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Trying to explain strange drift

graemlourens

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graemlourens

graemlourens

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Hi guys

Friday the 13th, perfect weather here in warsaw, so decided to go practise with the new equipment (first time polar aligning)

Using EQMOD / EQdir / CdC & Alignmaster.

Did initial PA with EQMod, then with Alignmaster, First few Goto's were very close to center, so my guess is PA was ok(ish)

Big mistake on my side that i found out later: when using Alignmaster you have to set alignment 'append on sync' do 'dialog based' so it doesn't update eqmod's target modelling.

I wasn't Guiding as i wanted to take it step by step and do it right first without guiding (2 min subs should be easy with AZ-EQ6)

Additional info:

- Mount was level

- OTA was balanced

- Ground was stable

- We were walking around the scope (it was a farmers field) but trying to not shake anything

- Huge amount of light pollution, but thats got nothing to do with drift :p

- 2 min exposures with ISO800

- Targeting Andromeda at around 21:30 on 13.02.2015

When looking at the images i noticed drift (that i could explain later due to eqmod adding alignment points to model), but strangely its not in every SUB. The even more strange thing is that the drift changes direction, and thats what i can not explain. Futhermore in around 5% of the SUB's theres no drift at all

Heres one thats more or less ok

post-39779-0-87714400-1424066959.png

Here one with a drift:

post-39779-0-57595500-1424066961.png

And this one with a opposite drift (just a few mins after the other one)

post-39779-0-57846300-1424066957.png

I don't think its the play in the gears when balance changes ( i had an slight overbalance to one side on purpose) as the shift would be dramatically more.

Has anybody got any ideas?

Regards, Graem

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graemlourens

graemlourens

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Hey Dave.

If i'm doing this right, north is bottom right hand corner approximate.

Attached is the stacked picture from the 'good' pictures i had (horrible result, but this is without flats, darks nothing, i was just sharpening my skills a little)

Regards, Graem

post-39779-0-20871800-1424081368_thumb.j

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graemlourens

graemlourens

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I uploaded a cutout of the center to the gallery in PNG, you should now be able to download it and zoom to see the drift.

Its the same everywhere (just the corners its not that visible as there is no field flattener)

The strange this is not the drift itsself, but that i have 2 kinds of drift, once bottomright, once bottomleft, that is what i can not explain.

As i had wrong alignment points (as stated at the beginning) i can account for a continuos drift in one direction, but not suddently a change in driftdirection.

heres the PNG

drift

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Thalestris24

Thalestris24

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I'm not convinced it's drift per se. It could be wind or backlash or flex or some combination. The way to diagnose drift, as I recall, is to do multiple subs of the same length and stack them in time order. I'm sure that used to be explained in the DSS Wiki but that appears to have expired. Maybe someone else can remember exactly how to do it?

Louise

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graemlourens

graemlourens

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Hi Louise.

Thank you for your thoughts.

My guess also, but the drift is so nice and 'linear' that wind i can not imaging (especially we didn't have much wind at all)

flex - as i'm not guiding i don't see where flex would occure - am i missing smth?

backslash - i agree, but then i wouldn't have it in nearly every image, except 5%, every image has this slight drift, either down left, or down right.

Just my thoughts, still trying to think what went wrong :/

Regards, Graem

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Thalestris24

Thalestris24

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Hi Louise.

Thank you for your thoughts.

My guess also, but the drift is so nice and 'linear' that wind i can not imaging (especially we didn't have much wind at all)

flex - as i'm not guiding i don't see where flex would occure - am i missing smth?

backslash - i agree, but then i wouldn't have it in nearly every image, except 5%, every image has this slight drift, either down left, or down right.

Just my thoughts, still trying to think what went wrong :/

Regards, Graem

Hi

I was just guessing a bit ;) - here's how to calculate drift using DSS:

"Deep Sky Stacker gives you the drift amount in dx and dy on the main screen when you register the images. If you order the images in the order you took them (ie by time), make the first frame the reference image and register them you can see the total drift over the imaging session. Divide this by the total integrated imaging time (minutes) and you get drift in arc seconds per minute."

(Annoying that the wiki has 'expired'.)

It might still be a balance problem. If you're too well balanced the scope can 'teeter' on the gear teeth. You need to be slightly unbalanced in both RA and DEC. I can't think of anything else, unless your PA was actually quite a bit out. Some combination of things maybe.

I'm sure someone else will come along with lots of wisdom!

Louise

Edit: I seem to remember something about plotting the dx, dy values in excel

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graemlourens

graemlourens

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Hey Louise.

I did have a slight overbalance both on dec and ra on purpose to prevent that kind of gear slack problem, so can pretty much rule this out, and that would not be that consistant.

Excellent idea with the DSS x/y shifts. Heres the image, it looks very consistant to me, but i can not explain the Y-shift at 22:10, as androma was moving constantly down/right (from an observers perspective), so why would the images start shifting to another direction.

post-39779-0-50193900-1424098695.png

But ok, that could be because of the wrong alignment data. Its also possible that its got smth to do with EQMOD maybe applied the models data. Thats not really my concern. the concern is that its not always the same stardrift.

Attached a video where i'm going through the pictures (in dateorder) and you can see the strange behavior)

drift.mp4.zip

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graemlourens

graemlourens

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Hey Dave

Its not about the image quality, more about testing new equipment :)

was my first time polar aligning, and i think that error would have occured anywhere in the sky? (just different drift then)

As i was not autoguiding, the image quality is not relevant i think as there are no corrections sent to the mount.

so my realy question is why the change in drift direction. I had drift, thats correct (as i stated, due to alignment points in sync of eqmod that shouldn't have been there) but that should be a linear shift, not suddently changing direction in each of the pictures (see the MP4 i attached in the ZIP)

Regards, Graem

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Davey-T

Davey-T

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I watched your video, it still looks like something loose somewhere if it's all caused by the same thing .

If the dovetail was loose on the mount it could slip one way then the other as the mount rotated.

Silly question I suppose you have got it in EQ mode, do you have to change something in the handset ?

Dave

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graemlourens

graemlourens

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Hey Dave.

I'll check th connections next time, but everything was rock solid, everything well tightened and rechecked a few times after i saw the first picture.

I'm not using the handset, i'm directly connected eqascom via eqdir cable to the mount. But yes, in EQ mode.

Regards, Graem

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graemlourens

graemlourens

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Hey Dave.

Thats a good point, i was on a grassy / muddy surface (with a matt underneath but still not very stable)

I still think that this would result in a continuous 'same' drift (not alternating), but i guess i'll now just have to go out and try again checking all the things that were now mentioned. This time i picked a spot with concrete solid ground far away from any vibrations, lets see how it goes.

About the legs: I once tightened the screws on the tripod, and then pushed down a little with my own weight, and it started slipping.

But i am very worried about tightening them too much, is there even a 'too much' on the tripod lets of the NEQ6/AZEQ6? (same tripod for both)

Regards, Graem

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