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Integrating Colour Data from a 10" RC tube
This is a strange one. that may not be caused by APP but I'm hoping that there is some ways to remove this really pronounce vignette (Picture Attached).
I normally use the ASI1600 with this telescope but I got this new toy (ASI2600) and tried it on this F8, 2032mm scope. The results where terrible... First of, the light and flats have these pronounce colour circular thing... see picture. And it doesn't all go away on calibration. I use SGP to get my data and I have to force the Bayer CFA to RGGB, Adaptive Airy Disc. (I hope this is right).
Any idea how to remove this? Does anyone understand this? Its true that the signal is weak in the city but still...
Here's the integrated light
And a single light
Any help, very much appreciated.
Difficult one, it almost looks like a lot of internal reflection or something to do with the way the new camera is set-up?
Check that your extension tubes are good - i.e. not shiny on inside surface or inside threads scattering the light
Indeed probably telescope stray light. Remove the camera, point the telescope at the daylight sky, put your eye at the spot where the sensor should be and look into the telescope. Move your eye around a bit over the field of where the sensor should be. You should see the telescope aperture but nothing else.
An example of stray light caused by a reducer: on the left without reducer, on the right with reducer, note the prominent extra bright ring in the right hand picture.
This effects could be reduced by improving the baffling in the telescope. This could be as simple as glueing flocking paper on reflecting surfaces, or adding an extra long dew shield, but it really depends on just where the stray light is coming from.
There's also the option of removing this in post processing but that would require knowledge of where the stray light comes from and something like a stray light correction algorithm. So far I've never seen this even mentioned in amateur astrophotography. It is not easy 😉
Here's another example of a GSO RC6 telescope. The images from left to right show the entrance pupil of the telescope as seen from the sensor position, as I moved the camera from the center of the telescope focal plane to the edge.
Note the narrow crescent in the dark disk of the secondary mirror, and the big crescent in opposite direction outside the primary mirror footprint.
Hi Ralph, Thank so much for the response and detailed explanation. I agree that it is definitely some light rays but might just be from light polution. I have done more data gathering yesterday using the VC200L Vixen. I had the same issue when stretching on images of M81. This is a new problem for me. This scope has always been very good to me when I lived in the UK a few months back. Now in Hamilton Canada. The sky pollution is much worst. Taking galaxies is proving to be very hard under these sky condition. Especially with a colour camera. I will try tonight with my mono ASI1600 and see the results. I believe since a mono is more sensitive that the SNR will be better and more signal should mean less stretching. The sad thing is that I want to use this new toy (ASI2600)...
I will do as you explained today to help me further understand the effect of light ray. I can definitely see the crescent.
In your experience. Given the limitation that I have (See attachment) with my light pollution. Wanting to take in the 2000mm range for galaxies. Is there a specific filter and setting that comes to your mind that would be most successful? For example, Would it be better to BIN2 for sensitivity and reduce Exposure time? I which there was a magic formula that would give you the best setting and exposure time :).
OK, long story here below...
There are many factors contributing to the "noise" of the image. Just to name a few:
- Read-out noise, present even with 0 second exposure time and 0 photons recorded
- Shot noise of the recorded signal. Photons arrive in a more or less randomly timed fashion. The more of them you collect, the better you know the average value. And to be complete correct, it's actually the number of detected electrons rather than photons. Photons are converted (with an efficiency called Quantum Efficiency) to electrons. But many other things also generate electrons, like dark current and amplifier glow. The shot noise is the square root of the number of detected electrons, so if you want to double the signal to noise ratio (SNR), you have to collect 4 times as many electrons.
- Dark current and bias. I'll skip over these as they are more or less covered under the previous two bullets.
- Flat field pattern. This is the part that's bugging you now. You have a uniform illumination, and you'll detect a non-uniform signal due to optics design, dust on filters and sensor, sensor non-uniform sensitivity, scattered light in the telescope, and plenty of other reasons. Now this is generally not considered a noise term, since it is nicely fixed and you can correct for its effects by taking a flatfield image. But, you'll never have a perfectly representative flatfield image, if you catch 99% of the variation than in general its doing a great job. So that leaves in this example a 1% error of the flatfield correction. I'll call this "flatfield noise" for now.
When you take photo's under dark skies you'll have e.g. 1 unit of sky background, and in your faint object 10 units of light (I'm just making up some numbers here). This means that 1% of 1 unit sky background is not corrected by the flatfield, so you have 0.01 units flatfield noise, on ten units useful light. That's an SNR of 1000 here for flatfield noise, and you're probably dominated by shot noise and read-out noise in your image, to say SNR = 100.
Now take your kit to town and take the same photo there: you could have 100 units of sky background, and still 10 units of useful light. Now, 1% flatfield noise is 1 unit, reducing the SNR = 100 under dark skies (dominated by other noise sources) to SNR = 10 under urban skies, dominated by flatfield noise. And the nasty thing of flatfield noise is that it is a systematic, and taking more exposures doesn't reduce it, whereas the contributions of shot noise and read-out noise do reduce when taking more exposures. So you're basically limited by the quality of your flatfield, and no binning or longer exposures is going to save you.
What you could do is reduce the flatfield noise by reducing the sky background (without reducing the useful light by the same amount). Light pollution filters will help you there, in particular narrow band filters. But the latter will limit you to emission line objects, since regular stars are also suppressed by narrow band filters.
Alternatively, you could spend a lot more effort on getting good flatfields. But that may mean carefully matching the illumination pattern of your flatfield exposure to the conditions you experience during light polluted night time photography. And this could mean matching out-of-field illumination in case of out-of field stray light, and matching the spectrum of the flatfield in case of spectrally dependent flatfield effects.
I chose to go for narrow band filters when doing photography in my back yard in the city, and travel to dark places if I want to do something else.
I had to read this a few times and scribble the maths just to make sense of it but I got it. Makes sense and I now understand the limitations. thank you! Yes narrow band is working but limited to nebulas and its galaxies that I want to take :). Luckily I have a chalet north of Ontario with much clearer sky. This gives me the excuse to go that I needed :).
Awesome stuff Ralph, thank you!