Center-Here not working for older Losmandy G-11

I have a mid-1990’s Celestron badged G-11 which I’m trying to control via SGP 2.6.x

I’ve found that the “center-here” feature in the preview frame, or the centering feature in the target properties dialog, does not work as expected.

I have it set to iterate up to 6 times with a target of 50px accuracy.

Whilst the plate-solving and sync appears to work, the error does not appreciably diminish during each iteration. It might vary by up to 20 pixels (up or down) but never reaches the 50px goal.

There is a similar thread that alludes to issues with the centering algorithm on some mounts: Syncless center (offset mode) doesn't seem to work for celestron

Does anyone know if this applies to the G-11, and if so, is there any workaround?

Auto centering is a key feature of SGP for me and allows multiple targets to be automated without human intervention. If each target requires manual iterative centering, then it greatly reduces its value.

This does work on a Tak EM-200 mount, so I’m curious about why the G-11 behaves differently? I suspect it may be something to do with how the mount reacts to “sync” commands, and how SGP determines the error. If gotos are inaccurate, but the error is not being taken in account, I can see how the centering error might never reduce. I’m talking about 500-800 pixel errors, so not insignificant, and enough to ruin the framing of most targets.

Thanks and any feedback.

John Wheeler.

John,
Are you using the “Sync” option with the G11? If I recall there was some weirdness about which coordinate set to pick with the Gemini. I would try changing to JNow or J2000 and seeing how that changes the results. If I recall the Gemini will always assume that sent coordinates are JNow but requested coordinates will be sent back however you have it setup.

Sorry, my memory is a little fuzzy on that. I believe this was specific to the Gemini 1 system (this is what I used for many years as well).

Thanks,
Jared

Thanks for the super fast reply Jared.

I have disabled the “syncs add to model” (or similar) check box because I understnad the model can interfere with the centering in SGP. There was a massive thread on this which I’m still reading through :slight_smile:

I’ll double check the JNow and J2000 settings to ensure they are consistent.

Another thing to try is to clear the Gemini model just before running the centering (or check all values are zero’d). It’s possible that previous plate-solves have somehow added to a model. The consensus seems to be that having a model can cause problems, and there’s no need for it if using plate-solving.

It seems that this is not an uncommon problem, even with high-end mounts: Platesolving not working

I’ll try some of these suggestions, and post my updates.

Thanks and best regards,

John Wheeler

I’m not using SGP anymore because centering is critical to my imaging - and sgp could not center my mount adequately.

I have since written my own code for image acquisition and it includes the syncless centering method that I described here numerous times. It is very simple and works extremely well and quickly.

For any mount that appears to get ‘stuck’ while centering in sgp - and keeps missing by the same amount - it is likely the same problem my cge-pro and other mounts have - and it has nothing to do with backlash or J2000 vs. JNow.

The process of centering has no inherent link to a need to sync - it’s just the way sgp chose to do it. All you need to do is keep track of the target you last used for a slew destination - and where the plate solve said you actually landed. In your next slew you make an adjustment to get you closer given the error you just measured - and there is no sync required.

In working with sgp on this issue some time ago it appeared the “syncless” centering option was very close to working and just needs some tweaking and I assume it would work.

It’s no longer a request from me - but I did spend a lot of time describing it, testing it, and testing the syncing options as they came out - so it would be nice if others could benefit - as I think they would once it works.

Frank

Hi Frank,

Thanks for your response. I was very interested in your comments in the long thread from 2015.

Have you developed your capture software for public release or just your personal use?

BTW, I’ve just discovered your MetaGuide software - I’ve been having a hard time getting PHD2 to give satisfactory results with my old G-11 and am looking forward to trying your software (with a ZWO ASI224 + OAG).

Best regards,

John

Hi-

I guess my code is off topic here but you’re welcome to follow up offline.

My main point is that sgp was overall working very well for me, but the inability to center accurately made automated runs impossible. I think I spelled out a way to do centering - but to my amazement it was criticized as a form of “windage” that was somehow unpalatable. Yet the entire process of iteratively syncing, solving, and slewing is itself a form of windage - with the added issue that it requires a sync - and many mounts do not like to be sync’d - or do so with undesirable side effects.

I think the current implementation of “syncless” centering just isn’t right - and I doubt it works in a useful way for anyone. I think it just needs to be revisited and tested a bit and it should work. It is a very, very simple concept.

Frank