Star Stretching to avoid bloated stars

[Peje]

On 26/04/2018 at 23:42, Oddsocks said:

I have had a good look at the new additions Pete.

The star's airy disk shape in the non-CC images looks good and the disk is nicely round, the diffraction spikes are sharp and well defined. Although there aren't many stars to look at in the image they look just like I would expect from a fast newt on a camera with large pixels. Apart from the coma there is nothing wrong with these images at all.

In the 'with CC' images the star's airy disk is a diamond shape, or square, depending on your viewpoint, and the diffraction spikes are ill defined.

 

I am no optical expert but with the limited experience I have to date I would say the CC you are using is possibly faulty, not a good match for the telescope or a materials QA issue.  

The image the CC produces is very similar to what you would expect with a 'pinched' mirror. Maybe the lenses are too tight in the CC body.

Try loosening the CC's lenses retainer rings and then a gentle retighten, the lens retainers should just touch the lenses, if you were to shake or tap the CC the lenses should be able to move very very slightly, if they are held tighter than that then the lenses may distort leading to the problems I see with the star shapes.

If you try the above and it makes no difference and if the CC is still under warranty try getting it exchanged, or try a different CC. I seem to recall the Quattro's had a dedicated flattener for these fast newts so maybe finding a reasonable priced alternative is not going to be easy. Hopefully just a tweak to the CC lens retainers is all that is needed.

HTH

William.

Finally got the SKywatcher f/4 CC. The difference is extremely noticeable. I'm comparing images from Tuesday with old CC to one from tonight with the new one.

[Oddsocks]

That’s quite a marked improvement Pete.

The forum image is fairly low definition but looking at the Aplanatic image and zooming into the small pairs of double stars in the upper-right, outer edge of the nebula shows a clear, dark separation between the pairs which is almost totally absent in the Baader MPCC image. Overall, the Aplanatic image appears much sharper with smaller, compact stars and improved contrast (though that might just be processing).

Hopefully another issue put to bed and if all your future images show the same improvement then the investment in the Aplanatic flattener will prove worthwhile.

William.

[Peje]

Thanks William.

These images are the raw 600s ha subs in SG Pro given a low stretch. The difference blew me away.

Initially I thought it might have been a focus mismatch but I remember spending quite a while getting good focus on Tuesday night.

[ollypenrice]

There are some radical solutions to the star size problem available in Layers based programmes, the most extreme being to de-star a copy of the final image entirely then produce a soft stretch from the linear data which you then paste onto the starless one in blend mode lighten. This sounds all very well in theory but, so far, I've never found a satisfactory way to de-star an image.

There are other ways to use layers. This one works very well for widefield images with galaxies and faint fuzzies within it.

1) Make a hard stretch to drag out all the faint stuff but be very careful to get the background sky to an exact and even value. (I aim for 23/23/23.) Don't worry about the big stars.

2) From the linear data make a soft stretch using a curve which lifts the bottom more than the top. You are aiming to replicate, exactly, the background sky of the hard stretch while keeping the stars small and colourful. (They lose colour as they brighten, of course.)

3) Place the soft 'small star' stretch on top of the hard stretch and set the opacity to zero so you can't see it. (Bear with me!)

4) Use the eraser to run over all the galaxies and faint fuzzies in the image, varying the brush size as you do so, so as to take off just what you need. Use a well feathered brush.

Finally you bring up the opacity of the top layer and so subdue all the stars without subduing your precious galaxies and fuzzies. Flatten and you're done.

The key is getting a near-perfect match between the two background skies.

An example: 

Olly

 

[wimvb]

4 hours ago, ollypenrice said:

There are some radical solutions to the star size problem available in Layers based programmes,

Also in mask based programs:

http://pixinsight.com/tutorials/NGC7023-HDR/index.html#High_Contrast_Small_Scale_Structures

You don't need to use the masked stretch in the example; any stretch process will do. The idea is the same as what Olly described, just adapted to PixInsight workflow.

Btw, to make diamond shaped stars round again, use morphological transformation with an x-shaped (or +, depending on the orientation of the diamond) structuring element (3x3).

[Peje]

4 hours ago, ollypenrice said:

There are some radical solutions to the star size problem available in Layers based programmes, the most extreme being to de-star a copy of the final image entirely then produce a soft stretch from the linear data which you then paste onto the starless one in blend mode lighten. This sounds all very well in theory but, so far, I've never found a satisfactory way to de-star an image.

Olly

 

I spend some time trying to do starless images but never found a method / result I was happy with.

[Peje]

I gave in to temptation and stacked 11x600s with the Baader CC then 9x600s with the Skywatcher. The image below is just a stack of calibrated lights, no processing.

I was surprised that the detail increase is a little more subtle than I expected, but the star size reducion has been really impressive. Also, the star shape seems to have dramatically improved.

Bloated stars were the main thing that displeased me in my images so I'm really happy that this very simple change has made such a difference. Now I just need some clear sky to start capturing some nice data