EQMOD/CdC persistent alignment error

[DougAtDallington]

I have encountered a problem with EQMOD pointing models that I create with the help of CdC.  They are causing consistent pointing errors.

I have an EQ8 mount permanently installed in an obsy that was originally polar-aligned using the supplied polarscope and very good pointing models created using CdC and EQMOD.

However, I recently started using Alignmaster in an effort to improve the polar alignment. And, as this forum can attest it isn’t as simple as it made out to be – especially if you forget to disable EQMOD’s pointing model - doh!      However, a reasonably good polar alignment has now been achieved even if I did take me most of last night to do it.

But now, as I use CdC to select target calibration stars I find that when I click Sync (EQMOD is set to ‘Append on Sync’) the FoV overlay shifts off the calibration star to a position a little to one side. And using the model this creates in EQMOD I find that all gotos are consistently off-target. I've tried deleting the model and starting again but the effect is the same.

To try and quantify the effect I noted the RA/Dec parameters displayed in EQMOD when a selected star had been slewed-to, and again after cantering the star in my CCD. The 'slewed to' parameters are almost exactly what they should be, and the star is precisely centred on my camera's CCD before hitting the Sync button.

The results are shown below and appear to display an interesting pattern of differences. For example, the Beta Aur and Beta Cas differences are remarkably similar, and the Alpha Ori and Beta Per differences match closely but with the RA and Dec differences swapped.

and I wondering if some of you clever people could explain what’s going on and how I could return my EQMOD/CdC combination back into the reliable control system it used to be.

Star name                          RA                                          DEC                       

Alpha Ori.            Deg        Min        Sec             Deg        Min        Sec                                                                        

Slewed to             5              56           2                7              24           22

After centering     5              52           44              7              28           35

Difference            0              3             18              0              4             13

Beta Aur.                                                                                            

Slewed to            6              0             42             44           56           27

After centering    5              58           11              44           49           49

Difference           0              2              31             0              6            38

                                                                                               

Beta Per.                                                                                            

Slewed to            3              9              14            41           0             44

After centering    3              4              42            40           57           22

Difference           0              4              32            0              3             22

                                                                                               

Beta Cas.                                                                                            

Slewed to           0              10            0              59           14            24

After centering   0              7              11             59           6              29

Difference          0              2              49             0              7             55

Hoping someone can help me….

Doug.

[michael8554]

Hi Doug

Given your experience with Alignmaster, I would do a quick check of your PA using another method.

Let it drift on a southern star for 5 minutes with a barlowed reticule eyepiece.

Different EQ8 and PC system times?

Not sure of the error this would give but I'm guessing it would be in RA only?

Michael

[wxsatuser]

I find that this happens if I manually move the mount, an HEQ5.

This puts the home position out as it's not possible to get it back smack on.

If I move the mount manually I delete the synch pointing stuff and set the new home postition.

Then do the pointing model.

[r3i]

Hi Doug

Given your experience with Alignmaster, I would do a quick check of your PA using another method.

Michael

Alignmaster can work well but I've had mixed results.  As you've got your mount permanently installed, I'd agree with Michael and double check the PA with a drift alignment check with something like PHD2.

[DougAtDallington]

Thanks Michael, Mike & Mike.

The PC and EQMOD times are sync'd, and the error is in both RA and DEC. So I'm afraid those can be excluded.

My EQ8 mount has auxiliary encoders that, provided EQMOD is running, follow where you manually point the scope, so I don't think that's the problem.

I've just downloaded and installed a copy of PHD2 which I'll use to check the PA - when/if the next clear night happens.

Kind regards,

Doug (in cloudy East Sussex)

[DougAtDallington]

Well, I tried PHD2 but I'm not having much success there either.

First attempt at calibrating PHD threw up error messages about differences in RA and Dc guide rates.

I took the option of continuing and the PA just got worse and worse. Searching the web I found people saying that PA had to be pretty good to start with or PHD2 would make a mess of it.

So I re-ran Alignmaster PA and managed to get an apparently v.good result.

I tried PHD2 drift align again, and got the same error message when trying to calibrate PHD2, so I gave up and went and had a cup of coffee.

Then I did a 4-star 'Align-on-sync' to build an EQMOD pointing model with exactly the same offset issue as before.

I'm stumped!

Any ideas anyone?

[Merlin66]

Doug,

Have you double checked the Park position in EQMod?

[DougAtDallington]

G'day Merlin66.

I've started the EQ8 with the hand paddle and told it to go to its home position, which it does, then reconnected to EQMOD and started from there - if that's what you mean.

It seems to remember the user-defined home position I use for taking flats.

Is there anything else I should check?

[Merlin66]

Not familiar with the EQ8 which has encoders(?)

Why not let EQmod park the scope for you and confirm it's pointing towards the pole.

[wxsatuser]

G'day Merlin66.

I've started the EQ8 with the hand paddle and told it to go to its home position, which it does, then reconnected to EQMOD and started from there - if that's what you mean.

It seems to remember the user-defined home position I use for taking flats.

Is there anything else I should check?

Why use the hand control when you have EQMOD.?

I wonder if this is where the error is, between the two, are both exactly at the home position?

[Merlin66]

Doug,

Did you resolve your problem????

[DougAtDallington]

Hi Merlin66,

Doug,
Did you resolve your problem????

No, not yet - still waiting for a night that's conducive to observing and doing drift-algnment. This is the UK after all!

I intend to check if there is a discrepancy between the home position set by EQMOD and that set by the position of the EQ8 encoders and the EQ* hand paddle (as suggested by Mike), and then see if I can get PHD2 to complete its calibration without throwing up error messages and do its own PA check.

Clear skies (I wish)!