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SplintUK

Zoro's been all over the sky

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I had a 2nd attempt at autoguiding on m31 tonight using my synguider on my travelscope 70, with my imaging setup an HEQ5 with Equinox 80 and a modded EOS 300d. I had shutter lockup on and delay set to 3s. This is the (frustrating) result of my 15m subs...

post-12157-0-86600600-1352589392_thumb.jpgpost-12157-0-86600600-1352589392_thumb.j

Anyone got any good ideas? All of my 10m plus subs seem to have similar Z or s shaped star trails. I know I'm doing something wrong, but not sure what...

Thanks for your help,

Dan

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Have you got your balance and polar alignment finely tuned? I know if I don't then I get wonky looking stars like yours!

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you could always try some shorter exposures, like 20 x 3 mins your core will not look as blown either with a shorter exposure

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Was your guide star close to M31? If not, that would couse field rotation.

Are you sure the synguider was locked on to the guide star throughout? I don't have one but I've read they do not signal when the guide star has been lost.

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It's hard to say if it lost then regained lock as m31 was basically straight up so looking at the back of the guider (or camera for that matter) was rather tricky. Plus I wanted to keep my distance from it during exposure so I didn't introduce vibrations.

I was wondering if its just a bad idea to be shooting close to zenith and perhaps that had a bearing?

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you could always try some shorter exposures, like 20 x 3 mins your core will not look as blown either with a shorter exposure

I'm actually more interested in learning how to auto guide than the actual result at the minute. My thinking is that once I've got it down pat, I can then start experimenting with length of sub etc, but for now I want to push the limits of exposure length and get good feedback of when the guiding goes wrong. From that standpoint I guess I learned a lot from last night.

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It's hard to say if it lost then regained lock as m31 was basically straight up so looking at the back of the guider (or camera for that matter) was rather tricky. Plus I wanted to keep my distance from it during exposure so I didn't introduce vibrations.

I was wondering if its just a bad idea to be shooting close to zenith and perhaps that had a bearing?

I have read that you should de-balance your scope slightly so the weight of the scope is pulling the rotation of the mount. This is specifically to counteract the tendency for the mount to wander at zenith (if perfectly balanced the motors will dither between pushing and pulling at zenith).

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It's almost certainly backlash. You don't have trailed stars, you have two sets of fairly good ones! I think that, here, we see the mount sitting on one side of mesh or the other and it oscillates quite fast from one position to the next. This isn't PA, I don't think. PA causes rotation. Here you have distinct sets of data, right and left in the image as seen. Slightly higher on the left than the right, so only one axis is affected and its the one that joins the dots. Can you tell which axis that was? (Dec or RA?) Because that's the one with the problem. For fun I'm going to bet on Dec because that's the hard one near the zenith. RA doesn't care.

It is common to run slightly out of balance, with the east side heavy in RA so that the gears are always pushing and you sit on one side of mesh. You can do something similar in Dec as well, say camera heavy, but near the zenith with the tube vertical that falls apart and oscillation is common.

You may have too much free play in the mesh as well. Astro Baby's strip down guide is a big help on this.

Olly

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I'm actually more interested in learning how to auto guide than the actual result at the minute. My thinking is that once I've got it down pat, I can then start experimenting with length of sub etc, but for now I want to push the limits of exposure length and get good feedback of when the guiding goes wrong. From that standpoint I guess I learned a lot from last night.

Seems an A*** about face way of doing it.....push the limits??? you can't even get a non star trailing image. No offense but surely you should be looking at getting a minute, two minute exposures first then once you have the workflow routine working and you know you are gonna get results, then you start to push the exposure. You appear to want to be able to run before you can walk which whilst I am sure you will get there eventually....you will spend a lot of time on your face.

I am in the same position as you and am slowly getting to grips with things.......it takes time and PATIENCE.

Edited by Rustmonkey

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Have you tried the same exposure with no guiding, it's just that to me that looks like an unguided image.

It looks to me like the drift left to right is dec drift due to polar alignment error and the drift up and down looks like periodic error.

I may be totaly wrong but it's Worth a try.

Mike

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Seems an A*** about face way of doing it.....push the limits??? ... You appear to want to be able to run before you can walk .......it takes time and PATIENCE.

You may well be right there. ;-)

I did manage to get a non trailing image at 5m exposure last week, so thought - 'right lets see what we can do.'

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Interesting that Mike and I have a totally different take on this one! That's why its a dark art of course...

Olly

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The biggest thing you need to do first is ensure your Polar alignment is really good then do a 3 star alignment. The better you do that the less your guiding system had to work which will make it more stable. Then see how far you can push it incrementally without guiding first. Once you have this down then set up the guiding and see how much more it helps. When you have that working, then you can start to push it a bit at a time.

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Interesting that Mike and I have a totally different take on this one! That's why its a dark art of course...

Olly

Below is how I see it, the only problem is the change in brightness of the star in DEC.

That could be due to the RA moveing faster in the direction of the drift in Ra for the protion of the dec drift that is dimmer ( I hope that makes sence :undecided: )

Mike.

post-730-0-44659800-1352645768_thumb.jpg

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As already been mentioned by Olly looks like a gearing problem, meshing or otherwise I've had this myself before but that was due to a cable snagging so rule that out first :D? are your goto's ok? sometimes at certain angles if your gearing is not meshed properly it can cause binding? Do you hear any clicking from the mount at that angle? as if a gear is jumping? When the mount is in the home position with the clutches locked hold the scope and give it a wiggle and see if there's any play in either of the axis?

Clear skies,

Matt.

Edited by SlipperySquid

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Below is how I see it, the only problem is the change in brightness of the star in DEC.

That could be due to the RA moveing faster in the direction of the drift in Ra for the protion of the dec drift that is dimmer ( I hope that makes sence :undecided: )

Mike.

Yes, I can see your thinking here. I didn't zoom in as much so you might well have it, Mike.

Olly

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