Odd drift in RA axis

[jambouk]

I attempted to image M57 the other night with my C11 and DSLR (with CLS filter, hence blue colour) on AZEQ6, unguided; 30 second subs,  collecting 143 minutes worth of data. Comparing the first and last sub I saw there was some drift in the target as I was expecting as I knew my polar alignment wouldn't have been perfect.

I stacked all the subs in the startrails software (http://www.startrails.de/html/software.html) to see if it would be easy to calculate exactly how much my polar alignment was off. The handset had suggested Mel was 15 arcseconds, and Maz 30 arcseconds or so; I'd done a handset polar alignment routine three times to tighten the polar alignment up using Vega and Arcturus.

The resultant image is a bit odd. The stars start their trail top right in the image and track towards bottom left, but after mostly forming a straight line, they start to curve off to the top left.

Is this what you'd expect with just bog standard inaccurate polar alignment, or is there something else? The gaps in the trails are some subs I'd deleted as they had motion artefact in them (wind I guess).

Thanks for any replies.

JD

[jambouk]

I'm quite pleased with the resultant image, though need to work out if the stars are a bit blobby due to poor PA, or a non-flat field, or poor focus, or something else.

I know it's not a scope for DSO stuff, but the seeing was awful and I wanted to capture something and have seen some impressive stuff done with a C11.

JD

[bogdog]

no expert here so take with pinch of salt..could it be the tripod got nudged? or gear slop where gears not meshed right then suddenly catch up? or balance

[ChrisLX200]

I'd say, because it is a non-linear progressive drift, that you have differential flexure twixt main OTA and guidescope. Ensure everything is tight and no cables are pulling (even their weight dangling can cause this).

ChrisH

[jambouk]

Thanks both.

Bogdog - this is a stacked image of 250 images, and there is a little bit of drit in EVERY sucessive image. The way I've stacked this image is so that the software doesn't try and align the stars, it simply stacks every frame on top of the last. Every sub has near pin point stars so there is no major image shift during each 30 exposure.

ChrisH - this was unguided, camera at prime focus and just tracking at sidereal rate on the mount. There were no tight cables, but there could be some shift in the mirror (mirror flop) as the OTA changes it position, but I thought that might be a larger and more dramatic change, but that is what I was hoping someone was going to clarify for me.

Thanks both for the suggestions.

JD

[ChrisLX200]

OK well there is something in there apart from simple drift (which would be linear) so either mirror shifting or weight (COG) moving. However if that is 120mins total worth of movement @ 2700mm f/l - unguided - then you have nothing to worry about.

ChrisH

[Guest]

That looks like a scope that has not been balanced correctly at all, I had an image like that once, be very careful as the blobby stars look like they are a result of the gears in the mount slipping, through excess weight on one side or the other, make sure you balance it perfectly it really does make a huge difference, I will look for my image when I had that problem and you will see it is very very similar, oncle I balanced I get 20 min guided subs now, or can if I need too, i rarely go,over 6 mins

MM

[Tim]

Who says it isn't a scope for DSO's?

I beg to differ

Tim

[hobsey]

Who says it isn't a scope for DSO's?

I beg to differ

Tim

Its certainly making me think about trying it with my C8.

[Guest]

Its certainly making me think about trying it with my C8.

I image DSO with a meade LX90 on a wedge, similar to the C8, no worries, need a focal reducer, and a DSLR for the big chip, but I have had some great images, so of course it can and should be done......

MM

By the way the picture on the left was taken with my Meade LX90 with FR and canon 1000D

[knobby]

I'd guess at balance... Its nearly impossible to balance a C8 or 11 as all the weight is at the end where the DSLR attaches... Stick the DSLR on and its even worse.

[Guest]

Well, a counterweight bar on the underside of the OTA is what is needed, this is how mine work

The total weight balances the guide scope and guide cam on the top, while the weight at the front balances the DSLR and focuser on the back, although not all fitted in this picture.

This is an old homemade counter balance system, I use the same as on the top now an ADM rail with threaded bar with weights on, and this can be slid forwards and backwards on the rail, and the weights can be moved further or closer to the OTA.

[knobby]

I'm trying to make (neatly) a weighted dew shield for mine

[jambouk]

Thanks for the replies.

Being incorrectly balanced is a real possibility. I will try it again next clear night after carefully balancing. I have to confess, I seldom [never] "carefully" balance!

Once I've tried again, I'll report back.

Out of interest, how do you guys guide your SCTs?

Thanks for the ideas.

JD

[Guest]

Well as you can see from my earlier picture I use an ST80 with meade DSI, and PHD software using serial connection to the handset and ascom pulse guiding, not ST4.

Works like a charm to be honest, would never change the way I do it.

MM

[Davey-T]

Thanks for the replies.

Out of interest, how do you guys guide your SCTs?

JD

I use an OAG, used a seperate piggy back guide scope for a while but find OAG is better for long (30 min) subs, also use what I believe they call a 3d balance system so that the weight can be moved sideways as well as back and forwards, helps when it's on a wedge.

Dave