Lightbox for Flats

[Marmo720]

Hi All,

I have been using the white t-shirt with iPad method to try get flats but I have continuously had issues with getting them right. My last session seems to have resulted in half the frame being washed out with white light for some reason and I believe it might be due to slipping iPad (which has in the past dropped on the floor from my attempts at flats!). 

So I am looking at creating a lightbox and already bought the tracing lightbox below from Amazon. I just wanted to check if anyone has used similar before? I can still return mine if there are better methods/tools. I opened the box to check and the LED lights are on one side of the panel (where the USB plugs in) and thought that would an issue even if dispersing the light with a few sheets of white paper? 

https://www.amazon.co.uk/Brightness-Adjustable-Ultra-Thin-Sketching-Animation/dp/B07GJBF4P3/ref=lp_3063633031_1_9?s=kitchen&ie=UTF8&qid=1575888195&sr=1-9

 

Thanks in advance for any advice and assistance.

 

 

 

 

[carastro]

I bought similar and it works well provided I put a piece of white typing paper in front of it to difuse the light as it is lit through "dots".  I also like the fact that it has a controller so i can turn the light up and down, though mine's not easily controllable and have to do it a few times to get the brightness right.  This saves having to put lots of typing paper in when I want a shorter exposure.  I also like the fact that it plugs into a USB.

Keep the box it came in and cut out a hole to fit over your aperture, this way it stops it slipping.

The rest of the advice is just about getting the brightness of the flats to around 1/3 well depth.  If it's too bright it will be white as you say and won't work. 

Carole

Edited December 9, 2019 by carastro

[Laurin Dave]

Like Carole I have a similar tracing pad, works a treat with a few bits of typing paper over it to diffuse/dim the panel.  Wish I'd kept the box now after reading Carole's response

Dave

[Marmo720]

2 hours ago, carastro said:

I bought similar and it works well provided I put a piece of white typing paper in front of it to difuse the light as it is lit through "dots".  I also like the fact that it has a controller so i can turn the light up and down, though mine's not easily controllable and have to do it a few times to get the brightness right.  This saves having to put lots of typing paper in when I want a shorter exposure.  I also like the fact that it plugs into a USB.

Keep the box it came in and cut out a hole to fit over your aperture, this way it stops it slipping.

The rest of the advice is just about getting the brightness of the flats to around 1/3 well depth.  If it's too bright it will be white as you say and won't work. 

Carole

 

Thanks @carastro and @Laurin Dave. I was worried about the LED lights being to one side but glad it works for you both so I will try it. 

My Achilles heel seems to be getting the Flats right. I think I am getting the correct Lights after some trial and error and advise from @RolandKol. Same with Bias and Darks. But every time I stack in DSS and check the image, I see either some dust bunnies (which are mostly removed now) or half the image being completely white compared to the other side. Hoping a better approach to Flats will give me that elusive first proper DSO image

 

[carastro]

Quote

half the image being completely white compared to the other side

What scope and filterwheel are you using, perhaps there is a light leak?

Carole 

[Laurin Dave]

Also What camera are you using  and how long are your flats..  maybe too short 

[carastro]

Quote

how long are your flats..  maybe too short 

Or maybe too long if they are coming out white.

Carole 

[Davey-T]

A pictures worth a thousand words, pinched the idea from Carole, needs a few sheets for LRGB and less for NB.

Dave

[Laurin Dave]

16 minutes ago, carastro said:

Or maybe too long if they are coming out white.

Carole 

Indeed Carole, was thinking if camera has mechanical shutter too short would give a shadow 

[carastro]

Quote

A pictures worth a thousand words, pinched the idea from Carole, needs a few sheets for LRGB and less for NB.

Dave

Thanks Dave, never taken a photo of mine.

Carole 

[carastro]

Quote

Or maybe too long if they are coming out white.

Carole 

Indeed Carole, was thinking if camera has mechanical shutter too short would give a shadow 

Yes so many permutations.  If he has a manual filterwheel it can cause light leak (as I found out to my cost in the past), due to the hole for the pushing of the wheel.  

Possible primary light leak if he has a Newtonian and doing flats in the daytime. 

Difficult to know until we know what kit he has.

Carole 

Edited December 9, 2019 by carastro

[carastro]

Quote

 LED lights are on one side of the panel (where the USB plugs in) 

I must admit my panel isn't lit from one side, I'd try it with paper to diffuse it and do some test flats. 

Carole 

[fireballxl5]

I purchased the OP's linked LED light panel and saw that the light intensity was heavily biased to one end. Together with the grid effect pattern I didn't like to look of it so returned it for a refund.

So I'm still on the lookout for an A4 sized LED panel that is evenly illuminated, ideally edge to edge😛

[Stub Mandrel]

2 hours ago, fireballxl5 said:

I purchased the OP's linked LED light panel and saw that the light intensity was heavily biased to one end. Together with the grid effect pattern I didn't like to look of it so returned it for a refund.

So I'm still on the lookout for an A4 sized LED panel that is evenly illuminated, ideally edge to edge😛

I use something very similar, if not identical.

I put a sheet of A4 paper over it which eliminates the 'grid', and I take 12 flats, rotating it about 30 degrees  between each one. This gives me excellent results.

[fireballxl5]

3 minutes ago, Stub Mandrel said:

I use something very similar, if not identical.

I put a sheet of A4 paper over it which eliminates the 'grid', and I take 12 flats, rotating it about 30 degrees  between each one. This gives me excellent results.

if that works for you then great, but I don't fancy rotating the panel between each flat exposure, especially when I like to automate the process across 7 filters (20x7 exposures🙃)

Maybe I've been spoilt by my 350x350mm EL panel (used with my 12" MC scope). I'm looking for something more convenient for use with my smaller refractors.

CS, Andy

[Marmo720]

Thanks for all the replies - apologies on my slow response as we had clear skies last night and I spent my time freezing trying to get the flats to work. 

@Davey-T - that is the exact same setup I did last night. I placed 6 sheets of white paper and set the light to the lowest level. Looking at the flats, I don't see any gradient caused by the LED being to one side so looks like it works for me even without rotating. 

@fireballxl5 - I thought it would also be evenly lit as well until I received it hence the post here. At a cost of £12, I was willing to try it with some white papers to diffuse the light. Seems to be working OK from last night test but will update once I have another clear night to test more. Not seen any that are evenly lit and assume might cost a bit. 

I did notice the light in the box looked "blue" but maybe that is due to everything else being in night vision. The histogram on APT looked fine as far as the colours go. 

@carastro - I forgot to add my setup to my signature, which would be helpful! I am using astromod Canon 600D with Altair Starwave 80 ED. No filter wheel etc. My current Lights setup is 60sec at 800 ISO. For Flats, I was using ISO 800 too which I now believe is the cause of my issues. My combined histogram for the Flats is pretty much dead centre or slightly to the right. I think it is meant to be about 1/3 from the left from reading online and discussion with you.

I was using ISO100 during the summer for Flats, which resulted in better flats but had a few stubborn dust bunnies that I was able to clear a few weeks ago. For some reason, I forgot to change the ISO down from 800 so hopefully that will resolve some of my headaches with Flats. 

@Laurin Dave - Is your tracing pad also lit from one side? No issues with that using a few papers to diffuse the light? The gradient to me seems less obvious when the dimness is set to the lowest but considering whether I need to rotate or not as per @Stub Mandrel suggestion? 

 

Many thanks all for the advise as always.

[Laurin Dave]

I wonder if the half illumination is  because your shutter speed for flats is too fast and the panel is flickering too fast for us to see but slow enough to mess with the flats.. what shutter speed are you using? I’ll check my panel type and let you know pretty sure its evenly illuminated though ..  also it’s very blue ..  I’ve used it with my EOS600D and it was fine

[Marmo720]

6 minutes ago, Laurin Dave said:

I wonder if the half illumination is  because your shutter speed for flats is too fast and the panel is flickering too fast for us to see but slow enough to mess with the flats.. what shutter speed are you using? I’ll check my panel type and let you know pretty sure its evenly illuminated though ..  also it’s very blue ..  I’ve used it with my EOS600D and it was fine

Thanks. I don't remember the shutter speed but will check. However, I did the flats again last night using the lightbox instead of the iPad method and I didn't have the issue with the one side illumination which suggests to me that it might have been ipad slipping slightly or similar instead of camera issue/settings. 

However, using the lightbox last night resulted in bright centre and darker outer image so hoping the change the ISO down to 100 will help fix that.

[Laurin Dave]

Bright centre dark edges is quite normal and shows the vignetting.. that’s what the flats correct for as well as dust bunnies

[Laurin Dave]

It’s a Huion tracing panel..  

[Marmo720]

25 minutes ago, Laurin Dave said:

It’s a Huion tracing panel..  

Thanks - not much more than mine and says even light. Will get it if the current one shows light gradient in my imaging.

The bright centre/dark outer + dust bunnies is what I saw after using the new light box to produce Flats so that is good news at least compared to iPad method I was using before which resulted in illumination on one side. I just need to get the correct Flat setting in APT to remove those effects. I hope!

 

[carastro]

Quote

I am using astromod Canon 600D with Altair Starwave 80 ED. No filter wheel etc

That's eliminated 2 possible light leaks then.  Just make sure there is no light leak at the viewfinder on the DSLR and then you should have eliminated all possible light leaks.  though it is starting to sound like your settings are more likley the problem.

Carole 

[bottletopburly]

Flat frames should be taken at same  iso setting as lights , I use an EL panel with a couple of sheets of A4 paper sandwiched between frosted acrylic absolutely no problems with flats now , turn dial on camera to AV mode before taking flats it will work out correct exposure and I take 25 flats each session .

[Stub Mandrel]

1 hour ago, bottletopburly said:

Flat frames should be taken at same  iso setting as lights

And the same bit depth... I've just spent and age generating cooled flats and darks but in MONO8 so DSS hast just thrown them all back in my face 😞

[bottletopburly]

6 hours ago, Stub Mandrel said:

And the same bit depth... I've just spent and age generating cooled flats and darks but in MONO8 so DSS hast just thrown them all back in my face 😞

 DSS has taken a back seat lately after trying Siril ,using the scripts so much easier  that way than DSS  literally a few clicks of the mouse .