Flats Advice?

[Astrosurf]

I've always been rather daunted by trying flats with all the myriad advice on how to create a light source, some being quite techie for me! I want to take flats with my 72mm frac. Can someone suggest a simple flats technique?

Alexxx

[spillage]

I'm sure you should have a clear sky today, so I would just use a white t-shirt and elastic band. I use apt and you put in your adu level and it works out the flat plan for you.

Mark.

[Astrosurf]

Thanks Mark, but my kit isn't set up still. I couldn't reproduce the optic train setting. What light source do you recommend?

I did an artificial flat using this tutorial! 

 

[happy-kat]

As only 72mm do you have a tablet there a great free app called lightbox.

Dark flat is identical to the flat but just stick the telescope cap on.

[Astrosurf]

1 minute ago, happy-kat said:

As only 72mm do you have a tablet there a great free app called lightbox.

Dark flat is identical to the flat but just stick the telescope cap on.

Great! Many thanks. 

[groberts]

+1 for daylight but I've also long successfully used a white t-shirt (double folded) stretched over a flower pot with a cheap cylindrical LED light attached with velcro on the base - I also have a layer of white t-shirt over this too.    I used to hold  it in front of the telescope aperture but now have it on an old tripod and place it close to the aperture so I can then just work through each flat using APT.  Works a treat + v low tech!

 

 

[Astrosurf]

I'd like to see your work flow. I can't picture the light attachment!

[groberts]

2 hours ago, Astrosurf said:

I'd like to see your work flow. I can't picture the light attachment!

Same as usual work flow for flats: place light source in front of telescope aperture + set ADU + calculate exposure time for each filter + take flat image set for each filter.

It's attached to the tripod with a rubber band! it is quite light so holds on OK.  I place it quite close to the aperture + central and then go to the computer and run through the sequence above for each filter. Works like a dream  

 

 

[Astrosurf]

Thank you! I did need that, but I meant how you put the light box together! 

[groberts]

OK, the pictures below show what I've done.  The LED pod at the base of the flower pot has velcro on the back which attaches to velcro on the bottom of the pot - so you can (a) take it out to turn on and off - the button is on the side and (b) attach small piece of white t-shirt fabric to dissipate the individual LED lights.  This sort of LED is cheap a readily available at Dyas or somewhere similar for only a few £.  I've painted the inside of the pot white to improve light quality. Then I just stretch two layers of white t-shirt fabric across the top of the pot, held on with a rubber band.  Finally, as you can see, with further engineering prowess the pot is held onto a spare bar with rubber bands fixed onto an old tripod.  You then need to experiment the best distance from the front of the telescope aperture to get the light value & spread you want - in my case I'm using 25,000 ADU and place it about 1cm from the from of the scope.   

OK it's a bit crude but is cheap + quick and easy to use and the resulting flats work well.  Should say I have a WO GT81 refractor but I suspect this might not work so good over 100mm, not sure? Maybe I'll get round to something more sophisticated one day but for now it works for me. 

Hope that helps?  Regards, Graham        

[Astrosurf]

That's excellent! Very many thanks. I've saved this to Favourites.

[newbie alert]

On 07/05/2018 at 14:53, groberts said:

+1 for daylight but I've also long successfully used a white t-shirt (double folded) stretched over a flower pot with a cheap cylindrical LED light attached with velcro on the base - I also have a layer of white t-shirt over this too.    I used to hold  it in front of the telescope aperture but now have it on an old tripod and place it close to the aperture so I can then just work through each flat using APT.  Works a treat + v low tech!

 

 

Does it create a even distriution of light as it looks alot brighter in the middle than it is at the edges...does it show on the flat like that?

[groberts]

The picture is probably not a good angle, as from the front illumination is quite even across.  Notwithstanding, there is a very slight diminution in light at the far edges - approx 0.5cm or less.  However, with the span of the pot larger than the aperture + the scope close up to the front at <= 0.5cm this gets lost and the result is OK i.e. make sure the diameter is larger than the scope and place close-up; it is also important to cover the underlying LED with white T-shirt material to dissipate the points of light within.

The images from my scope + ZWO1600 can sometimes show a little vignetting but with the flats taken with this set-up are fully corrected.

In the long run I would like to find something else larger but equally simple & convenient to use.  Any ideas?  

Graham

           

[Kevin Morefield]

I've found using sky-flats to be the simplest as it requires no equipment or other special process.  Why not simply shoot sky-flats?  

My process is to shoot one filter at dusk and one at dawn, but you can likely get in two filters each.  I use CCDAutopilot and it manages the exposures, timing, and dithering but I've also done this manually in the field with no issues.  You get the camera focused and cooled and point the scope at a point high in the sky maybe 20 degrees away from the zenith in the east and shoot exposures about 1/3 of the way from the left on the histogram.  Use a really big dither like 2 degrees.  Manually I just slew the scope randomly around.  Then when you combine them I use a bias frame and you can either use a median combine or outlier rejection to get rid of any stars that pop up.

Kevin

[ollypenrice]

On 03/06/2018 at 17:41, Kevin Morefield said:

I've found using sky-flats to be the simplest as it requires no equipment or other special process.  Why not simply shoot sky-flats?  

My process is to shoot one filter at dusk and one at dawn, but you can likely get in two filters each.  I use CCDAutopilot and it manages the exposures, timing, and dithering but I've also done this manually in the field with no issues.  You get the camera focused and cooled and point the scope at a point high in the sky maybe 20 degrees away from the zenith in the east and shoot exposures about 1/3 of the way from the left on the histogram.  Use a really big dither like 2 degrees.  Manually I just slew the scope randomly around.  Then when you combine them I use a bias frame and you can either use a median combine or outlier rejection to get rid of any stars that pop up.

Kevin

Kevin, I'm not sure that the 'master bias as dark for flats' works for CMOS chips. Like you, I use it with my CCD cameras, though.

Olly

[Kevin Morefield]

9 hours ago, ollypenrice said:

Kevin, I'm not sure that the 'master bias as dark for flats' works for CMOS chips. Like you, I use it with my CCD cameras, though.

Olly

Olly,

I’ve only just gotten my QHY367 but I *think* I used a bias when I assembled my flat in CCDStack.  What is the issue?  I’m betting you would need to match the gain but what other issues are there?

Thanks,

Kevin

[spillage]

I am sure that with a dedicated cmos camera bias frames can be an issue and I know allot of users of the asi1600 do not even bother with bias. Sure it is due to random noise from very short exposures and that you should make your flats exposure as long as possible. But then again I have read so much regarding this camera my mind does get muddled with all the info.

 

Mark.