First guided efforts - M42

Hey Carl,
Thanks for the tips. I'll be giving them a watch very soon. My flats were taken against a white wall. They looked pretty good. Is it best for flats to be in monochrome or does it matter?

Your exposure for taking flats should be just under have off your total ADU. On my mono camera has a total exposure of 65000 ADU so I increase or decrease my exposure time depending on the filters to 32000 ADU. My Luminance filter takes the shortest exposure to reach 32000 ADU .50 of a second where as my H –Alpha filter 5nm takes 3 to 5 seconds exposure to reach 32000ADU. I’ve never used a Nikon D3100 before but the method should be the same. There are lads here who use cameras like yours for astrophotography who would know better than I would.

Clear Skies,

Carl.

I haven't actually explored flats in that much detail yet. How do you measure them?

carlobeirnes wrote: Your exposure for taking flats should be just under have off your total ADU. On my mono camera has a total exposure of 65000 ADU so I increase or decrease my exposure time depending on the filters to 32000 ADU. My Luminance filter takes the shortest exposure to reach 32000 ADU .50 of a second where as my H –Alpha filter 5nm takes 3 to 5 seconds exposure to reach 32000ADU. I’ve never used a Nikon D3100 before but the method should be the same. There are lads here who use cameras like yours for astrophotography who would know better than I would.

I just take flats exposure duration so that the histogram shows the light to be roughly in the middle of the range on my Canon 400d, everything else the same as the lights. Newer cameras with better software control probably have better ways of doing it, but this basic approach seems to work for me.

My biggest issue with flats was being able to take the things! I'm never at the observatory at twilight and don't have a nice dome surface to use so I made a flat box over the weekend to simplify the task. The only issue with that is the LEDs I've used are slightly blue, so now i've just a different problem to solve

~Al

Hi lads,

To get good constant flats you really need a light box. At the start of an imaging run I put the light box on top of the scope then using maxim-dl I set the exposure for each filter. On the bottom right of maxim you will see the ADU number when you hover over the image this will change so set your exposure time this way. Remember use the centre of the image flats as your measurement as the edges will appear darker.
Al, I’ve tried twilight flats and they where a nightmare to get that’s why I went down the road of the light box. You’ll need to get white LEDs. Here are some handy links for light box

www.themcdonalds.net/richard/index.php?t..._Light_Box_for_SV80S
www.rc-astro.com/equipment/light_box/index.htm
www.nigelaball.com/Projects/lightboxforflats.html

Carl.

carlobeirnes wrote: Al, I’ve tried twilight flats and they where a nightmare to get that’s why I went down the road of the light box. You’ll need to get white LEDs. Here are some handy links for light box

www.themcdonalds.net/richard/index.php?t..._Light_Box_for_SV80S

Thats basically what I did. Except I'm using three of those lights mounted at the sides - woodies were selling them in threes
I'll write up a post about the construction later. The histogram shows the lights as slightly blue so I'll probably get some actually white LED's and replace those in future once I rule out that the diffusing plastic I'm using isn't diffusing the red away somewhere.

For now getting the flats, converting to luminance only and then converting back to colour again seems to do the trick. Certainly acceptable while I try and improve the myriad of other issues with my astrophotography!

~Al