Dual Esprit 150 imaging rig

[tomato]

1 hour ago, Laurin Dave said:

Indeed it is..  What cameras are you using ?  It'll make life somewhat easier if one camera has a larger sensor than the other as alignment will not need to be so precise.  I was also wondering whether you could get a bespoke thicker Losmandy bar made up that would extend the full length of both saddles and also move the scopes closer together.  The intention being that this would benefit both stiffness and scope alignment with the shutter aperture.

Dave

The scopes could be about 35mm closer together, which as you say, would give some multiple benefits. 

Will look at this when the ADM saddle comes off.

[Davey-T]

On the balance subject your bro could knock up a couple these and have one on each scope to maintain balance, weighs about 6 kilo with the saddle, not that weight is much concern to you.

The centre one has Losmandy clamp once adjusted and locked they go nowhere.

Dave

[tomato]

With a few hours of clear sky forecast last night, there was a quick thrash in the workshop to modify the mounting arrangements. The dovetail plate is now bolted directly to the Mesu, and the scopes have been mounted about 2” closer together.

Without the adjustment saddle the reference star was nowhere to be seen in the 2nd scope, so this was hastily refitted, @Tomatobro cradling the scope with camera and cables still attached in his arms while I put the adjustable saddle back on, fortunately, no mishaps.

A crude camera alignment was conducted and some test subs taken of M42, chosen to put the scopes in an orientation where tube flex/sag was more likely to occur.

The 5 x 300 secs images show a number of issues, I’m still getting bleed from the bright stars on the G2-8300 when downloading in SGP (not present in SIPS) and we need to follow Sara’s excellent guide on how to align the cameras properly. 

On close inspection, the star shapes look similar, indicating there was no significant flex, at least on this test. However, next step will be to fabricate some bracing bars across the top of the tube rings, while still retaining the adjustable saddle.

Guided Scope

 

Slave Scope

Edited January 4, 2020 by tomato

[tomato]

Ran another test last night, this time on the other side of the pier, and there is evidence of flexing/sag, with eggy stars on the slave scope. The materials are on order to enable a bracing frame to be made to bolt across the top of the scopes.

 

[Laurin Dave]

Based on my Esprit150 I’d suggest that you also check your focusers for sag/flex, either or both could also cause eggy stars

Dave

[Magnum]

Not had a chance to check my flexure issue since my last alteration, but it seems I only get the issue when imaging in the east. The following images were taken just after meridian flip, and only I tiny bit of elongation on the slave scope. both are 3 x 10 mins.

As you can see my fields of view are totally different so have zoomed out the 460 until it was roughly the same scale. im not really sure what my plan is going forward, at the moment im just using them as 2 different sets of data so I can at least get to grips with my 2 new cameras at the same time, and it gives me twice as much to play with in the British winter. if I removed the reducer from the 383/102 combo then they would be more similar but I think the 383 really needs the extra speed of f5.5 vs f7. Or I could could swap the cams around in which case the scales would almost match, but then im not getting the benefit of the large chip on the 383 LOL.

As you can see my alignment is pretty good with no adjustment devices.

 

Edited January 8, 2020 by Magnum

[ollypenrice]

What always strikes me as odd about dual rig flexure is that the slave scope can show trailing even when the primary scope is carrying a guidescope rather than an OAG. If the primary were guided by an OAG then, sure, we might predict trailing on the slave. But since, in most cases, folks seem to use a separate guidescope mounted on the primary (which is what makes it the primary) you'd expect focuser flexure to cause problems on that scope as well. If the problem always strikes the secondary scope (which seems to be the pattern and is something I occasionally get) then surely the flexure must be arising between the two imaging scopes. In that case the culprit must be in the mounting hardware, so the dual bar, alignment device or tube rings? I know Sara found that the clamshell alternative to rings in her original dual Tak was causing flexure.

Unless... there is some geometric effect we're missing concerning the location of the guidescope relative to the imaging scopes. I know I once tried centering the guide scope on the same FOV as the imaging scopes, something I wouldn't normally bother to do. Unfortunately I can't remember if it made any difference! Doh.

Olly

[tooth_dr]

22 minutes ago, ollypenrice said:

What always strikes me as odd about dual rig flexure is that the slave scope can show trailing even when the primary scope is carrying a guidescope rather than an OAG. If the primary were guided by an OAG then, sure, we might predict trailing on the slave. But since, in most cases, folks seem to use a separate guidescope mounted on the primary (which is what makes it the primary) you'd expect focuser flexure to cause problems on that scope as well. If the problem always strikes the secondary scope (which seems to be the pattern and is something I occasionally get) then surely the flexure must be arising between the two imaging scopes. In that case the culprit must be in the mounting hardware, so the dual bar, alignment device or tube rings? I know Sara found that the clamshell alternative to rings in her original dual Tak was causing flexure.

Unless... there is some geometric effect we're missing concerning the location of the guidescope relative to the imaging scopes. I know I once tried centering the guide scope on the same FOV as the imaging scopes, something I wouldn't normally bother to do. Unfortunately I can't remember if it made any difference! Doh.

Olly

I agree here. I had the same eggy stars in my second ED80 on occasions. It was due to not tightening the scope rings fully in the second scope after I had removed it on one occasion.  It sounds like a mounting issue between the scopes. 

[Laurin Dave]

28 minutes ago, ollypenrice said:

What always strikes me as odd about dual rig flexure is that the slave scope can show trailing even when the primary scope is carrying a guidescope rather than an OAG. If the primary were guided by an OAG then, sure, we might predict trailing on the slave. But since, in most cases, folks seem to use a separate guidescope mounted on the primary (which is what makes it the primary) you'd expect focuser flexure to cause problems on that scope as well. If the problem always strikes the secondary scope (which seems to be the pattern and is something I occasionally get) then surely the flexure must be arising between the two imaging scopes. In that case the culprit must be in the mounting hardware, so the dual bar, alignment device or tube rings? I know Sara found that the clamshell alternative to rings in her original dual Tak was causing flexure.

Unless... there is some geometric effect we're missing concerning the location of the guidescope relative to the imaging scopes. I know I once tried centering the guide scope on the same FOV as the imaging scopes, something I wouldn't normally bother to do. Unfortunately I can't remember if it made any difference! Doh.

Olly

I agree Olly..  guide scope alignment would presumably minimise any field rotation (which would be the same if the scopes were aligned).   I believe from the photos that Tomato is using an OAG so any focuser flex would only show on the slave.  This is what I figured out was happening when I initially went to dual scope (Esprit150 with piggybacked GTF81 then Esprit100) ..  the rather expensive remedy was a Feathertouch to take the 3 plus kg weight of the SX camera/filterwheel/OAG/Guidecamera/flattener on the 150..  the Esprit 100 focuser is fine with the much lighter ASI1600/EFW (the GTF81 focuser was a bit saggy). 

Dave

[ollypenrice]

1 hour ago, Laurin Dave said:

I agree Olly..  guide scope alignment would presumably minimise any field rotation (which would be the same if the scopes were aligned).   I believe from the photos that Tomato is using an OAG so any focuser flex would only show on the slave.  This is what I figured out was happening when I initially went to dual scope (Esprit150 with piggybacked GTF81 then Esprit100) ..  the rather expensive remedy was a Feathertouch to take the 3 plus kg weight of the SX camera/filterwheel/OAG/Guidecamera/flattener on the 150..  the Esprit 100 focuser is fine with the much lighter ASI1600/EFW (the GTF81 focuser was a bit saggy). 

Dave

Aha. Maybe the problem with an OAG in a dual rig might be that it can throw the full or combined flexure error onto the slave scope. A little flexure in the primary's focuser, for example, will be corrected by the OAG at the expense of driving the slave into incorrect positions. If you have a guidescope and the primary's focuser flexes a little there will be no guide input, which will leave the flexure uncorrected. Similarly any secondary focuser flexure will be uncorrected. But with an OAG the combined error will appear in the slave. (This assumes that the error of the doesn't happen to cancel that of the primary but that seems unlikely.)

Olly

[Laurin Dave]

29 minutes ago, ollypenrice said:

Aha. Maybe the problem with an OAG in a dual rig might be that it can throw the full or combined flexure error onto the slave scope. A little flexure in the primary's focuser, for example, will be corrected by the OAG at the expense of driving the slave into incorrect positions. If you have a guidescope and the primary's focuser flexes a little there will be no guide input, which will leave the flexure uncorrected. Similarly any secondary focuser flexure will be uncorrected. But with an OAG the combined error will appear in the slave. (This assumes that the error of the doesn't happen to cancel that of the primary but that seems unlikely.)

Olly

Yes that’s one of the dual scope problems with an OAG..  another is drift whilst the primary does autofocus..  (if of course you do it 😉) ..  although this isn’t much of an issue with the Mesu..  and another I have as I’m using SGPro is the inability to sync the exposures for dither,  in total I lose about 1 in 6 of the secondary scope subs..  (it runs 2 min or 5 min subs the ccd runs 10 or 20 min subs..  I dither every second exposure on the ccd..  )..  I’m also pretty sure that the relatively short exposures on the CMOS camera help mask any flex ..  so maybe Tomato that’s something else to examine ie whether for Galaxy season 10 or 20 minute lum on the primary and 1 or 2 minute RGB on the slave through a cmos camera (I seem to think you have one) would work ..  

Dave

[Jonk]

SGPro 4 is planned to include dual scope dithering, evidently very difficult to achieve but they say they’re working on it.

[Laurin Dave]

4 minutes ago, Jonk said:

SGPro 4 is planned to include dual scope dithering, evidently very difficult to achieve but they say they’re working on it.

So I understand..  although the latest I read (admittedly a few months back) was that it wasn't high on their priority.  As I run very different exposure lengths on the two scopes I'm not sure that for my setup it would make much difference in terms of improving capture efficiency, with more equal length exposures it clearly will.

[tomato]

Yes, the Guided scope has an OAG, and the Slave scope currently has the SW manual focuser. When @Tomatobro made the stepper motor drive for his Esprit 150 he also fettled the focuser and it was considerably more stable than when it was supplied. Building a motorised drive for scope #2 is on his to do list....

[tomato]

SGP is dithering between subs on the Guided scope, the slave scope is just blindly (!) bashing out RGB subs using Artemis Capture.  The Guided scope is taking exclusively Lum subs, soI am not refocusing frequently, the dither doesn’t upset the guide graph as far as I can see. 

There will be two ASI 178s going on there for Galaxy season.

[Laurin Dave]

Can I suggest leaving the Moravian on the guided scope..  it'll be much easier to align...  you'll be able to go deep with long subs... your stars will be lovely with those little spikes on bright stars from the  KAF sensor.....you'll get a much wider field mono image as well..   and at 1" pp you'll be at or below the seeing limit anyway, binning the ASI178 2x2 will give the same resolution..   Plate solve and frame on the 178 equipped scope through a second instance of SGPro

Dave

[tomato]

Thanks for the suggestion, I was impressed with the results obtained from the 178 when using it last Spring. 

It would be great to try all camera combinations but no doubt the UK weather will not co-operate.

[Magnum]

23 hours ago, ollypenrice said:

Aha. Maybe the problem with an OAG in a dual rig might be that it can throw the full or combined flexure error onto the slave scope. A little flexure in the primary's focuser, for example, will be corrected by the OAG at the expense of driving the slave into incorrect positions. If you have a guidescope and the primary's focuser flexes a little there will be no guide input, which will leave the flexure uncorrected. Similarly any secondary focuser flexure will be uncorrected. But with an OAG the combined error will appear in the slave. (This assumes that the error of the doesn't happen to cancel that of the primary but that seems unlikely.)

Olly

Yes I think you are spot on here Olly, im using an OAG on the main scope, and the situation you described was also going through my head. 

[swag72]

I avoid an OAG on my dual rig.... Focusing would be a pain with using one.

[Jkulin]

On 01/01/2020 at 14:49, ollypenrice said:

Of course, the other solution would be two mounts...  Eek.

That was exactly what was going through my mind Olly, I know it means an extra mount and pier, but surely would be far more accurate and less problematic?

[swag72]

27 minutes ago, Jkulin said:

That was exactly what was going through my mind Olly, I know it means an extra mount and pier, but surely would be far more accurate and less problematic?

If I had my time again I would have had separate mounts for each scope....... far easier I think.

[Davey-T]

9 minutes ago, swag72 said:

If I had my time again I would have had separate mounts for each scope....... far easier I think.

For the price of only three Mesus you could have a 10Micron GM2000HPS, no need to worry about guiding and non of that fiddly Sitech stuff 😂

Dave

[swag72]

21 minutes ago, Davey-T said:

For the price of only three Mesus you could have a 10Micron GM2000HPS, no need to worry about guiding and non of that fiddly Sitech stuff 😂

Dave

I dont use my Mesu for the dual role...... a simple EQ8 does that

[tomato]

Two mounts would be nice but I only have room for one dome.....🙄

[tomato]

The top bracing plate has been made with posts to bring it to the same height on the scope without the adjustment saddle. There are threaded adjuster on this so the idea will be to get the cameras aligned, take up the slack on the plate then tighten it down. We will then see how much the FOV has moved. 

A small sliding weight has been incorporated which can be screwed into either end of the non adjusting saddle to balance the scopes without moving them in their tube rings.

Just need a clear night (not the horizontal rain currently coming down in Shropshire) to try it out.

Edited January 13, 2020 by tomato