Bad blue stars - Forums

Hi! I am coming to you for your support, my problem is the following: I am not happy with my blue stars, the yellow and red ones come out perfect but the blue ones are not. As you can see in the image, they come out as two colors and when I want to highlight the color I can't get the blue color to be uniform throughout the star. My equipment is: William Optics FLT91, WO 68III Flattener, Atik 414 EX Mono Camera, ZWO Filters and Wheel. Generally the exposure time I use for the RGB channels is 300 seconds under a Bortle 2 sky and between 1 and 2 hours of integration per channel. Should I drastically reduce the exposure time? Is there a defect in my tube or filters? I process them in PI and use the most up-to-date tools available. I notice that since I combined my RGB channels, the resulting image already shows signs that my blue stars are wrong. I would greatly appreciate any suggestions please. Thank you and clear skies!

Probably is the optics, assuming it is lateral color aberration and it is a function of the distance from the optical axis. Without a full view and better still, having the RGB raw frames is hard to tell.

Welcome to the world of chromatic aberrations. We had a proverb for this while studying optics "Je blau, desto brech". Meaning freely translated, blue wavelength are refracted the strongest (in the visible spectrum), which might be why you only see this issue with blue stars.

I would guess chromatic aberration from a budget refractor (like mine lol) but you seem to have Newtonian diffraction spikes on some brighter stars? A wo scope shouldn't do that.
Also you shoot mono which makes this triply weird.
Look forward to reading the cause and fix

TiffsAndAstro:
I would guess chromatic aberration from a budget refractor (like mine lol) but you seem to have Newtonian diffraction spikes on some brighter stars? A wo scope shouldn't do that.
Also you shoot mono which makes this triply weird.
Look forward to reading the cause and fix

I guess you can still suffer from CA with a mono, if you get different focus size for different wavelength from bad optics?

andrea tasselli:
Probably is the optics, assuming it is lateral color aberration and it is a function of the distance from the optical axis. Without a full view and better still, having the RGB raw frames is hard to tell.

Thanks for your reply. Is there anything I can do in case it is my problem? Thanks!

D. Jung:
Welcome to the world of chromatic aberrations. We had a proverb for this while studying optics "Je blau, desto brech". Meaning freely translated, blue wavelength are refracted the strongest (in the visible spectrum), which might be why you only see this issue with blue stars.

It's a very sad world! Thanks for your reply.

Kristof Vandebeek:
How do you focus?

The focus was done automatically with my EAF and it was done with each filter change. Thanks for your reply.

TiffsAndAstro:
I would guess chromatic aberration from a budget refractor (like mine lol) but you seem to have Newtonian diffraction spikes on some brighter stars? A wo scope shouldn't do that.
Also you shoot mono which makes this triply weird.
Look forward to reading the cause and fix

My tube, although not one of the most expensive, is not very cheap either. It is strange that my WO refractor throws spikes, they are not artificial at all, they are natural, but as you say, only in large stars. Is that serious? Thanks for your reply.

It does indicate a problem. You can get spikes with long integration subs and hard stretching in a refractor but it's usually sort of random in terms of angle. Here you have 4 perfectly placed diffraction spikes, like a Newtonian. The spikes are short and fat so there are at least two things intruding into the optical path, 90 degrees apart. The first thing I would do is pull the camera and have a look up through the instrument and see if there's anything obvious. The next thing to try is imaging a bright star without the flattener for filters. If that's clean, add the flattener and finally the filters. It's a matter of just trouble shooting one step at a time. Good luck!

Tony Gondola:
It does indicate a problem. You can get spikes with long integration subs and hard stretching in a refractor but it's usually sort of random in terms of angle. Here you have 4 perfectly placed diffraction spikes, like a Newtonian. The spikes are short and fat so there are at least two things intruding into the optical path, 90 degrees apart. The first thing I would do is pull the camera and have a look up through the instrument and see if there's anything obvious. The next thing to try is imaging a bright star without the flattener for filters. If that's clean, add the flattener and finally the filters. It's a matter of just trouble shooting one step at a time. Good luck!

Thank you very much for your comments. I will do what you suggest and I will let you know the result. Clear skies.

Enrique Ojeda:
Thanks for your reply. Is there anything I can do in case it is my problem? Thanks!

Get BlurXterminator, that should sort it.

Really depends what the problem is Enrique but I would say that anything short of "that's what the objective does" should be fixable.

Besides the obvious chromatic aberration, I'd be looking at why you're getting those spider-vane-like diffraction spikes. It's a refractor - there should be absolutely nothing in the optical path causing those diffraction patterns.

Jonny Bravo:
Besides the obvious chromatic aberration, I'd be looking at why you're getting those spider-vane-like diffraction spikes. It's a refractor - there should be absolutely nothing in the optical path causing those diffraction patterns.

Yes, I've been told that this is something extremely rare. I will do the tests that Tony tells me and at the same time I will send an email to WO to see if they will help me and give me feedback. I have never hit my tube, nor have I dropped it, nor have I altered it in any way. In my stacking I apply Drizzle since the combination of camera and optics gives me an undersampling. I mention this in case perhaps the problem was something between the stacking and processing, which I don't think is the case. I will try with another camera from a colleague to see if the same thing happens, I don't know if certain combinations of cameras and tubes simply do not make a good team ='(. As soon as I have news I will let you know. Thank you very much.

andrea tasselli:
Enrique Ojeda:
Thanks for your reply. Is there anything I can do in case it is my problem? Thanks!

Get BlurXterminator, that should sort it.

It's what wo say for their new scope lol ;)

that spikes effect happens to me when mi cat drop some hair in the lens , but not perfect aligned like a cross , thats is weird

that spikes effect happens to me when mi cat drop some hair in the lens , but not perfect aligned like a cross , thats is weird

I tried with a zwo asi 294mm pro camera and the spikes don't come out and the blue stars come out much better. Could it be my Atik 414 ex mono that is the problem? Thanks!

I am not sure how it could be the camera but any obstruction between the sky and the chip can be the cause.

Tony Gondola:
I am not sure how it could be the camera but any obstruction between the sky and the chip can be the cause.

Thanks Tony. The weather has not allowed me to do the tests you suggested. As soon as I can, I will do them. Regards.

Enrique Ojeda:
that spikes effect happens to me when mi cat drop some hair in the lens , but not perfect aligned like a cross , thats is weird

I tried with a zwo asi 294mm pro camera and the spikes don't come out and the blue stars come out much better. Could it be my Atik 414 ex mono that is the problem? Thanks!

Regarding the blue stars: Probably the asi294 has a UV blocking window in front of the sensor and the other camera proabably has a clear glass window. That would be my guess. By blocking smaller wavelengths you can get rid of the chromatic aberration problem. If you have a UV/IR cut filter, you could try this and see if you still get bloated blue stars.

Regarding the blue stars: Probably the asi294 has a UV blocking window in front of the sensor and the other camera proabably has a clear glass window. That would be my guess. By blocking smaller wavelengths you can get rid of the chromatic aberration problem. If you have a UV/IR cut filter, you could try this and see if you still get bloated blue stars.

I don't think so. Firstly because the ASI294mm hasn't got a UV filter and secondly because the colors are created by filters with UV/IR cut-off. Besides, UV would probably not reach the same focus of the RGB filters at all and would only contributes a glare in the luminance.

andrea tasselli:
Regarding the blue stars: Probably the asi294 has a UV blocking window in front of the sensor and the other camera proabably has a clear glass window. That would be my guess. By blocking smaller wavelengths you can get rid of the chromatic aberration problem. If you have a UV/IR cut filter, you could try this and see if you still get bloated blue stars.

I don't think so. Firstly because the ASI294mm hasn't got a UV filter and secondly because the colors are created by filters with UV/IR cut-off. Besides, UV would probably not reach the same focus of the RGB filters at all and would only contributes a glare in the luminance.

I think you may be right. I Just checked the QE curves of the cameras and both are pretty sensitive at 400nm and below. If the same filters are used, then both cameras should show the problem, if it is CA. However, On my Samyang 135 I also had this problem with a Hoya UV/IR cut filter which filters at about 400nm and with an Astronimik L-3 which has a cutoff at 420 the problem was gone. 20nm can be a lot, but I did not investigate that too much to be honest. In any case this obviously does not explain Enriques problem as you said.

Tension optics, you should loosen the cell screws.