Taking flats, all new to me...

[LunarView]

Hello,

I am new here and have a question that I could do with some help with..

I Have recently treated myself to a CCD camera and Atik, which has a shutter, I am trying to take some flats, Never used them before, and wonder what sort of duration I need as with the shutter it’s causing issues and they are always too bright, I have an EL panel as my flat field light source which is not adjustable, so any help would be appreciated..

thanks for looking

Dean

[Tomatobro]

Hi. Can you post the camera model and which software you are using?

[symmetal]

You need the exposure to be around 0.1 to 1 second to avoid shutter issues so the EL panel output need to be dimmed either by putting neutral density filters in front of it, or you can use a white T-shirt and/or sheets of white paper in front of the display to achieve the same effect. 

Alan

[ollypenrice]

34 minutes ago, symmetal said:

You need the exposure to be around 0.1 to 1 second to avoid shutter issues so the EL panel output need to be dimmed either by putting neutral density filters in front of it, or you can use a white T-shirt and/or sheets of white paper in front of the display to achieve the same effect. 

Alan

That seems short to me though I think it's the value recommended by Atik.  The folks I know using this camera go for longer, more like 2 seconds. I don't have one so have no direct experience. As you say, typing paper works a treat and is highly adjustable! 

It's a good idea to apply darks for flats, AKA dark flats, but the good news is that a master bias will do this job perfectly well. There is no need for dedicated flat darks with CCD cameras and short flat exposures.

Olly

[LunarView]

3 hours ago, Tomatobro said:

Hi. Can you post the camera model and which software you are using?

Hi

it’s the Atik 383l+ and was trying with Artemis...but am happy to use other software, I have just got APT too.. 

[symmetal]

Using the white T-shirt I forgot to say this is stretched over the front of the scope rather than draped over the panel which may have creases in view. 

2 hours ago, ollypenrice said:

That seems short to me though I think it's the value recommended by Atik.  The folks I know using this camera go for longer, more like 2 seconds. I don't have one so have no direct experience.

Do you know the reasoning why the folks think 2 seconds is better than 1 second Olly? 

Alan

[LunarView]

17 minutes ago, symmetal said:

Using the white T-shirt I forgot to say this is stretched over the front of the scope rather than draped over the panel which may have creases in view. 

Do you know the reasoning why the folks think 2 seconds is better than 1 second Olly? 

Alan

I know the question is not aimed at me...but

Is it to do with the shutter being seen in the image and causing a shadow...as I was having that issue with under 1 sec exposures...

[symmetal]

6 minutes ago, LunarView said:

Is it to do with the shutter being seen in the image and causing a shadow...as I was having that issue with under 1 sec exposures...

Interesting. Thanks LunarView. I'm surprised it's still visible near 1 second but you learn something every day. 

Alan

[ollypenrice]

1 hour ago, symmetal said:

 

Do you know the reasoning why the folks think 2 seconds is better than 1 second Olly? 

Alan

Yes, it's to avoid shutter wipe but if Atik say you can use far shorter flats subs then I guess they will be right. There seems to me (from memory) to have been an orthodoxy suggesting at least 2 seconds. This could be incorrect and here I am perpetuating it!

Olly

[symmetal]

Thanks Olly. Lunar replied he had the shutter in view at exposures less than 1 second (not sure how much less) but 2 seconds seems a good value to ensure it's not a problem. 

Alan

[Star101]

First off....I am OK at processing. Not great, just OK   ( My disclaimer ) 

If ? the camera needs 2 seconds for the shutter, then a light source ( could be daylight) and covering of the aperture will be needed.

First, cover the aperture of the scope with a white sheet, try to avoid wrinkles/shadows. If possible, point the scope at a light source.

The idea of Flats is trying to create a blank picture that only contains a picture of dust motes/particles along with the shadows your scope and camera display, as in vignetting.

When I create Flats using Artemis, I check the histogram graph and aim to get the WHITE number between 27000 and 32000 for my FLATS. These seem to work OK for me. 

 

Hope this helps.

[LunarView]

I was using exposures close to a second, will try lower, the problem is I have a flats panel, but it’s too bright at just under a second, and any ,lower I get the shutter shadow...so 2 seconds will be worse, so guess I need a new way to try and take the flats...

[Davey-T]

8 minutes ago, LunarView said:

I was using exposures close to a second, will try lower, the problem is I have a flats panel, but it’s too bright at just under a second, and any ,lower I get the shutter shadow...so 2 seconds will be worse, so guess I need a new way to try and take the flats...

How big is the panel ? you can dim it with some sheets of printing paper.

Dave

[Star101]

1 hour ago, LunarView said:

I was using exposures close to a second, will try lower, the problem is I have a flats panel, but it’s too bright at just under a second, and any ,lower I get the shutter shadow...so 2 seconds will be worse, so guess I need a new way to try and take the flats...

I use a thick white t shirt over the end of the scope when pointing at the light panel. Sometimes doubling the thickness so as to dim the brightness of the light. 

[symmetal]

If you need to stop a lot of light a neutral density gel filter may be worth considering. Here's a sheet of 4 stops ND costing £7 that Oddsocks posted in another topic. The sheet is 122 x 53 cm so you can cut it up to make several layers to stack giving you 8 or 12 stops etc. ND. numbers can be a bit confusing as it's a logarithmic scale. Here's a chart matching ND numbers to number of stops and transmission. 12 stops only allows 0.024% of the light to pass so you should find stacking a few layers will enable you to get an exposure of a second or more. Probably easier than 3 or 4 T-shirts on the front or a ream of paper in front of the panel. 

Alan

[Laurin Dave]

I have a shutter camera and use a Huion tracing panel and between 2 and 10 sheets of A4 typing paper to get exposures of 5-15 seconds for my flats ..  30000adu  works a treat.   takes a while to do all but no trace of shutter lag 

Dave

[LunarView]

1 hour ago, Davey-T said:

How big is the panel ? you can dim it with some sheets of printing paper.

Dave

Hello,

i tried that and although it does dim the light, it also shows as a pink light through the paper, whereas without its a blue white light, so will that affect my flats...? I also tried a piece of 5mm white acrylic I had and that does the same, creates a pink light through it....not sure why this is the case, or maybe it’s normal... 

[Laurin Dave]

 Is your camera mono or colour ?   Mine is mono 

[symmetal]

1 hour ago, LunarView said:

i tried that and although it does dim the light, it also shows as a pink light through the paper, whereas without its a blue white light, so will that affect my flats...? I also tried a piece of 5mm white acrylic I had and that does the same, creates a pink light through it....not sure why this is the case, or maybe it’s normal... 

The shorter wavelengths will be absorbed more than the longer wavelengths passing through mediums like many sheets of paper or thick acrylic as they are not designed for their uniform filtering properties. Blue being the shorter wavelength will be absorbed most followed by green, with red the least affected. So it will look pink. You can compensate with longer exposures for green and blue compared to red if you have a mono camera. For a one-shot colour you would need a more neutral colour so that the three peaks corresponding to the three colours are not clipped at the edges of the histogram. ND filter gel, being thin and designed for filtering would be your best option with a one-shot colour in my opinion.

Alan 

[Star101]

I'm sure someone will correct me on this but with Flats, colour is not so important as they are treated as grey/mono, is my understanding.

 

[LunarView]

8 hours ago, Laurin Dave said:

 Is your camera mono or colour ?   Mine is mono 

It is the mono version, I was hoping to be doing some plain mono imaging while saving for some narrowband filters, as my budget is not great ATM..

[LunarView]

2 hours ago, Star101 said:

I'm sure someone will correct me on this but with Flats, colour is not so important as they are treated as grey/mono, is my understanding.

 

I hope you are correct on this as i could fit the acrylic in place on a more permanent basis, rather than keep adding paper...so I hope so,some can confirm... thanks.

[carastro]

I used to own an Atik383 and was having problems with shutter shadow.  Contrary to what others have said above Atik told me to take around 3 secs flats and dim the light source.  

Obviously if you have a panel which doesn't adjust, you'll need to dim it down with sheets of typing paper between the light panel and the scope.  

Worked for me.

Carole

[LunarView]

2 minutes ago, carastro said:

I used to own an Atik383 and was having problems with shutter shadow.  Contrary to what others have said above Atik told me to take around 3 secs flats and dim the light source.  

Obviously if you have a panel which doesn't adjust, you'll need to dim it down with sheets of typing paper between the light panel and the scope.  

Worked for me.

Carole

Thanks Carole, yes this seems to be the opinion, my issue now is that the paper makes the light from the panel more of a pink glow rather than a blue white glow, and wondering if this will be an issue with the flats...?

[carastro]

I don't think the colour paper will matter.  

Carole