Poor Collimation, Guiding or Spacing

[Dec]

Hi All,

I'd really appreciate some advice on my new imaging setup. To date most of my imaging has been with an Altair ED80 triplet but I have recently invested in a 10" skywatcher Quattro to use with my Atik 383L as I'm really interested in imaging galaxies.

My mount is an NEQ6 and my imaging train is a 383L, Atik EFW, and a SW Aplanatic coma corrector (with spacing adaptors to set the CC at 55mm from the CCD).

I took some test subs last night (a stretched 300s Lum sub is shown below) but I appear to be getting egg shape stars round the outer edge of the image.

My question is, are the stars a result of poor collimation, incorrect spacing of the Coma Corrector, poor guiding or a combination of these.

I suspect it may be mostly down to collimation as I have never owned a Newtonian before and have been really struggling collimating this scope despite reading almost every collimation thread/tutorial on here.

I know some of the more experienced imagers on here will be able to tell what the problem is just by looking at the attached image.

Any feedback or advice is really appreciated.

Thanks

 

dec

 

[Oddsocks]

20 minutes ago, Dec said:

I know some of the more experienced imagers on here will be able to tell what the problem is just by looking at the attached image.

Unfortunately the attached image is just too compressed to evaluate. Zooming in, the stars are cubes and you can't tell too much from that!

Try reposting with a link to the raw FIT in a free, shared, Dropbox folder or free, shared, Google drive folder, or an uncompressed TIF here in the forum.

Just looking at the uneven illumination across the image with vignetting only evident in the bottom right and left corners that suggests collimation is not quite right.

When you look at the eggy stars in your raw FIT are they egg shaped in a radial or tangential direction, i.e. all with tails pointing away from the centre of the image or all seemingly aligned to an invisible circle that is running around the outside of the image? if so that is most likely corrector spacing not quite right.

If the eggy shapes are all aligned to a common axis across the image but appear worse towards the edges then that is tilt.

If guiding is suspect the eggy shapes would appear aligned to common axis and be present equally in all stars in the image.

 

 

[Dec]

15 minutes ago, Oddsocks said:

Unfortunately the attached image is just too compressed to evaluate. Zooming in, the stars are cubes and you can't tell too much from that!

Try reposting with a link to the raw FIT in a free, shared, Dropbox folder or free, shared, Google drive folder, or an uncompressed TIF here in the forum.

Just looking at the uneven illumination across the image with vignetting only evident in the bottom right and left corners that suggests collimation is not quite right.

When you look at the eggy stars in your raw FIT are they egg shaped in a radial or tangential direction, i.e. all with tails pointing away from the centre of the image or all seemingly aligned to an invisible circle that is running around the outside of the image? if so that is most likely corrector spacing not quite right.

If the eggy shapes are all aligned to a common axis across the image but appear worse towards the edges then that is tilt.

If guiding is suspect the eggy shapes would appear aligned to common axis and be present equally in all stars in the image.

 

 

Thanks Oddsocks. Appreciate your help. Hopefully this link will work.

https://www.dropbox.com/s/a9102p3iicdc8no/L_Luminance_Bin1x1_300s__-20C.fit?dl=0

The egg shapes do appear radial (although not uniformly). Could this be a mixture of collimation and corrector spacing.

[tomato]

Not much help, but the elongation doesn't appear to be down to one definite cause,  There are a couple of stars in the top left quadrant that appear to be elongated  at 90 degrees to each other, but I'm not sure how this could happen on single sub.  

As Oddsocks has pointed out, Is the vignetting only on one side of the image significant?

[Dec]

2 minutes ago, tomato said:

Not much help, but the elongation doesn't appear to be down to one definite cause,  There are a couple of stars in the top left quadrant that appear to be elongated  at 90 degrees to each other, but I'm not sure how this could happen on single sub.  

As Oddsocks has pointed out, Is the vignetting only on one side of the image significant?

Thanks tomato, this may be down to my attempts at collimation. Despite following several tutorials and using a Cheshire collimator and collimator cap, I am still struggling to achieve accurate collimation.

 

 

[tomato]

I sympathise, my first scope many, many moons ago was an 8" F4 Schmidt-Newtonian and I struggled like crazy to get it properly collimated. I also used to regularly chuck it on the back seat of my car to take it to a dark site which definitely didn't help.

However, please don't despair, there are plenty of fast Newtonian users  on SGL who have mastered the skill, sometimes reading every thread going on the subject can result in info overload. Maybe giving one of the folks at FLO a call might help?

[Dec]

2 minutes ago, tomato said:

I sympathise, my first scope many, many moons ago was an 8" F4 Schmidt-Newtonian and I struggled like crazy to get it properly collimated. I also used to regularly chuck it on the back seat of my car to take it to a dark site which definitely didn't help.

However, please don't despair, there are plenty of fast Newtonian users  on SGL who have mastered the skill, sometimes reading every thread going on the subject can result in info overload. Maybe giving one of the folks at FLO a call might help?

Thanks tomato. That's a good call. I'll give the guys at FLO a bell.

I'm hoping if I can eliminate poor collimation from my setup it'll make it easier to overcome some of the other issues.

 

 

[Oddsocks]

There are quite a few different issues apparent in the dropbox fit, looks like a mixture of collimation and tilt.

In this case the best starting place is to use coma to help you diagnose where the problems lie and begin with no flattener fitted.

Take an image of a rich star field at short exposure time and without the flattener fitted and you should clearly see coma in the frame edges, if the image train is properly collimated the coma tails will all point away from the frame centre and be equally distributed around the frame edges.

If the system is not properly collimated then coma will be not equally distributed, it may be more severe on one side or one corner, the direction of the coma tails will indicate which direction is tilted if tilt is present.

Make your collimation adjustments and once the image shows an equal distribution of coma then fit the flattener and adjust the spacing.

For fast Newts an autocollimator will help enormously, I use one for my OO AG8.

I will send you a PM with some collimation documents that you may find helpful.

[Dec]

Thanks Oddsocks. Just noticed this morning that my motorised focuser might be the reason for the tilt as it's tight up against one of the adjustment bolts for the spider vanes.

Next clear night I'll remove the coma corrector and check the direction of the coma.