Wind, Vibration and Argh!

[Chloegeek]

Hey guys

Been working on imaging for ages, I am using a Celestron Nexstar 8SE, mounted on the standard tripod with a heavy duty wedge. I have got my polar alignment as close as is possible with the equipment I have at hand. My tracking is on close enough that I can get a few minutes per exposure.

I am having something of a problem though. This problem appears to be wind.

Even in light wind (Sub 5mph) I seem to have trouble getting decent exposures, only around 10% of my exposures don't have atrocious amounts of wobble in them. I've attached an example of each.

I imagine the issue I'm having is because I'm using the standard tripod, which to me seems to be not amazingly steady. I have placed it on some very stable and firm hard standing. There are walls either side to help reduce the wind.

So my question is this:

Do you guys think wind is my problem? Or something else.

And if you have had similar issues in the past, what have you done to solve it? 

[kalasinman]

PE?

[ronin]

Unfortunately it makes sense, the 8SE has a long focal length, any movement means a big shift across a sensor.

Short stick and move it through 1 degree and the other end moves a little, use a long stick and move it through 1 degree and the other end moves a lot. You have a long focal length (long stick).

Add to which the long focal length makes a bigger image and that image is therefore dimmer, so you need longer exposures to collect sufficent light and that increases the chances of any breeze coming along and spoiling everything.

[frugal]

I would have thought that wind would have caused movement in multiple directions, rather than just along a single axis. I wonder if it is periodic error, where the gears in the mount are not perfect, so sometimes they move too fast, and sometimes too slow.

[Chloegeek]

That does all make a fair bit of sense. I thought it might be PE but it seems to come in batches at random intervals and for random lengths of time.

I suppose either I need a stronger fork arm or tripod.

Or perhaps do my imaging through a smaller refractor mounted onto my 8 and use the 8 for auto guiding.

Sent from my SM-N9005 using Tapatalk

[glowingturnip]

just a thought, but the trails there look a bit barbell-ed, more at each end than in the middle.  Maybe it's too well-balanced and is rocking from one side of the backlash to the other.

I think that's movement in RA you've got, so try unbalancing it a little by moving the weights to make it east-heavy, then it will stay on the driving side of the worm gear and not flop over to the other side in the wind.

[kalasinman]

If you are considering guiding, I'd recommend a sticky here on SGL that covers the issues. http://stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/ . I'm starting out with guiding and reading it kept me on a track to, hopefully, success. Before I can use my new guide rig on the new scope, I'll need a low profile focusser, but I'll get it sorted. Pay particular attention to the segments on pixel scale and matching imager and guider scale.--Jack

[Chloegeek]

just a thought, but the trails there look a bit barbell-ed, more at each end than in the middle.  Maybe it's too well-balanced and is rocking from one side of the backlash to the other.

I think that's movement in RA you've got, so try unbalancing it a little by moving the weights to make it east-heavy, then it will stay on the driving side of the worm gear and not flop over to the other side in the wind.

That actually makes quite a bit of sense. Would adjusting the backlash compensation perhaps fix this problem? I have noticed the backlash in my scope is absolutely atrocious. In the end I need to fix that backlash anyway. I'm not using an equatorial mount, so shifting weight is quite difficult, the best I could probably manage is changing the position of the scope in the dovetail, which I imagine could possibly help.

If you are considering guiding, I'd recommend a sticky here on SGL that covers the issues. http://stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/ . I'm starting out with guiding and reading it kept me on a track to, hopefully, success. Before I can use my new guide rig on the new scope, I'll need a low profile focusser, but I'll get it sorted. Pay particular attention to the segments on pixel scale and matching imager and guider scale.--Jack

Cool! I would love to start guiding soon, but I need to get myself a Guide Port to USB adapter still, and I'm a bit short on cash at the moment so it might be a little while before I pick one up. I'll subscribe myself to the thread and read through it this evening.

[glowingturnip]

That actually makes quite a bit of sense. Would adjusting the backlash compensation perhaps fix this problem? I have noticed the backlash in my scope is absolutely atrocious. In the end I need to fix that backlash anyway. I'm not using an equatorial mount, so shifting weight is quite difficult, the best I could probably manage is changing the position of the scope in the dovetail, which I imagine could possibly help.

I don't think backlash compensation will help, all that does is move the motors faster when the mount knows it is reversing direction and is in the backlash, so it's for quick response when observing.  It wouldn't do anything to help if you are rocking around in the backlash.  When you get into guiding, the PHD developers say you should have backlash compensation turned off so it doesn't confuse PHD.

So it's an alt-az goto mount on an equatorial wedge ?  Hmmm...  I guess you could try unbalancing it slightly with the dovetail - if you made the scope slightly top-heavy then it would always be leaning on one side or the other I guess.