose the edges where they don't overlap. Having a smaller and a larger FOV makes alignment much more tolerant. The high end alignment devices are very expensive. With different fields of view you might get away without one just by shimming the slave scope into alignment.

Registar, though certainly not free, is a stunning program which makes light work of all resizing/co-registering and cropping tasks so that they become a total non-issue.

Olly

[Marmo720]

Thank you for all the feedback - very informative and helpful. 

@vlaiv Thanks for the breakdown. I had seen snippets of this in different places but good to have it together to think through. Unfortunately, I don't think the Altair Wave 80 has a reducer as I would have got it anyway for wider fov imaging. Unless other reducers work with it? Altair suggest otherwise from their support chat.

At the moment I have the 1600mm and the 183mm. Planned to sell the 1600mm but as the money had already left my account and I was so close to a dual rig, I am now contemplating spending more money to get a dual rig setup 🤣 This hobby!!!

@carastro Thanks Carole - I was inspired by your setup on your website as well as Sara. I am going to use the SW guidescope mount to align both telescopes. 

@ollypenrice Thanks for the guidance. Good to know with regards to the split for RGBOIII on one camera and LHa on the other. I believe Sara also does something similar. The reduced resolution with RGBOIII being not a huge significance is key in my thought process. My plan was:

1) For any target that could fit in the smaller FoV, I would use both telescopes where the higher resolution captures LHa and the lower resolution would compliment with RGBOIIISII. The field around the target would general be stars and so don't need to crop.

2) For any target that was larger than the smaller FoV, I would just use the wider FoV telescope combo and move the filter wheels around as required to capture complete data set. 

Thanks,

Mo

 

[carastro]

Quote

 I am now contemplating spending more money to get a dual rig setup  This hobby!!!

Tell us about it lol

[carastro]

Actually I think we did "tell you about it" when you first started - lol.

Carole 

[Marmo720]

2 minutes ago, carastro said:

Actually I think we did "tell you about it" when you first started - lol.

Carole 

You certainly did. Evidence of me asking if £1k was enough is somewhere in this forum hahaha. Oh those days  

[Louis D]

There was a recent APOD image of Thor's Helmet where it's obvious that images of different resolutions and sizes were successfully combined for an overall pleasing result:

[vlaiv]

1 hour ago, Marmo720 said:

@vlaiv Thanks for the breakdown. I had seen snippets of this in different places but good to have it together to think through. Unfortunately, I don't think the Altair Wave 80 has a reducer as I would have got it anyway for wider fov imaging. Unless other reducers work with it? Altair suggest otherwise from their support chat.

At the moment I have the 1600mm and the 183mm. Planned to sell the 1600mm but as the money had already left my account and I was so close to a dual rig, I am now contemplating spending more money to get a dual rig setup 🤣 This hobby!!!

I think that Altair Wave 80 is the same as TS 80mm APO? I use TS x0.79 reducer / flattener with it with ASI1600 for wide field.

https://www.teleskop-express.de/shop/product_info.php/info/p5965_TS-Optics-REFRACTOR-0-79x-2--ED-Reducer-Corrector-fuer-APO-und-ED.html

On my "upgrade" list is this - and I think it will work also, but you'll have to dial in the spacing yourself:

https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html

 

[Marmo720]

1 hour ago, vlaiv said:

I think that Altair Wave 80 is the same as TS 80mm APO? I use TS x0.79 reducer / flattener with it with ASI1600 for wide field.

https://www.teleskop-express.de/shop/product_info.php/info/p5965_TS-Optics-REFRACTOR-0-79x-2--ED-Reducer-Corrector-fuer-APO-und-ED.html

On my "upgrade" list is this - and I think it will work also, but you'll have to dial in the spacing yourself:

https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html

 

Yes it probably is the same. I have seen some people ask whether the 0.79 reducer would work on the Wave 80 despite the Altair website only mentioning the larger wave telescopes. The website suggests it might work though but with less spacing tolerance:

https://www.altairastro.com/altair-planostar-3-inch-079x-3-element-reducer-for-102-115-130mm-altair-triplet-refractor-285-p.asp

 

[Marmo720]

@carastro Just wanted to check if you had any issues with weight with your setup? I have most parts I need but realise now my total weight once all setup would be 11kg. I haven't gone past half way for my HEQ5 Pro before so wondering if I need to reduce the weight or it would be OK? I normally do 60sec subs and not sure if I need to reduce that or OK to go longer subs if needed in the future?

Thanks

[carastro]

I don't think I have weighed my set up Mo, but I so long as everything is balanced I don't have issues.  

I must admit to trying out my ED120 and ED80 as a dual rig on one occasion on the HEQ5 and even that seemed to work OK as well but I decided in the end I was pushing my luck on that combi and have not used it again. 

Since I don't use a CMOS camera I can't advise on sub length, but with my CCDs I can do 600secs no problem.  

Carole 

[sloz1664]

Unfortunately, I don't think the Altair Wave 80 has a reducer as I would have got it anyway for wider fov imaging. Unless other reducers work with it? Altair suggest otherwise from their support chat.

Is your Wave 80 the triplet scope?

Steve

[Marmo720]

1 hour ago, sloz1664 said:

Unfortunately, I don't think the Altair Wave 80 has a reducer as I would have got it anyway for wider fov imaging. Unless other reducers work with it? Altair suggest otherwise from their support chat.

Is your Wave 80 the triplet scope?

Steve

Yes, the one linked to below but from a couple of years ago. I have the 1x Planostar flattener with it.

https://www.altairastro.com/altair-wave-series-80mm-f6-super-ed-triplet-apo-2019-457-p.asp

 

[Marmo720]

5 hours ago, carastro said:

I don't think I have weighed my set up Mo, but I so long as everything is balanced I don't have issues.  

I must admit to trying out my ED120 and ED80 as a dual rig on one occasion on the HEQ5 and even that seemed to work OK as well but I decided in the end I was pushing my luck on that combi and have not used it again. 

Since I don't use a CMOS camera I can't advise on sub length, but with my CCDs I can do 600secs no problem.  

Carole 

That is great to hear. I am sure balancing will be interesting but I intend to keep things in place once setup so hopefully works out long term. 

[carastro]

I probably looked up the weight limits and weighed the kit when I first started with the dual rig, but I have forgotten the figures now. 

Carole  

[Marmo720]

I did the same but forgot to add in the weight for 2 cmos cameras, 2 EFW and the Skywatcher finderscope saddle is not light! But everything is solid and symmetrical so hopefully easier to balance.

I am still waiting on my 1600mm order to get the dual rig working. No clear nights has helped with the wait  

[sloz1664]

35 minutes ago, Marmo720 said:

Yes, the one linked to below but from a couple of years ago. I have the 1x Planostar flattener with it.

https://www.altairastro.com/altair-wave-series-80mm-f6-super-ed-triplet-apo-2019-457-p.asp

 

I have the same scope & purchased this Reducer and works well, although it is micro sensitive regards spacing to obtain a flat field.

Steve

 

[Marmo720]

1 hour ago, sloz1664 said:

I have the same scope & purchased this Reducer and works well, although it is micro sensitive regards spacing to obtain a flat field.

Steve

 

Thank you! That is good to know that it works. No one on the Altair Facebook group responded so assumed it wouldn't work.

[sloz1664]

2 hours ago, Marmo720 said:

Thank you! That is good to know that it works. No one on the Altair Facebook group responded so assumed it wouldn't work.

Yes I had the same response. I spent a very long time looking for a matched FR for the 80 triplet.

Btw here's my dual scope rig:-

Steve

 

[ollypenrice]

On 15/06/2020 at 16:08, Louis D said:

There was a recent APOD image of Thor's Helmet where it's obvious that images of different resolutions and sizes were successfully combined for an overall pleasing result:

If it's obvious is it successful? I'm not going to knock this image but I will say that, personally, I wouldn't process in this way since there is a huge mismatch in sharpness between the area around the progenitor star and the rest of the field. It looks like two images to me, one wrapped around the other. While it can be a tough decision to sacrifice resolution when blending high res with widefield I think it is sometimes necessary to do so. APOD or not, I'm afraid I don't like this composite image because it looks composite.

Olly

[Anne S]

1 hour ago, sloz1664 said:

Yes I had the same response. I spent a very long time looking for a matched FR for the 80 triplet.

Btw here's my dual scope rig:-

Steve

 

Do you need to shim the upper scope to get the fov matched? I tried running an Altair Wave102 with an Altair Starwave 70EDT a year or so ago and found they were fairly close, particularly as the upper scope had a larger fov. Also, do you have any problems with higher altitude targets and the upper scope? I've the same dome as you, the early Pulsar 2.1.

I gave up with the dual rig at the time as I had an issue with the reducer on the Wave. I want to try again now I've fixed that and upgraded my mount.

[tomato]

It’s a bit off topic but an interesting debate. Folks combine subs of different exposures to bring out star detail in the core of M42 for example, and as far as I can see, this is a perfectly acceptable technique. I guess the exposure combination is less obvious in the final image, and certainly gives a more pleasing result. I do agree that an experienced imager could spot that data of different resolution has been combined in the APOD,  but it is nevertheless a spectacular image.