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Drift Alignment...

RichieJarvis

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Whippy

Whippy

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Autoguiding doesn't fully compensate for bad polar alignment. You can get visible field rotation in your images...

Really? I've never done drift alignment, TBH not even really accurate polar alignment and never got field rotation even on subs of 10 minutes. If you're getting that then I'd hazard a guess that the guiding isn't doing it's job properly.

Tony..

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RichieJarvis

RichieJarvis

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Well - Its not good enough to image tonight, so the final results will have to wait...

The software itself is very easy to use. You target a star in a southerly direction, give it your longitude and the Dec of the star, put it on the cross-hairs on the screen using the handbox, then click start - the software then tracks the movement. You can leave it as long as you like - it recommends 300 seconds (5 minutes.)

Once you decide to calibrate, you click a button, and the software displays 2 lines, a red line and a green line on either side of the screen. You move the star to the red line with the handbox, and then move the star to the green line by adjusting the mount.

Then you can select a star to the east, and repeat the process to setup the other axis.

You can repeat as much as you like - i did the N/S axis about 5 times whilst learning the software, and the E/W once. My E/W is now spot on - the N/S is alot tighter than it was. I need to redo, but the clouds came over :clouds2:

Once strange effect of the software is that you cannot copy/paste whilst its in use - kinda strange, and really annoyed me whilst trying to paste a URL into an email while I was waiting.

All-in-all, I am very happy - this is the first time I've ever drift-aligned, and it made the whole process alot easier, and explains the steps required, both on-screen and in a well written manual.

I give it 4 stars (out of 5) - it only drops 1 because of the cut-paste issue.

And as for whether you need to drift to image - I partly agree with Whippy on that - until last week I was on a tripod, and never drift-aligned, but relied on guiding to take out the errors - I never saw any field rotation, despite using 10 minute exposures.

I am hoping that now its drift aligned, it should improve the guiding, although that'll be hard, as I was able to get a consistent < 0.3 pixel error without drift aligning!

Cheers,

Richie

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MikeP

MikeP

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Due to problems attributed to my web cam I was never able to get WCS to work and could not guide with the web cam either. It now appears that the problem was with my (so called) powered USB hub which has been replaced. The problems pushed me in the direction of PolarAlignMax and the weather has prevented me trying WCS since, but it did look very useful to me too. Well done Richie.

On the subject of field rotation, would it show up when stacking a series of subs? In other words would the object "move" in the FOV over the course of an hour's total exposure?

Mike

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SteveL

SteveL

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If you're getting that then I'd hazard a guess that the guiding isn't doing it's job properly.

Autoguiding cannot compensate for a bad polar alignment, not matter how good the autoguiding is. As an extreme example, autoguiding an AltAz mount (if you could!) will not eliminate field rotation, even if the guide star stayed absolutely dead centre. The same goes for EQ mounts.... if your axis of RA rotation isnt close to the NCP, then you will see field rotation, even with autoguiding.

Autoguiding only adjusts RA and DEC, it cannot adjust the rotation of the camera.

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MartinB

MartinB

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The software presumably needs to know the focal length of the scope as well?

The longer the focal length the quicker field rotation will show up. You certainly can't run 30min Ha exposures at 2500mm focal length without very accurate polar alignment!

Drift alignment is one of those things that gets much quicker with practice. You soon learn to judge the mount correction required based on the speed of movement of the alignment star. The hardest thing about drift aligning is making sure you move the mount the right way, gets very confusing at times!

Sounds a neat bit of software Richard

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Kaptain Klevtsov

Kaptain Klevtsov

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The software presumably needs to know the focal length of the scope as well?

IIRC the focal length isn't required. The output scales automatically as you move the target on the screen after the calibration period. The software doesn't need to know if you have a big error and short focal length or a small error and long focal length, as it sees the error as being the same. The difference is how much you need to adjust the mount to move the star to the new position on screen. I think it works like that, but I could be wrong as its a long time since I had a go with it. Very good and easy to use software.

Kaptain Klevtsov

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SteveL

SteveL

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Another option if you have MaximDL is PoleAlignMax, from the same guys who bought you FocusMax - http://users.bsdwebsolutions.com/~larryweber/. You need to instal the PoleAlignMax (aka PAM) from the above website, plus the updates from the Yahoo group - http://groups.yahoo.com/group/FMaxUG/

It uses the plate solving ability in Maxim. You centre a star, it takes an image, plate solves, and it works out where it is... its slews East (or west) 15 degrees and plate solves its location... its does this again, and from that can determine how far out you are. You then centre a star, and it moves the mount in RA, and you move the *mount* so it is centered. It moves in DEC, and you move the *mount* to put it back in the centre.

I havent actually tried this yet, because ever since I found and installed the software, its been blumming cloudy :clouds2:

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RichieJarvis

RichieJarvis

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On the subject of Field Rotation: SteveL - your right - but only if the mount is significantly away from the NCP.

From what Whippy and I said, both of us are within a gnats whisker of NCP anyway. I achieve that by using the EQ6's polar scope, and don't see much drift. Its therefore not a huge job for autoguider software to compensate, and I suspect it would take many many hours of exposure before field rotation became an issue.

From my imaging, where I am typically taking 2-3 hours worth of subs without drift aligning, there is no rotation between subs that I can detect - honest mate, you'll just have to trust me on this!

My reason for drift aligning is simply to improve things so that the autoguider software can reduce the number of corrections. As I said though, even without, its not working that hard, so my alignment using the polar scope can't be that bad.

I've had to take the mount down several times since I got it as well, as having the pier adaptor made required me to take it over to the fabricator for measurements, and then fitting. After each time I had to re-align using the polar scope, and I was doing long exposure imaging again immediately afterwards with no ill effects from field rotation.

I'll have to have a look at the PoleAlignMax software as well, and do a comparison - at least I'll be able to use my SXV-H9 with that, without having to switch cameras!

Cheers,

Richie

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glennbech

glennbech

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Well... A picture speaks a thousands words? I suspect field rotation during guiding is in play in this 300 second sub of the bubble nebula. My entire set is like this and the stack is practically useless. rAs you can see all edges have elongated stars, the center is fine. A 0.8 Reducer/Flattener especially designed for the Megrez 90 is used, I usually have perfect round stars all the way to the edges...

Is there any other explanation for this than field rotation? (I was guiding with PDH/using a 80mm Helios Startravel).

8325_normal.jpeg

(click to enlarge)

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MartinB

MartinB

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Richie, I don't see the point of you doing anything other than using the polar scope if it gets you close enough not to suffer from field rotation. There is something to be said for not having the alignment absolutely bang on since it means the dec correction stays in the same direction eliminating backlash errors. It's a different matter as you move up to 2000mm focal length and beyond though

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RichieJarvis

RichieJarvis

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Hi Martin,

Yeah - I have often wondered about that :(

I can't see any field rotation in this though, and thats 3 hours worth of data, the longest sub was 5 minutes!

2008-09-02-NGC7635-Small.jpg

I guess if it ain't broke, don't fix it probably applies :D

Still - I am sure I will find the software very useful off pier @ star parties!

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