Thoughts on guidescope mounting vs OAG non-parfocal focus issues?

[pipnina]

The finder shoe on my 8" photon isn't good enough, as I have discovered (though many would have warned me of such), as I have been getting issues that can only be put down to differential flexture.

I have the astro essentials 50mm f4 finder, which has two tube rings and a mini vixen style dovetail. So I have found it hard to attach to many dovetails ontop of a scope before due to the rather short screws (I had to file my shorter skywatcher dovetails down to make it fit my previous scope! Owch!)

At first I figured that I could escape this and save some weight on my scope (and make cable management easier) by switching to an OAG. Sadly this was not the case, as my narrowband filters have some very large offsets from my LRGB filters, so when shooting HA or SII (OIII is actually quite close) my guide cam would be looking at big donut stars!

I could maybe attempt to sit an OAG behind the filters, but I only have 36mm diameter and I think the 20mm spacing from sensor to filter that currently exists may already be pushing it. Not to mention I might be needing 10s+ exposures on the guide cam when imaging on Ha/SII/OIII...

Not able to think of a solution to that, I am just going to put my guide scope in a better spot. The finder shoe is in a bad place for balancing the scope anywho!

I think the simplest option is using a dovetail on the top of my scope, using the second dovetail mounting spot available on my tube rings. This puts the scope in a much nicer spot for balance, although it increases the torque needed on the part of the mount to issue corrections as it adds weight and moves it away from the fulcrum.

I was looking at this because it is flat (good for my finder rings as the screws are short) and of the same size as my existing dovetail, so it should fit the current tube ring spacing.

https://www.firstlightoptics.com/dovetails-saddles-clamps/stellamira-universal-vixen-style-dovetail-bar.html

 

But what if I should use this as an opportunity for an even better solution? I had been eyeing an upgrade to losmandy mounting my scope anyway, but if I use the ADM dual saddle for my HEQ5, and then get this dovetail for my scope: https://www.firstlightoptics.com/dovetails-saddles-clamps/stellamira-universal-losmandy-style-dovetail-bar.html would I then be able to simply attach the guidescope by its rings to the side of the losmandy dovetail with some kind of adhesive? It would then be very close to the fulcrum of the scope, while also rigid. Plus my scope itself will be much more rigid on the mount, as I don't think the vixen style connection is sufficient for this 10KG + setup anyway. (I cannot polar align to any good tolerance, the scope shifts in the saddle between west and east side I think)

My bad artist's depiction of what I think I could try?

 

What do you guys think about this? Until I thought hard about it today I figured an OAG was the be-all-and-end-all of guiding, but now I am not so sure!

[CraigT82]

Are you sure the guidescope is moving and not the primary mirror? That’s the issue I had with my 8” f/6. Solved with an OAG. 

[pipnina]

2 minutes ago, CraigT82 said:

Are you sure the guidescope is moving and not the primary mirror? That’s the issue I had with my 8” f/6. Solved with an OAG. 

I suppose I couldn't rule it out entirely, but I have applied the silicon sealant mod to my mirror and that has helped to keep the collimation accurate so far. Before I did the mod, I would see my laser shift position quite badly when moving from parked position to east or west pointing.

[michael8554]

You need to address the soft-tipped screws holding the guidescope in the rings, a lot of the Diff Flex will be due to those.

I would favour your first solution, gluing sounds so "last resort".

Ditch the short guidescope mounting bar and mount the rings with longer bolts through the new bar.

Michael

[Varavall]

19 hours ago, pipnina said:

But what if I should use this as an opportunity for an even better solution?

Hi

I did similar by going to my local scrapyard and got a piece of 10mm aluminium alloy plate (cost about £1) and drilled and tapped some holes then attached my TS 80mm finder/guider scope to the top of the main scope. Rock solid! You can see it if you look past the cables (now tidied up) in the photo. The Zwo guide camera has gone and been replaced with the lovely QHY5III462C which is almost a mono camera when used with the IR pass filter.

Adrian

[Ratlet]

Could you rotate the scope so the focuser and camer hang down to offset the weight of the guidescope being on the rings up top.  

Alternatively just get 2 vixens and screw them to the losmandy plate to they extend our and then attaché the guide scope to that.

Personally I would go for sticking it on the top.  Things sticking out to the side in my experience quickly become things that got snapped off.

[andymw]

Just seen this and wondered why your filters would have any real affect on the OAG?  My OAG sits just after the coma corrector on my 200PDS so gets clear light from the OTA.  My filters are also not parfocal, however for guiding the delta in the focus would not really be significant.  I'd say, don't give up on the OAG approach just yet.

FYI: My setup is:  8" 200mm F5 Newtonian (Skywatcher 200PDS).  Coma Corrector (which has 55mm backfocus) connected to focusser tube-> ZWO OAG V2 (ASI290MM mini guide camera connected to OAG via a helical focusser) -> 9mm spacer -> ZWO 8 way mini filter wheel (with 1.25 inch filters) -> ASI1600MM Pro (which has 6.5mm focus back space).

Edited September 13, 2022 by andymw

[andymw]

On 03/09/2022 at 16:04, Varavall said:

Hi

I did similar by going to my local scrapyard and got a piece of 10mm aluminium alloy plate (cost about £1) and drilled and tapped some holes then attached my TS 80mm finder/guider scope to the top of the main scope. Rock solid! You can see it if you look past the cables (now tidied up) in the photo. The Zwo guide camera has gone and been replaced with the lovely QHY5III462C which is almost a mono camera when used with the IR pass filter.

Adrian

Just saw this photo and thought I would mention that you can flip those focusser wheels so that your EAF doesn't stick out of the front.  You could also ditch the finder scope and add an OAG thus getting rid of the guide scope.  

Edited September 13, 2022 by andymw

[Varavall]

33 minutes ago, andymw said:

Just saw this photo and thought I would mention that you can flip those focusser wheels so that your EAF doesn't stick out of the front.  You could also ditch the finder scope and add an OAG thus getting rid of the guide scope.  

Thanks for the comments, but the EAF works just fine that way round, but to use an OAG with my guide camera would just put extra stress on the focuser, not to mention all that work with back focus and no doubt more expense. As my setup gives me PHD2 RMS of 0.3-0.9, depending on seeing, I can do 15 minute subs without problems. So as they say, if it ain't broke don't fix it!