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unexpected spike of light on strong stars

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8 replies to this topic

#1 michele

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Posted 20 February 2019 - 07:47 AM

Hello friends of Cloudnights!
This is yet-another-question about my camera weird behaviour. The camera is a mono SBIG STF 8300, I bought it second hand a couple of months ago.
Now the problem: there is a sort of spike (or blooming? not sure) on the brightest stars in my frames and I'd like to understand from where does it come and how to get rid of i. I don't want to be forced to remove it by hand with clonestamp-like tools.
Here one example

 

https://www.dropbox....__-10C.fit?dl=0

(bin 1, G filter, 300s exposure)

 

This is another example:

 

https://www.dropbox....__-10C.fit?dl=0

(4 seconds bin 1, L filter)

 

Please note, same strange spike in two very different telescopes: a small apo (WO Star71) and a 8" f4 newton (skywatcher wide photo). Images were taken on two different nights.
One thing worth noting is that it seems to affect only G and L filters, there are no traces of it in frames taken on the same subject with B and R filter (B and R frames were taken in a different night and the histogram peak is slightly higher for G filter, nothing really relevant I would say, 2500 ADU in R and B vs 2700ADU in G)

any idea?

thanks

 

Michele



#2 AIP

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Posted 20 February 2019 - 09:04 AM

typical effect of dirty filters with grease. Verify that you don't have fingerprints
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#3 michele

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Posted 20 February 2019 - 12:37 PM

Thank you! I'll check ASAP

Michele

#4 michele

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Posted 24 February 2019 - 02:40 PM

Reopening this issue because, apparently, I was wrong, the same spike appears on all 4 filters

 

https://www.dropbox....Tb9-xq7Hda?dl=0

 

the camera is new (bought used, actually) so I didn't have enough frames to check all filters properly, but this afternoon I cleaned L and G filters (without touching the others) and then I made a test on Rigel for all four filters and I found the same spike in all of them.

 

So, to recap:

 

- in every filter

- with two different telescopes

- the fact that the M45 picture showed the spike only with L and G filters may lead to two different conclusions: maybe the problem appeared between the two sets of pictures (taken in several nights, L and G were the last) or maybe the brightness of the subject wasn't enough to reveal it

 

it looks like a problem in the camera itself, any suggestion?

 

thanks in advance

 

Michele



#5 AIP

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Posted 24 February 2019 - 02:45 PM

Rotate the camera 90ยบ and take a new photo. If the spike is in the same position the problem is in the camera.

Verify that you don't have fingerprints or dirt in the optical glass of the camera


Edited by AIP, 24 February 2019 - 02:46 PM.

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#6 Monkeybird747

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Posted 25 February 2019 - 09:08 PM

The rotation test AIP mentioned should help narrow it down. What, if anything, is common between the two telescope setups besides the camera? Could it be some internal reflection off of an oag prism, adapter ring, filter wheel casing, etc? At first I though blooming blooming due to saturation of bright stars, but I don't know if blooming feathers out like that.

 

What kind of filters?



#7 michele

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Posted 26 February 2019 - 12:12 PM

thank you both,

   yesterday I made another test and ... no spike anymore?!? actually there were high clouds so they may have hidden it, I'l make another test tomorrow and if I spot it (not difficult: 5s exposure on Rigel) then I will rotate the camera to see what happens

 

Michele


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#8 happylimpet

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Posted 27 February 2019 - 04:36 AM

something has (or had) a smear on it, or a hair, or something like that....


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#9 michele

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Posted 27 February 2019 - 02:35 PM

Hi friends of cloudynights,

    I have news!

 

Here are the thest exposures:

https://www.dropbox....fsO9HUM6wa?dl=0

 

the experiments I made using the camera rotator:

 

A. Spike is present in my reference position/rotation, with "wide" side on DEC

B. Spike is present and more bright if I rotate clockwise the camera to have the "wide" side on RA, it does not change its orientation with respect the image frame

C. Spike is not present (there is a faint trace actually) if the rotation occurs counterclockwise - it is a pity because in this position the filter wheel cables interfere with autoguider!

 

 

spike is only on brightest starts, for example on pleiades I have one star with it with 10 minutes subs on L or G, Rigel needs just 30s, Rosette field does not show any trace of it with 10m in R, G and B

 

this is really a strange result that leaves my without many ideas!

 

For sure, the spike is related to something internal to the camera, because it does not rotate, at the same time this being linked to the rotation position may be ... an external source of light maybe? there are some

 

any other idea?

 

thanks

 

Michele




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