17 replies to this topic

[TamsyMan]

Hi All,

 

I recently upgraded my setup to monochrome and have found a weird dark ring found in my master light and flats. My equipment is an ASI533MM Pro, Redcat 51, Antlia 4.5nm Edge Ha 1.25" filter screwed in like this: https://www.cloudyni...-1613438879.jpg . I took 5 * 5 minute guided exposures using the ASIAir. I then took 20 flats and 20 dark flats and didn't take dark frames because I didn't think it was necessary for the camera.

 

In the individual stretched light frames, this pattern doesn't seem to exist. I would like to know what has happened here so that I can decide whether to return some of my equipment (within the 30 day period). I have attached some pictures to show what it looks like. I stacked in DeepSkyStacker and did just an autostretch in Siril for each of the images and then took a low res copy and posted them here.

Attached Thumbnails

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Edited by TamsyMan, 12 February 2024 - 03:37 AM.

[happylimpet]

You cant do a flat correction without subtracting darks, or failing that a bias, if you think that there is no dark current or amp glow. But just take darks.

 

While the above is definitely an issue, and will cause problems on its own, flats do look weird. Ice on filter or some optical component maybe? Hmm. No reason to think camera is problematic.

[Mike7Mak]

I then took 20 flats and 20 dark flats and didn't take dark frames because I didn't think it was necessary for the camera.

If you're not gonna use darks you need to at least subtract a bias from the light frames in order to have the flats work correctly.
 

[TamsyMan]

I don't think it is ice because I had a dew heater band on, it didn't get below 0 degrees last night and the flats look the same when taken indoors also.

[TamsyMan]

If you're not gonna use darks you need to at least subtract a bias from the light frames in order to have the flats work correctly.
 

I took a dark flat. Does that count as bias or not? I am going to start making a dark library now then.

[acrh2]

DSS is known to produce rings.

Here's my thread about it.

 

https://www.cloudyni.../#entry12186799

 

This only started happening after I switched to mono. Perhaps it has something to do with a very low background signal in mono images.

There are several other threads like mine.

After switching to stacking in Siril, I haven't seen any more rings.

 

(Also, you have to have darks for flat calibration. If you aren't taking darks (not recommended,) you need to make sure that the software uses flat darks or biases in place of darks. PixInsight does this automatically, but other software like DSS may not.)

[TamsyMan]

Thank you all very much for your help! I am going to start by making a dark library (being captured now) and I will try stacking in Siril to see if that helps. Many thanks and please keep the suggestions coming - who knows what the problem is!

[TamsyMan]

I made my dark library of 20 darks at the same temp, gain, exp length and applied it and stacked in Siril. But the image is not any better. I think it might be due to the filter. Does anyone know anything about how my 1.25" filter might cause this?

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[TamsyMan]

Hi guys. I've done some further testing. When I take out my 1.25" Antlia filter, the pattern goes away and when I move the filter forwards in the imaging train (towards the telescope), the pattern also disappears, as show in the attached image. I don't know what to do. Should I return the filter or does the flat frame below look good to start imaging? Thanks for all your help.

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[acrh2]

I have never seen that behavior.

If you post examples of light, dark, flat, flat dark fits files (linear data,) I can take a look and see if there are any issues. 

[Marcelofig]

I have the same camera and filter and I have never seen anything like it, I literally have zero vignetting.

 

One question, how exactly are you taking the flats, what is the exposure? For narrow band I usually lower the panel brightness until I get an exposure of at least a couple of seconds.

 

One test you can do is to stack the lights without any calibration and see if the problem is still present.

 

And by the way, the 533MM does not need flats darks, it works perfectly fine with bias.

[JF1960]

It must have something to do with the Petzal design of the RedCat51 and the fact you are using 1.25” filters.  But, you’d think mounting the filter closer to the sensor should fix the problem not make it worse…

[TamsyMan]

First of all, I have uploaded my final image of the Rosette with only 35 minutes of data, which I thought looked very good. I have uploaded a light, dark, bias, flat and stacked and calibrated light here: https://drive.google...oYr?usp=sharing . My flats are 10s long because I use an iPad and t-shirt.

 

I would like to know if I should return this filter and get maybe a 31mm version and a filter wheel (I am within the 30 days money back guarantee) or is it just something weird that has now been solved by moving the filter forwards. Finally, in the future, I am going to upgrade to a filter wheel and a full set of LRGBSHO filters. Given that when using a filter wheel, the filters are mounted very close to the sensor, should I return this filter as it might not work when I upgrade?

 

I am currently thinking that I should return the filter but what should I get instead - a 31mm filter and filter wheel? Please give your advice and reasons for why this ring occurred.

 

Thanks for all your help!

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[TamsyMan]

Oh and here are the images of the Horsehead with the ring problem: https://drive.google...TIN?usp=sharing

[acrh2]

Oh and here are the images of the Horsehead with the ring problem: https://drive.google...TIN?usp=sharing

I looked at the files you posted.

 

1) The old flat for the Horsehead image is bad. It seems to be the sole cause of the ring artifact - it's present in the flat but not in the lights.

Flat_10.0s_Bin1_20240211-222713_0020.fit

 

Not sure what happened. Maybe the focus was way off? Ideally, you want to be in focus for infinity when taking flats, at that temperature. A little bit off is ok. Short of retaking flats, you could try and use the flat from the Rosette image. It should work fine if your imaging train hasn't changed. I usually reuse flats when I don't change my imaging train, and I haven't seen a dust mote move but once in 3 years.

Moving the filter probably did nothing. Taking new flats is probably what changed - check the new flat against the old one and see if the ring is absent. 

 

2) You are underexposing your flats, by a factor of 3 to 5. It's not optimal, but it's all not a huge deal.

 

3) The filter is not bad. It's not great, it has a dark pattern that you would normally not expect. But this pattern does calibrate out fine, like it did in the Rosette image.

 

4) With regard to getting a mono setup. Consider that in the future you might want to go with a larger sensor, like APS-C. You'd want 36 mm filters for APS-C. 

There's a difference in filter cost of about $40-$50 per SHO filter between 36 mm and 1.25". But if you resell smaller filters, you'd probably be losing money anyway.

In my opinion, a filter wheel is well worth the investment - it's convenient, it's the only way to achieve full automation, you can use filter offsets for faster autofocus (if you use a PC) etc.

[TamsyMan]

Hi Mercury Atlas and thank you for all your help - it has been fantastic. I retook flat with an exposure at 50% along the histogram , infinity focus, -10c cooling etc. However, the ring artifact is still present. Although I could just use the flats from the Rosette image, I think that this is too inconvenient and not what I paid for. I am therefore going to return the filter and, instead, I am going to go for the 36mm filter and filter wheel. I was going to buy a filter wheel anyway and, despite the filter being £50 more expensive, it's good for future proofing and, although I'm not going to buy a new camera any time soon, I don't want to deal with any more issues so 36mm is just a safe option!

 

Thank you all for all of your help and, despite the issue not being fixed, I now know I am going to return the filter. Many thanks for your time and effort.

 

TamsyMan

[happylimpet]

I think the bright ring could be from reflections from the shiny inside of the filter ring, hence changing the distance changed the location of these.

Edited by happylimpet, 13 February 2024 - 04:37 PM.

[TamsyMan]

I think the bright ring could be from reflections from the shiny inside of the filter ring, hence changing the distance changed the location of these.

That sounds very possible. Would this be solved by getting the 36mm filter and filter wheel? Also, should I get the Antlia 4.5nm filter again or should I go for a 3nm one (I live in a Bortle 9 zone).