Since years I encounter problems to sync my dome to my scope using the MaxDome II INDI driver indi_maxdomeii .

My equipment :
- Scope Celestron EdgeHD 1100 mounted on a CGE Pro mount
- Dome Sirius Observatories 2.3 m.

I measured carefully the dome parameters as required by KStars (version 3.4.2).
- Home + Park azimuth = 67.00°
- Ticks per turn = 178
- Autosync threshold (deg.) 0.5°
- Radius (m) 1.150 m
- Shutter width (m) 0.600 m
- N displacement (m) 0.000 m
- E displacement (m) -0.050 m
- Up displacement (m) -0.220 m (the intersection RA-DEC axis is lower than the dome center)
- OTA offset (m) 0.410 m

I used the Celestron hand control to set precisely 5 scope orientations :
1- North position Azm = 90° Alt = 90° The scope is parked in this orientation

2- Zenit position Azm = 0° Alt = 135* The scope points to zenith on the East side (the weight bar is horizontal)
3- Zenit position Azm = 180° Alt = 45* The scope points to zenith on the West side (the weight bar is horizontal)

4- South position Azm = 0° Alt = 180* The scope points to South on the East side (the weight bar is horizontal)
5- South position Azm = 180° Alt = 0* The scope points to South on the West side (the weight bar is horizontal)



Results for the 5 scope orientations using the standard dome parmeters above (i.e. OTA offset = 0.410m) :
1- North position Azm = 90° Alt = 90° Scope in dome aperture but not perfectly in the middle (dome absolute position = 346.10° while it should be about 360°)

2- Zenit position Azm = 0° Alt = 135* Scope perfectly synced
3- Zenit position Azm = 180° Alt = 45* Scope only halfway in dome aperture

4- South position Azm = 0° Alt = 180* Scope only halfway in dome aperture
5- South position Azm = 180° Alt = 0* Scope completely aside


Results for the 5 scope orientations using the standard dome parmeters above (except for OTA offset = 0.050 m) :
1- North position Azm = 90° Alt = 90° Scope perfectly synced in the middle (dome absolute position = 358.24°)

2- Zenit position Azm = 0° Alt = 135* Scope perfectly synced
3- Zenit position Azm = 180° Alt = 45* Scope OK but dome aperture should be best rotated by 180°

4- South position Azm = 0° Alt = 180* Scope perfectly synced
5- South position Azm = 180° Alt = 0* Scope completely aside


CONCLUSION :
Using the carefully measured dome parameters, the sync dome-scope is not successful
After changing the OTA offset = 0.050 m (which is a NON-SENSE !) the sync dome-scope is perfect for 4 out of 5 scope orientations

I am surely making something wrong but that's years that I try to find what !
I would really appreciate if somebody could indicate a direction where to look.
I attach the 2 logs related to the 5 scope orientations using (OTA Offset=0.410) or (OTA Offset=0.050)
Thank you

Sorry I forgot to attach one file

I used quite approximate value for slaving for a couple of years and they worked ok for the most part but had issues near zenith. When upgrading my mount, I decided to do a bit more systematic calibration of the values. I couldn't accurately measure RA/DE axis intersection distance from dome center so I entered rough estimates as the parameters and followed somewhat systematic trial-and-error method. My dome controller has encoder so the step count and thus dome azimuth control is accurate and mount pointing model was fairly accurate too.

First I pointed the OTA at north-ish horizon on both pier sides and adjusted OTA offset and W/E displacement until the dome opening was centered on both sides. Then I pointed the OTA at east horizon and adjusted N/S displacement until the OTA was again centered in the opening. At this point only the up/down displacement was left and I adjusted that by pointing the OTA at around 45 degree angle south-east and again adjusted until OTA was centered. After this I haven't had issues pointing anywhere in the sky, including zenith.

Noticed your controller has quite low azimuth resolution, mine has 3240 steps per revolution so it can point at almost 0.1 degree accuracy, yours can only with 2 degrees though it shouldn't matter if the position is accurate (no backlash etc).

My equipment is in the signature, dome syncing parameters are:
Dome radius 1.0m
Shutter width 0.6m
N displacement 0.04m
E displacement 0.0m
Up displacement 0.2m
OTA offset 0.33m
Auto sync threshold 3 degrees

Hi,

I programmed the dome sync routine quite a time ago, so may be there are some hidden bug already there.
Calculating dome position is a bit tricky, and the smaller is the dome, the more sensible are to the input parameters.

I wrote a Spreadsheet to debug the calculations. You can make a copy and play with it to see how parameters affects the outcome:
docs.google.com/spreadsheets/d/1FWN0S7kG...HIo/edit?usp=sharing

You can enter the values in blue cells, and output are in the orange cells.
You enter your telescope parameters, and your latitude. Also enter target declination and target hour angle (angular distance to the meridian, in hours).

I already put your parameters in the blue cells, when your telescope is parked (Dec = 90, HA = 6 hours), the counter weight is in the lowest position, and the tube points to north.
In that case, SpreadSheet calculates a target Azimuth for the Dome of 356,8. If you set the East disp. to 0, the the target Azimuth is 360 as expected. Any change in the OTA parameter, didn't change target Azimuth.
May be your telescope is just a bit of north(?)



The side of the pier is determined bay the sign of the OTA parameter (+ is west, - is east)

I posted a reply to jpaana but it didn't get through :

I followed very carefully your procedure without success.
I thought that maybe a careful polar alignment of the scope is a requisite to a successful slaving.
My polar alignments have always been sloppy and I'm waiting clear skies to do a better one.
This idea is reinforced by the remark of fcasarra : "May be your telescope is just a bit of north(?)"

fcasarra said "sign of the OTA parameter (+ is west, - is east)"
As I understand it the OTA offset is defined as the distance between the intersection of the RA and DEC axis and the scope optical center : it should not have a sign + or -
I am wrong ?

Well, sort of.
The OTA offset and the pier side are connected and if the pier side is incorrect this can be compensated for by changing the sign of the OTA offset. One way this could happen is in the Southern hemisphere where the dome is upside down.

Here's a process that might help:

1. Make sure that the dome azimuth movement and position are correct and repeatable. If the azimuth is incorrect or not repeatable you will never get a good sync. Good means to withing a couple of degrees.
2. Align the mount. For a celestron GEM that's polar aligned a quick align should be good enough but once again you can't get a good sync without a good alignment. Within a degree should do.
3. Set the dome radius to a sensible value, Do not change it again.
4. Set the mount to hour angle 6h, declination 90. This is the Align position, looking at the pole with the counterweight shaft down. In this position the OTA offset will have no effect on the dome azimuth, not will the NS position of the mount.
5. Adjust the mount EW offset until the scope is looking through the centre of the dome slit.
6. Move the mount so the counterweight shaft is horizontal and the OTA is looking at the Southern horizon. In this position the main things that affect where the dome needs to be are the EW mount offset and the OTA offset. We have already set the EW offse so set the OTA offset. Try this with the mount on both side of the meridian and iterate as required to get the scope able to see out in both cases.
7. set the scope so it is looking at the East or west horizon, this should be with the dec counterweight shaft pointing down. Adjust the Ns offset so the OTA can see out. Do this both looking East and West.

That should get the EW offset, the OTA offset and NS offset set and each one is set independently of the others.
I'm not sure how to manage the up/down offset but maybe adjusting things with the mount pointing at an altitude of about 60 degrees would do this.

The important thing is to only adjust a parameter when you are in a position where it has a big effect.

Hope this helps.

Hi Ferran,
Thanks for sharing the spreadsheet .
I'll play with it to-morrow and I'll keep you informed

OTA sign is for the SpreadSheet only!
In INDI, OTA is always positive.

Last night was dark enough to improve the polar alignment of my scope (I was really out !).

I’m not very good with this kind of exercise but autoguiding does miracles.

Once the polar alignment was better, I followed the process provided by Chris ROWLAND above to set the dome parameters … and the dome synced perfectly with the scope in all directions !

I improved also the park azimuth of the dome which was some degrees off.

Thanks to jpaana, Ferran and Chris for their help.

I'm having a similar problem with my NexDome - but one that is giving me a headache because it's only a problem with the INDI dome driver, the dome driver that comes with TheSkyX that I am using my trial license works perfectly using exactly the same dimensions.

I will try at @chris-rowland suggestions to make sure I'm not misunderstood how INDI's dome module differs from TheSkyX. There's only one other thing that I think that could be confusing things and that is that the Paramount driver might be reporting detail that is causing the problem eg: due to pier side calculations.

I'm Southern Hemisphere just in case that means I need to sue different witchcraft - the documentation around this stuff isn't exactly clear or that easy to find!