"Colorful" Stars - RC needs collimation? - Forums

Topic starter

Mar 26, 2018

Hey fellow RC-Users,

another thing gives me a headache these days. Please take a look at my M67:
M 67
The stars are blueish in the upper left corner, greenish in the lower left corner and orangeish (<- is this a correct english word?) in the lower right. I wonder where this comes from. You can see it - more or less - in nearly every picture of me. I remember when i got the RC i collimated the secondary using the following steps:
- Slew to a bright star and defocus a little bit.
- The star turns into a ring
- That ring should be round. If not:
- Use the screws on the secondary to adjust it until it is round.

Maybe the ring i got was not really round? Or is my collimation okay and the issue has nothing to do with collimation? Any thoughts? How can i check if my collimation is REALLY okay?

Thanks and CS,
Michael

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Jose Carballada

IOTD Staff

Mar 28, 2018

Michael:
Hey fellow RC-Users,another thing gives me a headache these days. Please take a look at my M67:
M 67
The stars are blueish in the upper left corner, greenish in the lower left corner and orangeish (<- is this a correct english word?) in the lower right. I wonder where this comes from. You can see it - more or less - in nearly every picture of me. I remember when i got the RC i collimated the secondary using the following steps:
- Slew to a bright star and defocus a little bit.
- The star turns into a ring
- That ring should be round. If not:
- Use the screws on the secondary to adjust it until it is round.

Maybe the ring i got was not really round? Or is my collimation okay and the issue has nothing to do with collimation? Any thoughts? How can i check if my collimation is REALLY okay?

Thanks and CS,
Michael

Seems to me a little bit burned, maybe could you try with gain 0?
And for what reason do you bin? I think is better not bin cmos cameras specially because on this cameras is not a hardware binning is a software bin and it's better to do it after deconvolution.
Another info, in my experience I saw some troubles with colors when you are compiling one channel before meridian flip and another after meridian flip, I hope this help you.

Michael Timm

Topic starter

Mar 29, 2018

Jose Carballada:
Seems to me a little bit burned, maybe could you try with gain 0?

Seems like a good idea to me. I wanted to try with gain 0, but i am afraid i have to expose too long :-(

Jose Carballada:
And for what reason do you bin?

Same reason like above. Exposure times drop by about factor 3-4 when binning. The combination of the very small pixels (2.4 um) with the relative long focal length lead to over-sampling. The resolution is about 0.3" per pixel. When binned, its about 0.6" per pixel, which seems to be better. Well, maybe i'm wrong with binning?

Jose Carballada:
Another info, in my experience I saw some troubles with colors when you are compiling one channel before meridian flip and another after meridian flip, I hope this help you.

There was no meridian flip involved. Thanks a lot for your reply, of course this helps ;-)

I have another thought:
Look at the smaller stars in this picture:
M 17
They show the same behaviour and this picture was taken with my small newton. Conclusion: The strange colors have nothing to do with the collimation of the RC. Maybe they are in fact a little bit burned, that some of the color-information "jumps" to the pixels in the neighbourhood? Or this is a side-effect of over-sampling? Anyways, on the long run i will buy a ASI 294 Pro which will provide me with bigger pixels, cooling and a lot higher full well :-)