Does anyone have experience with dome slaving?

[w0mbat]

Hi all,

I am having a frustrating time with slaving my dome. I have a Sirius 2.3 metre observatory which has Maxdome 11 dome control. The pier is centred in the observatory. I have repeatedly carefully measured all the distances required in Ascom Dome Control but cannot get accurate slit position in all directions.

One difficulty is the North South pivot point offset. The manual says that a north offset is positive and south negative. My belief is that this should be reversed for the southern hemisphere as the mount is facing south. I asked Diffraction Ltd this and they answered that it stays the same. But if I use N+ and S- it just doesn't work properly. If I make the pivot point offset positive for a southerly offset (which is what I have due to having the DDM60 mounted on a central pier) then it is much closer but still not good enough. By increasing the pivot point southerly offset figure to around twice the actual measured offset I then get good slit position everywhere EXCEPT close to the zenith.

I have been checking the aiming by shining a laser into the DSLR viewfinder on my N10 astrograph which gives a nice image of the main mirror on the inside of the dome slit cover. As far as I can see this should give me an accurate indication of the telescope field of view.

I realise this is probably an issue that it will be difficult to help with. But I have run out of ideas and have wasted heaps of time on this and was hoping someone might give me a new idea.

I would particularly like comments on whether you agree with my assessment of the correct sign for North South pivot point offset in the Southern hemisphere.

Thank you,

Regards

Ian

[TimC]

Hi Ian,

 

Yes, this is a frustrating and time consuming pursuit at times. I have both a 2.3m Sirius dome and a 3.5m Sirius dome in the Southern Hemisphere. In the 2.3m dome I have the dome scope slaving done by TheSkyX Dome add on controlling MaxDome2 as the mount in this dome can only really be run by TheSkyX as it's driver. I therefore put the  dome geometry and scope offsets in TheSkyX. In the 3.5m dome, I am running the DDM85 premium, and have had good success putting the dome geometry and offsets in to POTH so that POTH handles the issue. I am now using TheSkyX in this dome however to control the mount driver and use TheSkyX's dome add on to control the dome and that is giving me adequate scope slit slaving alignment.

 

Whatever software you use, as you know, it is important to set the dome's home position  azimuth accurately into the MaxDome driver before doing anything else. I then check that the dome slit azimuth lines up well with the scope azimuth when the scope is asked to point due east and due west. I have found negative numbers in any of the software for the mount RA/DEC intersection to the optic tube centre do not work, but in all the software I have used, a positive number woks for me. I point the scope to the north and south and make the tube flip and unflip experimenting with the offset until I get the best results. The offset has been close to, but not exactly the measured distance from the RA/DEC mount intersection and the middle of the optic tube. I had to experiment for a while to get the correct offset. Of course it is important to enter the correct dome radius of 1.75m and the correct lat. and long. I think POTH was the easiest to setup. I have never just relied on MaxDome2 by itself to do this.

 

Cheers,

Tim

Edited September 3, 2014 by TimC

[w0mbat]

Hi Tim,

The offset I am referring to is the North South offset of the pivot point within the dome. As my pier is central there is an offset introduced by the mount itself.....around 85mm. I have found just one reference to this in another forum from a southern hemisphere person who stated that the sign of this offset needed to be reversed for the southern hemisphere, that is, a south offset becomes positive rather than negative as it is in the northern hemisphere. This seems to make sense to me and my slaving is certainly better when I enter a positive number.

I am a bit confused by your reference to "never just relied on Maxdome2 by itself to do this". I use Ascom Dome Control which I thought was part of the Ascom package although written by Diffraction Ltd. My planetarium software is C2A. Slew commands from C2A are routed through Ascom Dome Control to Autoslew. I do not use POTH. Everything works fine except that the slit positioning is not always accurate. I also have MaximDL and the slaving parameters required in it are exactly the same as in Ascom Dome Control. MaximDL, Ascom Dome Conrol and Maxdome are all from Diffraction Ltd. Are you implying that there is an issue with their dome control?

Thanks for your input.

Ian

[TimC]

Hi Ian,

 

Sorry, we are talking about he same thing. I was talking about the ASCOM Dome Control in my last sentence -  that utilises the MaxDome2 driver. You will, of course, need to input all the offsets...offsets of the mount pivot point from the absolute centre of the dome,  and also the optical offset, assuming you are using a GEM??

 

Sorry to confuse the issue.

 

Cheers,

Tim

[w0mbat]

Members (especially those of us in the Southern Hemisphere) may be interested to know that I wrote to Diffraction Limited about my dome slaving concerns. I was amazed when they answered that they are not certain of how to set dome slaving parameters in my circumstances! There only suggestion was essentially to fiddle with settings until I find what works best. This seems to tie in with your experience Tim.

Ian