How do you counter field rotation.

Hagar (Doug) · #1 13-01-2009, 04:09 PM

Hi everyone, as some of you will be aware I have been dabbling in blending Ha data into RGB data to enhance a RGB image.

I have struck a little problem. I usually tru to capture and stack the RGB data then do the same with the Ha data. This is fine but when I take bothe final images into photoshop to blend the two images. What I am finding is that the time between the RGB and Ha datais causing a small difference in the final position of the stars due to field rotation I think.

If I was to just stack the to images I wouldn't have a problem but trying to blend the two images is a problem. I cannot for the life of me find anyway to anchor a point on an image and rotate the image around this point untill it matches the RGB image.

ANYONE HAVE ANY IDEAS????

Zuts · #2 13-01-2009, 05:51 PM

Quote:

Originally Posted by Hagar

ANYONE HAVE ANY IDEAS????

Hi,

I beleive a lot of people use registar. This will rotate and shrink/expand any sub to fit any other sub. For example you can use it to combine images from say a 3m FL SCT and a 500 mm FL apo.

It is a bit expensive as the price is in USD but it works great and there is a free trial. From memory you cant save with the trial but it does demonstrate the full potential.

Cheers
Paul

Hagar (Doug) · #3 13-01-2009, 08:01 PM

My problem isn't stacking the images. I have a problem rotating an image around a fixed point to allow image blending in photoshop when the images are taken some hours appart and show some field curvature.
I am sure it can be done but I have no idea how.

Zuts · #4 13-01-2009, 08:26 PM

Quote:

Originally Posted by Hagar

My problem isn't stacking the images. I have a problem rotating an image around a fixed point to allow image blending in photoshop when the images are taken some hours appart and show some field curvature.
I am sure it can be done but I have no idea how.

Registar will expand/compress and rotate any two images at different focal lengths and with field rotation. An image is aligned to another image and can then be saved.

This is not done to stack it is so it can be blended in for example photoshop.

For example, you have a series of rgb images which have been stacked and processed and some Ha data. After stacking and processing the Ha data you would align it to the stacked RGB data in Registar and then save the rotated/compresed/expanded Ha image.

Then in photoshop you would blend the Ha data which now fits exactly to the image scale and orientation of the RGB data.

Try it and see

Cheers
Paul

Hagar (Doug) · #5 13-01-2009, 08:48 PM

I'll have a look and give it a go. Thanks Paul

KenGee (Kenith Gee) · #6 14-01-2009, 12:17 AM

Hagar if your fund manager is in a good mood then investing in either Astroart or maxim DL will allow you to do just about anything you would want to do.
I should also say if your getting field rotation and you haven't touched your setup I'd be checking my polar alignment as well.

Zuts · #7 14-01-2009, 01:09 AM

Quote:

Originally Posted by Hagar

I'll have a look and give it a go. Thanks Paul

No worries, I forgot to post the link

http://www.aurigaimaging.com/

Cheers

kinetic (Steve) · #8 21-01-2009, 08:42 PM

Quote:

Originally Posted by Hagar

I cannot for the life of me find anyway to anchor a point on an image and rotate the image around this point untill it matches the RGB image.

ANYONE HAVE ANY IDEAS????

Doesn't Nebulosity do this Doug?
When you select Align and Combine....instead of
just clicking on Translation, select Translation and Rotation.

You then use a 2 star reference, (select stars as far apart as possible)
There is even a setting Translation/Rotation/Resize as well.

That should do it?

As Paul suggested also, Registax does it well, I've used that for
years for slightly misaligned shots near the SCP (lots of misalignment),
and also for comet shots from different nights (misalignment mainly due to
piggyback differences).

regards,
Steve B.

pjphilli (Peter) · #9 22-01-2009, 10:31 PM

Hi KenGee
I noted your point about bad polar alignment being a possible cause of rotation. Is it true that even if you are guiding well in both RA and DEC that poor polar alignment will still give rotation?
Cheers Peter

pvelez (Pete) · #10 24-01-2009, 12:37 PM

For what its worth, Images Plus also lets you do this. Just align the RGB and Ha files and use the aligned files in Photoshop.

I had the same issue - trying to rotate in PS (which was a right pain).

Pete

rat156 · #11 24-01-2009, 05:38 PM

Quote:

Originally Posted by pjphilli

Hi KenGee
I noted your point about bad polar alignment being a possible cause of rotation. Is it true that even if you are guiding well in both RA and DEC that poor polar alignment will still give rotation?
Cheers Peter

Yes it will.

Hagar, I use CCDStack, which also aligns the rgb and l shots, then save them and blend in PS as usual. Registar also works, but CCDStack has other advantages.

Cheers
Stuart

renormalised (Carl) · #12 24-01-2009, 06:26 PM

Doug, to make it even easier than using software to derotate a starfield, why not invest in a field derotator. You just add it to the back of your scope and plug it into your mount. As your mount moves through RA an DEC, the derotator compensates for any movement by rotating your camera, or whatever, in the opposite direction. Meade make one for their own scopes but I also know you can get them for other scopes.