NEQ6 star trails

[thegableguy]

Hi there

Got my newly acquired NEQ6 mount with ED80 aboard. Trying to figure out how to eliminate these damn star trails.

Even 30-second exposures the stars are about three times as wide as they are high, if you get me. Not insignificant.

I've leveled the mount perfectly, aligned it as closely as I know how to the celestial south pole, entered my exact coordinates and done a three-star alignment.

I understand what drift alignment is, but haven't tried it yet. I'm laboring under the belief that the above should be enough for it to sort itself out... guess not, huh? Is the star trailing entirely down to my polar alignment not being good enough?

[Somnium]

Hi Chris, you will need to drift align. aligning to magnetic pole or even "close" is not good enough. try drift aligning.

[bugeater]

Sounds like you need to get the polar alignment a bit more accurate. For 30 second exposures the function built into the handset might be sufficient.

[Marko of Oz]

You didn't mention setting the latitude scale on the mount to your location.:question:

[raymo]

2 or 3 star alignment only gives the mount a model of the sky for Go To
purposes, nothing to do with PA. I have found the built in PA function fiddly and time consuming.
I suggest you Google DARV, like I did. It is the easiest way of polar
aligning I have come across. Another possibility; are you lining the mount
up with true south, as you should, and not magnetic south.
With an ED80 on a reasonably aligned mount you should get an
acceptable percentage of subs at around 90 secs, and with really good
PA, about 100-120 secs, or even longer.
raymo

[Atmos]

It can also depend on what the periodic error is in your particular mount. If it has a really wide periodic error it may not give good 30s shots without guiding.
Some are built better than others.

[raymo]

I don't think P.E.C. would be a problem Colin; at 30, or even 60 secs you
are only sampling a small portion of one revolution of the gear, therefore
only a small proportion of the total periodic error.
raymo

[thegableguy]

Thanks folks.

I'm going to exemplify how utterly clueless I am about all this, by way of an embarrassing admission:

I thought the mount's 3-star alignment process was mainly to compensate for my rough-as-guts polar alignment. Just figured out how wrong that is.

Somnium, the (extremely nice & helpful) guy who sold me the mount, talked me through the drift alignment process and I've read several articles describing it. I was just being lazy in neglecting to actually try it, thinking that the mount would solve all those problems. Yeah not so much!

All good. I'm hoping that drift alignment only seems hard the first half-dozen to dozen times you do it, then just becomes part of the routine.

[rustigsmed]

Hi Chris

I would firstly update the handset with the latest firmware from the skywatcher website.

Then read this

http://www.iceinspace.com.au/63-498-0-0-1-0.html

Setup as directed and then do a two star alignment.
Then you need to do the polar alignment routine on the handset, this will result in physically adjusting the mount.
Then two star alignment again and then polar alignment routine again repeating till it gets closer and closer. Check the handset manual for further details if unsure.

Good luck!

Russ

[raymo]

When doing the star alignment you only need do a 2 star one, unless your
cone error is large[look up cone error if you need to], in which case a 3 star one helps counteract that. I strongly recommend that you Google DARV and
watch the short tutorial, definitely much quicker than traditional drift aligning,
which, if you are looking for really accurate PA, can take quite a while, a
painstaking business.
raymo

[thegableguy]

Yes Ray - this is the type of drift alignment that Aidan talked me through when I picked up the mount.

[doppler]

Here is the link for DARV
http://www.cloudynights.com/page/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760

[kens]

Not entirely so. The NEQ6 has a worm period of around 8 minutes. That's 4 minutes worth of PE in one direction and 4 minutes in the other. So a 60 second exposure could see 1/4 of the peak-to-peak PE. If that's a typical 30 arc seconds you've got over 7 arcseconds of trails. Even with 30 second exposures it's over 3 arcseconds. So maybe not trails there but certainly elongated stars.
Nevertheless, a PA error of 2 degrees would give around 10 arcseconds dec drift over 30 seconds and 20 over 60 seconds.

The first thing that should be asked when analysing star trails is: what direction are the trails? Are they aligned with RA or Declination?
The second one is how long are the trails? Preferably in arcseconds but at least in pixels.

[jenchris]

If you have a camera running, just set up phd2 (free download )and use the drift alignment routine there. It is easy to do and can get you within cooe in a few minutes.
On other matters, pec. I had no idea typical pe was 30 arcseconds.
I can see a cycle in my phd log, but it appears to be less than 1 arcsecond. So I haven't bothered with doing it. Would it.make much difference. My total rms.error is about 1 arcsecond
.
Apologies to op for hijacking his thread.

[kens]

I assume the 1 arc second is guided. Guiding can greatly reduce all but the worst PE as it is normally a slow cyclic drift. I've played with PEC but never found it made a noticeable difference.
To find the true PE you need to do an unguided run of at least two worm cycles. My EQ6 pro has 17 arcseconds p-p but I think it had been highly tuned by the previous owner

[Tony_]

Hi Chris,

Scroll through your subs either on the camera or your PC. If the stars in each frame seem to jump back and forth a lot - then it is probably caused by PE. If the movement is consistent in one direction then it is probably drift from poor polar alignment. You will possibly get both.
Another way of telling whether it is drift - see how accurate your gotos are at the end of the session compared to at the beginning - the difference is most likely due to drift.
I have a CGEM - I am lucky to get 30 second subs (at a focal length of 2300mm) without egg shaped stars from PE, I don't have much problem with drift. I use the hand controller to do the PA.

Regards,
Tony.

[raymo]

@ 2300 mm its not surprising that you are limited to around 30 secs, but
Chris's scope is only about a quarter of the focal length of yours.
raymo

[Tony_]

At 500mm focal length I can get 2 minutes unguided (possibly longer).
I use an autoguider now - my subs are now limited by light pollution not PE.

[thegableguy]

Thanks everyone - streaks are entirely due to my polar alignment or lack thereof. PEC doesn't make the slightest difference.

Finding it harder than I care to admit to figure out the drift alignment process. I'm kinda guessing more than anything else. Gonna have to do some more research before it really jibes for me unfortunately. Feeling sort of dumb, but still determined to figure it out. I just can't quite visualise what movements I need to make. Pretty sure I'll get there in time but yeah, frustrating for now.

I'm still limited to maximum 30-second subs, though they're at least a bit sharper than my previous efforts. I'm slowly getting better at it, though my adjustments are still largely guesswork at this point...

[bugeater]

How are you trying to align? I found the process built into the handset to be quite easy, though perhaps not the most accurate method. Start simple and work from there. Guessing is unlikely to work :)

[Camelopardalis]

Golden rule of drift alignment...

watch a star near zenith for azimuth adjustments
watch a star near the horizon for altitude adjustments

and then iterate a couple of times (they interact) until you nail it.

When it comes to which way to turn...it doesn't matter...try turning one way, and if it makes it worse, turn it the other way :D

I use the DARV method as implemented in APT (Astrophotography Tool), quick and easy to get it done. With the DARV method, you "watch" a star as above but with the camera shutter open, basically track East for a short period of time and then reverse and track West for a similar time. If your polar alignment is good, the trail of the star will follow it's own path back. If your polar alignment is off, you'll get a V-shape. Adjust the relevant axis until the V-shape shrinks and becomes just a line. Then slew to the other part of the sky - horizon if you just did zenith, zenith if you just did horizon - and repeat. Just remember only to touch the adjustments according to the rules...

[thegableguy]

I've been taking several 30-second exposures and seeing which direction the stars are moving, but then scratching my head in mystification as to which direction things need to move. Took hours before I got close and still it was only good for 30-40 seconds. I'd really like to get 2-3 mins.

Guess I'll keep practicing..!

[thegableguy]

That's the shortest version I've ever seen! Thank you!!

I tried the DARV version but neglected to leave it still for the first few seconds so I had arrows but didn't know which direction they were going...! Will try again when I can. Thanks again.

[Camelopardalis]

Hope it helps!

Well...holding it still for a couple of seconds helps but ultimately it doesn't matter...it's either a V-shape or it's not :D the adjustments you make should either make the V-shape wider or narrower.

[thegableguy]

Quick question - do they have to be anywhere specific on the horizon and the zenith (meridian, equator etc) or will any low star & high star work?

[raymo]

They need to be on or near to the meridian, and low in the east or west.
raymo

[thegableguy]

Hmmm. Thought so. That's gonna make it tricky. Hmmm.

[Camelopardalis]

Or as low as you can get, east or west (it doesn't matter which) in practice. The further you get away from the horizon the less pronounced the effect of inaccurate altitude adjustment becomes.

It doesn't have to be any particular bright star either. Your camera just needs to pick up stars.

[jenchris]

Should end up with a result similar to this

[jenchris]

Duplicate sorry