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Cross shaped stars in image

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#1 Vikr

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Posted 29 November 2021 - 01:00 PM

Hi everyone, I am just beginning to start astrophotography. Recently I tested out my equipment and I have a couple of questions.

 

Firstly, I use a Nikon D90, Skywatcher 130 PDS and an EQM 35 pro mount as my setup.

 

 I decided to point my telescope at the Double Cluster last night just to see if everything was working properly.

 

I took 50 or so 10 second subs at ISO 800. However, upon examining the photos I noticed there are cross shaped stars in the image, mainly towards the fringes. I have attached a close up of the bottom right part of one 10 second sub. Is this an issue with tracking or vibrations, or perhaps an issue with collimation? problem.jpg

 

Another question: when I tried processing the image in GIMP, I noticed a discernible darker ring around the fringes of the stacked image. Is this coma and should I invest in a coma corrector or can I correct in GIMP?

 

Thanks everybody for your help!



#2 astrokeith

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Posted 29 November 2021 - 01:11 PM

Looks like simple (very) bad collimation.

 

How are you doing your collimation, and do you star test afterwards?



#3 Vikr

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Posted 29 November 2021 - 02:27 PM

Looks like simple (very) bad collimation.

 

How are you doing your collimation, and do you star test afterwards?

Hi Keith,

 

Thanks for the quick reply. I am doing my collimation using the collimation option AstroPhotography tool. Thanks for pointing this out, I was unsure as to whether this was collimation so I will try to recollimate. I neglected to do a star test this time.

 

Best,

Vikram



#4 mayhem13

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Posted 29 November 2021 - 09:40 PM

That’s Coma……and absolutely normal for a Newtonian. You’ll need a Coma corrector to continue in astrophotography……but be advised, a quality coma corrector costs more than your scope…..you may want to start with replacing the scope instead with a wide field doublet or triplet refractor since your just starting out…..less to worry about when imaging….where there’s literally everything to worry about! Lol



#5 DivisionByZero

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Posted 29 November 2021 - 10:13 PM

That does look like coma. My recent astrobin images are largely uncropped and you can compare with my corners (coma corrector is on my wish list). The thing about coma is that it can help you check collimation. Make sure all the tails point to the image center and you have it!

#6 Kevin_A

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Posted 29 November 2021 - 11:04 PM

Coma.



#7 michael8554

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Posted 30 November 2021 - 08:40 AM

Coma is usually only seen at the edges of the images, yours is also in the centre of the image.

 

So this is Coma caused by poor Collimation.

 

If after a good Collimation you still see Coma at the edges, then you will need a Coma Corrector.

 

These star shapes are often called "seagulls" or "teardrop shaped"



#8 alphatripleplus

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Posted 30 November 2021 - 09:10 AM

Coma is usually only seen at the edges of the images, yours is also in the centre of the image.

 

 

I think the OP mentioned that the image shown is just a close-up of the lower right part of the field of view. If it looks like that at the centre of the field of view too, it's probably poor collimation.


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