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Squiggly lines in long exposure


accrama
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Hi everyone,


I am new here and new to astrophotography, but not to telescope use (I have been dealing with all kinds for 15 years). I purchased a new Celestron VX 8" Schmidt Cassegrain and Celestron VX 6" Newtonian. 


I want to take long exposure astrophotography. One minute exposures (for stacking) are more than enough. I have balanced the equipment, pointed to the north star, aligned with 2 stars and have 2 more for calibration.


I did a long exposure to Procyon. As you can see, there are trails, so the alignment wasn't very good.

I didn't move the telescope for a while, I did not push the camera button (I used a wireless bulb), I did not pace around the telescope.


What worries me, and I would like your input please, is the squiggly line path that the star trail is making on the long exposure? Why is that? Is it a faulty motor/gear? Is is vibration from the mount? Is it the balancing? How can I correct for that? 



Thank you very much for your help.

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Not sure of your focal length your at but it's very hard to get even the most expensive mount to track well over 2 minutes with good results at longer lengths. Until you try auto guiding with your mount you won't really know how good it tracks and how the gears are. Give it a go.

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I may be wrong but based on a description of how they work I suspect this is typical of an alt-az tracking mount. to get smoother tracking you need an equatorial mount? maybe I am completely wrong and you are using an eq mount! in which case ignore me.

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Hi accrama

firstly can advise you to buy the book "Making every photon count". Everyone will tell you this and the reason is - its a damn good primer.

Like you I came to astrophotography after a long period of regular photography. I will give it to you straight. You need to think about guiding.

You may think that a minute is as long as you want to go. However, when you start doing longer exposures, there will always be a target that you could do

if you could get longer exposures. And you will become more discerning about star trails.

The star trails you have are due to the mount not tracking precisely. It doesn't mater what mount you have, you can twiddle and fiddle but there will

always be tracking errors. I have been down this road. Bite the bullet and go for guiding. I set up from scratch each night I go out. It's not that difficult.

This astrophotography game is like a drug. Once you've got your best M42, there will be other challenges and you wont be happy until you have the kit to

do them.

Good luck

cheers

gaj

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The mount isn't tracking accurately enough. If it's an equatorial mount I would check my polar alignment routine as it should be possible to get in excess of 1 minute exposures without the need to guide.

If it's an ALT-AZ mount then  they aren't really suited to long exposure astrophotography.

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From the Celestron website, it seems these are EQ mounts

http://www.celestron.com/browse-shop/astronomy/telescopes/advanced-vx-8-schmidt-cassegrain-telescope

http://www.celestron.com/browse-shop/astronomy/telescopes/advanced-vx-6-newtonian-telescope

Please confirm these are the correct items. 

It may be you have balanced the system overly well, as counter intuitive as that sounds. If the system balance is 100% bang on, then you can suffer from backlash in the gears, and the tracking suffers. You want to slightly (and I do mean slightly) weight the system to the rising side. This ensures a 'tension' on the mechanisms and stops the backlash. (I've hit this myself with an AltAz mount. Slightly tail heavy, I was able to track for 1 minute easily. Bang on balance, and I couldn't even achieve 10 seconds). The longer focal length of the C8 SCT is going to magnify any errors, and make them far more visible on any image, so if this was shot with the C8, and you didn't see this on the C6 Newt, this may well be the reason.

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I have found the polar align app by George varros an excellent help... and have managed up to 1.30 mins unguided on a low payload mount.

Im a beginner but what ive found helpful is to make sure an align the scope roughly fully loaded with all the equipment then find my target do another polar alignment then check frame and focus then a final alignment before i take any exposures.

I also found it quite good to kick out the legs of my trypod before doing any of this to make sure they were under tension and also made sure when i released all the clutches on my mount nothing moved and was well balanced.

As i said im a beginner but this routine seems to work for me so far.

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Hi everyone,

Thank you for your multiple answers. I didn't expect so much input.

I am using a Celestron Advanced VX German Equatorial Mount for both the Newtonian VX 6" (focal length 750 mm) and for the Schmidt - Cassegrain VX 8" (f = 2000 mm).
[see here: https://www.astronomics.com/images/Product/large/20045.jpg]

I have seen the problem with both telescopes attached to the mount. Are there images that exemplify the "tail heavy" - "tension" balance that is prefered for the telescope?

Polar alignment has been done previous to the photography session.

Thank you all  :grin:

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Thanks for the additional info. That is quite a trail for just over 2 minutes, I have to say. The 'ripple' could be a combination of seeing conditions and not having had sufficient time to cool down to ambient temperature?

As John says above, balance is important and a slight offset to the RA axis to make the rising side a little heavier is worth doing to keep a constant gear mesh.

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Could we see the whole image rather than a crop? (There is an obvious difference, once you have your eye in on these matters, between a guiding drift and polar misalignment-induced field rotation.)

Are you sure the mount is set to track at sidereal rate? Other rates are available on most mounts. And (best to ask!) it was set to the correct hemisphere?)

Olly

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And (best to ask!) it was set to the correct hemisphere?)

I wondered about this too, Olly but apparently a star alignment routine was successful indicating that the mount knew what hemisphere it was in - those do seem like long streaks though for such a relatively short exposure with a driven mount.

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Hi everyone,

I see. I'll try about making it "heavy" so to force the drive.

I had a correct two-star alignment for the telescope and it was finding objects relatively well. The controller is set to sidereal rate and I am using the on-board site directory to set my location (which should be OK).

I don't have the full sized image with me any more (deleted it), but all the stars in the field showed exactly the same trail.

So, another question, for a well-balanced, well-aligned, and well-weight-balanced electronic Celestron VX equatorial mount, what exposure time would you expect BEFORE trailing showing?

Thank you :)

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Depends entirely on the pixel scale (which, for a given camera, means the focal length.) The shorter the focal length the longer you can go. Using your long focal length guiding is really a 'must' and, even guided, may be far from dead easy.

Olly

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I've just copied this from the Celestron website on the section devoted to your mount.

You’ll be able to track through long exposures using permanently programmable periodic error correction.

I don't know what others think of this claim, which makes no mention of autoguiding, but I know what I think of it. I think it should land Celestron in court.

Olly

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