Guidescope within a dome

[mhard26339]

Hi guys, I have a problem and need some assistance.

I run a Celestron 9.26 on an NEQ6 with the Celeston 80mm guide scope package on top, a sizable combination I know, all of this is housed in a Pulsar 2.2M dome. The problem I have is that on occasions with the angles that the equatorial mount gets to, and the size of the SCT/guide-scope combination the guider does not look out of the shutter of the dome due to the offset. I have tried adjusting the measurements that go into the pointing model for the shutter but I either often have a choice of full SCT or guide-scope visibility, not both. OK with Hyperstar I use short exposures but the difference between guided and unguided is clearly visible (better since I did the Rowan belt mod)

What I really want is a mechanism to always keep the guide-scope on top but I have not been able to work anything out yet or find anything commercial. Just throwing it out into the ether to see if anybody has had (solved) the same problem or any ideas. 

Regards

Mike

[Tinker1947]

I would think it will throw up all sorts of guide errors having the guide scope in effect move around the main scope, the easy solution is only image where the guide scope can see the stars, there's plenty to have a go at which ever way you best direction is, a slightly harder solution take the hyper star out and guide with a OAG, you could then go for a Mono CCD, Filter wheel, Auto focuser, you properly know how it all works Mike but you can pop over and have a look at my bits. 

[mhard26339]

Hi Ray, you really do like spending my money!!    But it is so annoying ; I may be forced to put the camera at the other end and buy your OAG but could try a wider angle smaller guide scope say a 60mm

See you soon.

Regards

Mike

[ChrisLX200]

Yes you don't need an 80mm guidescope, something much smaller will do - a 50mm scope mounted close to the dovetail bar so offering the smallest offset.

ChrisH

[Tinker1947]

I have  used a Finder Scope came off my C11 guided the C11 ok, i have the adapter that converts it( remove the eye piece and screw in a Guide CCD) if you want the adapter bring you finder scope to check the thread, 

This is what it looks like...

[kens]

Grab a saw and make the dome shutter wider

[ollypenrice]

Tricky problem. The issue with a smaller guidescope is potential loss of guiding resolution. It's at this point that we have to be wary of the 'round stars' test because it isn't an adequate test. Round stars can appear if guiding errors are random on both axes, as they can be. (I've had this. When we were setting up the guide parameters on the Mesu we always had round stars but we reduced the RMS to a quarter of its original value. Prior to this we were losing a lot of resolution, as was visible in the images.) It's necessary to calculate the average guide error in arcsecs per pixel and compare that with the resolution in the same units at the imager. I can't say that a finder guider won't pass this test with an SCT but I'd want to be sure. I'd always want to guide an SCT with an OAG, myself.

If you look at the geometry of your setup might there not be an alternative single location for the guide scope which might work throughout?

Alternatively, could you have a saddle plate in a second loction allowing a single quick change of the guide scope/cam when necessary? You'd obviously need to choose a new guide star but if the guide scope saddle plates held the guidescope in the same relative rotation as each other there would be no need to re-calibrate. Various people make mini saddle plates. I have them by Altair Astro and Baader. I can use them to allow me to swap a guide scope for a solar scope, for instance.

Olly

[perfrej]

Why not try to get the guide scope closer to the main tube?

/per

[michaelmorris]

Olly has stolen my idea!

I too think the simplest (and probably cheapest) solution is going to be to have a second guidescope mounting point.   Swapping it over should be pretty quick if it's on a dovetail and then all you would need to do is recalibrate the guiding program.

[ChrisLX200]

Use an ONAG?  http://www.innovationsforesight.com/products/on-axis-guider-onag/

ChrisH

[ollypenrice]

Use an ONAG?  http://www.innovationsforesight.com/products/on-axis-guider-onag/

ChrisH

Gosh, new one on me! Rather clever. I can see continuus autofocus catching on as well... 

Olly

[mhard26339]

Tricky problem. The issue with a smaller guidescope is potential loss of guiding resolution. It's at this point that we have to be wary of the 'round stars' test because it isn't an adequate test. Round stars can appear if guiding errors are random on both axes, as they can be. (I've had this. When we were setting up the guide parameters on the Mesu we always had round stars but we reduced the RMS to a quarter of its original value. Prior to this we were losing a lot of resolution, as was visible in the images.) It's necessary to calculate the average guide error in arcsecs per pixel and compare that with the resolution in the same units at the imager. I can't say that a finder guider won't pass this test with an SCT but I'd want to be sure. I'd always want to guide an SCT with an OAG, myself.

If you look at the geometry of your setup might there not be an alternative single location for the guide scope which might work throughout?

Alternatively, could you have a saddle plate in a second loction allowing a single quick change of the guide scope/cam when necessary? You'd obviously need to choose a new guide star but if the guide scope saddle plates held the guidescope in the same relative rotation as each other there would be no need to re-calibrate. Various people make mini saddle plates. I have them by Altair Astro and Baader. I can use them to allow me to swap a guide scope for a solar scope, for instance.

Olly

Hi Oly

 

Thanks for your input and yes ideally I would like to use an OAG  but the set-up currently utilises a Hyperstar and an OSC which obviously negates this, one day I will "turn it around" but at the moment it suits.  To further muddy the water for approx 8 months of the year the dome is operated remotely from Spain so everything has to be set up and tested before I go with no changes mid term.

 

Below is the set-up showing the Celestron 80mm guidescope, a large beast whic has of course caused the problems, a great guide scope but big and heavy, My friend Ray (Tinker1947) loaned me a Skywatcher 50mm scope and bracket to try, that worked OK but the C/L of the scope was still surprisingly high and away from the main scope C/L axis. Further research came up with this. https://www.amazon.co.uk/gp/product/B016AM86RK/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1 , I know that you do not like ring attachments but screwed to the existing Celestron dovetail bar, moved forward and the 50mm guidescope the package is as close to the main scope as I can get.

 

 

First light was the other night and all went very well, the clarity of the guiding image was not as good as the Celestron for the same exposure but an increase in exposure length in PHD soon fixed that with numerous stars to choose for guiding wherever the mount was aiming. Additionally, the helical focusing worked like a dream; the Orion would probably give sharper image but at half the price this works well.

 

The wider angle of the guide scope coupled with the centre-line of both scopes being much closer together appear to have solved the problem without cutting up the dome!!.

 

Thanks for all of your inputs.

 

Regards

 

Mike

[Pompey Monkey]

I just bought the exact same guide scope from ebay: http://www.ebay.co.uk/itm/111840711328?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

(£19 cheaper - sorry!).

It's an excellent bit of kit, giving me, I estimate, a 2-3 degree fov with my QHY5. The optics are much better than my old Meade 9X50, so there are more stars available to guide with.

I agree that the two-stage focuser (a coarse slide and a fine helical) is also really neat, even if I only ever need to use it once!

Edit: Also, I mounted the rings on my own custom "dovetail", a 10 mm X 30 mm aluminium bar. This meant that I could place them slightly closer together and get the dew band completely longitudinally aligned with the objective - a shortcoming in the supplied set-up.

FWIW, I don't see the problem with guide rings on the guider, particularly if it is small and light. I think the big problem with "guide scope rings" is where they are used around the main OTA and consequently have a larger diameter with a smaller inner to outer diameter ratio. They also need to support more weight (that of themselves and the guider/dovetail) and rely on the incompressibility of a large, thin-walled tube...