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piprees

seems like a tracking problem.

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Hi, lately I seem to be getting what seem to be very small star trails when imaging with a Canon 40D on a Skywatcher Explorer 200. Have a look at the attachments, they're 120 secs, 102, and 90, all at ISO 800. My EQ5 has a motor drive with Right Ascension, polar alignment is spot on, the images have a 10 second delay between each exposure plus a 10 second delay after shutter opening for mirror lockup. After taking these shots I changed the camera for a 7mm eyepiece and centred on Vega. After watching half of Bake off, went back and Vega was still centred. I'm absolutely baffled :confused: as the longer the exposure the longer the trailing but not really too long and the central object doesn't seem to be affected. Any comments will be gratefully received. Kind regards. P.post-31395-0-78564400-1412800586_thumb.jpost-31395-0-94752200-1412800621_thumb.jpost-31395-0-71196200-1412800660_thumb.j

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It could be PE (periodic error) in your worm gear. Wouldn't necessarily show up in an ep as by nature the gear would return to it's original position, hence periodic.

Someone more knowledgeable may have a better explanation.

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greetings

try a set of 10  or more 30 second exposures with no delay or cool down look at them in sequence and check for trails if trials are in all reduce exposure to 15 second up iso to 1600 repeat

if trails appear  at regular interval depending on exposure time it points to a periodic error issue. if no trails increase exposure ....  it's a timing thing..

Andy

Edited by andyukv
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Thanks guys, looking a little closer, the 'trailing' seems to be between 3.00 and 12.00, if you see what I mean.

If it's a periodic issue how can it be resolved?

P.

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With your telescope focal length(1000mm) and your mount(EQ5), and if you do not use guiding, it is very hard to not get stars trials using exposures longer than 30s... On my setup (SW150/750 + HEQ5) I can get good images, without stars trials with 30s. When I am using longer times on most images I have star trials. For 50s I have about 6 - 8 out of 10 good images. For 90s only 1-2 out of 10 exposures are ok. Generally: longer exposures = more images with stars trials.  It is because mounts are not able to follow stars precisely(errors), and this is why we need to use guiding ;)  

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Yes, I'd love to have a guided scope, problem with dosh!!! I'll try loadsa 30 - 60 secs exposures and see how things work out. It's odd that up until a couple of weeks ago I was able to take lots of 4 or 5 minute exposures with no problems, Bodes and cigar, whirlpool, pinwheel, even Leo triplet. It's a hobby that throws up strange problems, with a steep learning curve and a hungry appetite for knowledge. Thanks folks. P.

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You have some coma, shows up best top right.

Some of the trails look hooked, something moved?

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Can't think of anything knocking the mount. Maybe the motor could be a bit jerky, really difficult to tell. Guess I'll have to live with it.

Thanks again.

P.

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With your telescope focal length(1000mm) and your mount(EQ5), and if you do not use guiding, it is very hard to not get stars trials using exposures longer than 30s... On my setup (SW150/750 + HEQ5) I can get good images, without stars trials with 30s. When I am using longer times on most images I have star trials. For 50s I have about 6 - 8 out of 10 good images. For 90s only 1-2 out of 10 exposures are ok. Generally: longer exposures = more images with stars trials.  It is because mounts are not able to follow stars precisely(errors), and this is why we need to use guiding ;)

The length of good subs possible varies with the quality and adjustment of the mount. I recently bought a ZEQ25GT and being a total beginner, I was at first getting star trails at any length. As I progressed, I learned polar alignment and hyper tuned my mount. I recently made the following of Pleiades 20 x 300s with zero throw aways unguided.--Jack

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Edited by kalasinman
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Vega is trailing, look at its diffraction spikes. You have double spikes top right and bottom left.

Have you done anything that would effect the balance of the mount?

Also does it track better on one side of the meridian to the other?

Sent from my iPad using Tapatalk

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Firstly the fact that Vega stays centred for ages tells you nothing, really, since any mount should do this. That includes mounts with too much PE to use for AP. It's the small scale tracking precision that matters.

In my view, what we see in your images arises from backlash above all. You have 'double stars' rather than trails, so the mount is sitting on one side of mesh then oscillating quickly onto the other side, giving two positions. Both positions give their own separate stellar image.

You can try to tune out the backlash by following, say, Astro Baby's strip-down guide or you can load the mount to run east heavy. Whether east is counterweights or scope depends on which side of the meridian you're on and the imbalance should not be much, just enough to keep the gears pushing as they drive.

Olly

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Thanks everybody, my viewing area is severely limited, north east to south east and about 30 deg to perpendicular with a small area towards the west. This is due to atrocious light pollution and nearby buildings. Lordy D4N, your eyesight is far better than mine, I hadn't noticed those. The balance and backlash sounds interesting Olly. I'll give it a stripdown and see if things improve. Thanks again and Jack, that image of Pleiades is first class.

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Just an update, I've adjusted the motor so that the cogs are more tightly meshed and dropped the counterweights a little on the bar. The difference is quite noticeable. Many thanks again for all of your advice. P.

post-31395-0-97510600-1412941834_thumb.jpost-31395-0-19869200-1412941868_thumb.j

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Just an update, I've adjusted the motor so that the cogs are more tightly meshed and dropped the counterweights a little on the bar. The difference is quite noticeable. Many thanks again for all of your advice. P.

attachicon.gif120 seconds.jpgattachicon.gif150 secs after mount adjustment.jpg

Looks like you got rid of the backlash. The next thing on the improvement agenda is that both your focus and collimation (optical axis is low and to the right of centre) is off. It's one of the eternal constants of astrophotography, until all your error sources are smaller than the seeing/diffraction limit there is always at least one more thing to improve...

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Thanks Glappkaeft, I thought about doing a collimation check thinking that it was really difficult and needing special, (very expensive!!!), tools. After reading about it on various forums I had a go and found that what you've said was exactly correct and it was fairly easy to rectify. Just waiting for a clear night to see what the difference is. Yes, your also right in that it's a continual battle against anything that can go wrong as it's certain that the thing that's forgotten will creep up and bite yer bum!!!

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