How do I cheat and get round stars?!

[Astrosurf]

My autoguiding is still dodgy as I continue learn its foibles. I have some subs with tracking errors and wondered if there's a function in Photoshop (I have 7) or Noel's Actions that can clean them up? (I'm still waiting to get Noel's Actions emailed to me)

Alexxx

[3ggman]

Here is two tutorials that might help.

http://stargazerslounge.com/imaging-tips-tricks-techniques/47061-6-reducing-star-trails.html

http://stargazerslounge.com/imaging-tips-tricks-techniques/47075-7-how-make-stars-smaller-sharper.html

I have never done the first but the second one is very good for getting tighter stars and increasing their color. I would imagine the first one is the one you want.

[Astrosurf]

That's great. Many thanks.

[ollypenrice]

To Martin's method I would add one refinement to simplify life. Begin by rotating the canvas so that the trails are either vertical or horizontal. That way you only ever ever move the offset one way.

Olly

[kookoo_gr]

check this video (turn subtitles on)

[iwatkins]

If they are really bad, select the whole image and use a free form Transform to stretch/compress the image to squish the stars back into shape.

Sometimes works, sometimes doesn't.

Cheers

Ian

[Astrosurf]

Excellent. Thanks guys. I didn't know there was a subtitles feature on YouTube. It's very good.

[Quatermass]

Depends how bad they are if they are a bit elongated just duplicate the image change blend mode to darken and use the arrow keys to move the layer down till the stars shape up. Next apply a hide all layer and paint them back in where they are needed. That is a simple method but it works well.

[Tim]

dont forget Alex, if the stars are all mucked up, so will everything else be.

In other words, you can't make a silk purse from a sows ear :s

Sometimes it is best to cut your losses and ditch the data and start again, using it only to learn what not to do next time.

Cheers

Tim

[ollypenrice]

Tim's right, of course, and I would never fix my own trailed data. However, sometimes folks do want to rescue a night's work and it is possible.

There is another trick, too. This one is good for edge of field stars showing a little coma or BIG stars which, after being reduced, have lost their shape. You can record it as an action and save it to a single key but it can only be applied one at once.

Magic wand to select star, then record;

Select, Modify, Expand by (try 4)

Feather selection (try 3)

Blur, Radial Blur, Spin, Best, Maximum.

Deselect.

Star size, defect size, pixel size, all vary so the expand and feather values are trial and error jobs. I don't have the thousand dollar flattener for my TEC and live without it thanks to this routine for a few corner stars per picture.

Olly

[Astrosurf]

Phew, this is getting very rich in info! I'll have to spend some time going through this. I'm getting better at PS but it's just the tip of the iceburg! I've had a book for ages I haven't gone through yet. Must do.

I would cut my losses but this was a session to catch M95's supernova, so I won't get another chance. It stacked OK but still quite rough.

Thanks for all your help guys, you're brilliant.

[RikM]

I have used the blend mode set to darken and shift the layer over technique with DSLR shots from a static tripod. It can save otherwise irreproducible images such as conjunctions.

[Astrosurf]

I have used the blend mode set to darken and shift the layer over technique with DSLR shots from a static tripod. It can save otherwise irreproducible images such as conjunctions.

Nope, didn't get that, sorry! Must get that book out . . .

[ollypenrice]

Nope, didn't get that, sorry! Must get that book out . . .

It's the same as the method outlined in Maritn B's tutorial on SGL.

Olly

[Astrosurf]

It's the same as the method outlined in Maritn B's tutorial on SGL.

Olly

Thanks Olly

[Ju_Cooper]

I know that the question was how to do this in Photoshop, but I've recently been playing with PixInsight's Morphological Transformation process for this type of issue.

For example these trailed stars:

Were transformed with these settings:

With this result:

This was a more exaggerated example than I would normally use it for and some of the residual background noise has been "eroded" out of existence (I had clipped the background a lot deliberately in the example so that the trailed stars were more obvious). The shape of the structuring element needs quite a bit of trial and error to get right, but in general the the white part of the structuring element should be parallel to the long axis of the trail. In regular use it is pretty much essential to mask the image so only the stars are transformed.

[Astrosurf]

Thanks Julian. I've heard of this sw and it sounds interesting. Of course using these techniques will also affect the DSO's appearance. As I haven't tried any of these techniques on my poor images I don't know if they'll work! Oh, for more time.

[Ju_Cooper]

Of course using these techniques will also affect the DSO's appearance.

This is definitely true, but can be almost completely avoided with the Morphological Transformation method by using a good star mask. Unfortunately, I can't seem to make a decent mask in PixInsight: I end up making the mask in Photoshop and transferring it back to PixInsight.

[Astrosurf]

This is definitely true, but can be almost completely avoided with the Morphological Transformation method by using a good star mask. Unfortunately, I can't seem to make a decent mask in PixInsight: I end up making the mask in Photoshop and transferring it back to PixInsight.

That's true. If I choose a day to dedicate soley to PS and be disciplined, I'll get something done. It's all too easy to have weekends taken up with family stuff. Evenings are for rest!