HA Flats technique

[Marky1973]

Just got a Huion light panel and was playing this afternoon to see how it works with getting flats. I tried with my Baader L filter in the imaging train and got the following flat image - this is one image of around 0.3s, the light panel was about 2ft from the telescope with a white t-shirt doubled up and stretched over the dew shield. I've autodeveloped the image in star tools

However when I tried with the HA filter in the light train I got this.

The lighter edges of the flat are evident and not something that appears on the L or the RGB flat frames. The "borders" were there regardless of whether the light panel was flat against the dewshield or a couple of feet away - with the latter, the exposure getting longer and longer. The image is autodeveloped again. The borders are not obvious on the individual images until autodeveloped or stretched, but I was concerned that stacking lots of them would heighten the effect.

So is this an issue with the light being too bright, or just to be expected with the narrowband performance of an Ha filter?

I dunno if these flats are any good, but I was just testing, but I am wondering if there are any additional techniques that should be used with Ha Flats over and above those used with broadband filters, or is the appearance of the flat what we would expect anyway?

Will keep experimenting!

 

[Chris-A]

How long did you expose the Ha flats for? Just wondering if they're well underexposed - I generally find I have to expose my Ha flats at least 10x the exposure length of L flats to get a decent level histogram, ie. 20000 or so ADU peak. 

[Davey-T]

Put your HA flat frame in Maxim and get an average reading of  146.107 should be something around 25000 so needs longer exposure

Ignore the bit that says dark frame at the top 

 

Dave

 

[Davey-T]

Here's one I did earlier 

Ignore the lack of detail, just a Maxim thing

Dave

[Thalestris24]

Personally I don't put anything on the end of the scope but put a clean, white A4 sheet on the light panel. I find it's best to take flats in the dark when the panel is the only source of light and is fairly close to the end of the scope. I can't remember how long I expose Ha flats and can't check until I've fixed the broken PC!

Louise

[Chris-A]

My Ha flats are 60 seconds, for info - that's with an EL panel with a sheet of opaque perspex, placed right against the end of the scope.

[Marky1973]

Looks like I might have been confusing expectation with reality again. I was exposing to 22,000, about a 1/3 of the way from the left of the histogram in ARTEMIS so something obviously went wrong with that Flat or I posted the wrong one Dave! But it looks like I need to just be less fussy with what appears on the screen... :-)

11 hours ago, Davey-T said:

Put your HA flat frame in Maxim and get an average reading of  146.107 should be something around 25000 so needs longer exposure

Ignore the bit that says dark frame at the top 

 

Dave

 

 

[Marky1973]

11 hours ago, Chris-A said:

How long did you expose the Ha flats for? Just wondering if they're well underexposed - I generally find I have to expose my Ha flats at least 10x the exposure length of L flats to get a decent level histogram, ie. 20000 or so ADU peak. 

Chris, I guess I need to play around again and try some longer subs, I just wasn't thinking I would need to go to that length of sub. The LRGB filters all have different requirements obviously, but were all under 1s! Obviously the HA filter is a VERY different beast!

[Marky1973]

10 hours ago, Thalestris24 said:

Personally I don't put anything on the end of the scope but put a clean, white A4 sheet on the light panel. I find it's best to take flats in the dark when the panel is the only source of light and is fairly close to the end of the scope. I can't remember how long I expose Ha flats and can't check until I've fixed the broken PC!

Louise

Thanks Louise, might try that with the light panel, might be a bit more sensitive than the t-shirt - one or two layers makes a massive difference to the light coming in, so a few sheets of paper may be more easily adjusted!

I was set up in dark room yesterday with just the light panel (and the laptop screen hidden) so I, hopefully, at least got that bit right! :-)

[Davey-T]

My one was 5sec exposure but I use a home made light box illuminated with halogen bulbs and a dimmer switch so increase the illumination considerably to shorten the exposure.

Dave 

[Marky1973]

Cheers Dave - I am obviously barking up the wrong tree with the Ha flats though, so clearly need to expose for longer than I thought I needed to - I was just concerned with the stretched image but I guess that is a very rough and ready stretch and a stack of 100 flats will probably compensate for that!

[Davey-T]

20 minutes ago, Marky1973 said:

Cheers Dave - I am obviously barking up the wrong tree with the Ha flats though, so clearly need to expose for longer than I thought I needed to - I was just concerned with the stretched image but I guess that is a very rough and ready stretch and a stack of 100 flats will probably compensate for that!

Don't stretch your flats when you use them for calibrating though, just produce a master flat or load them all as they are depending on your processing software.

No point doing a 100 flats I only do 16 to 20 after that the improvement is minimal.

Dave

[Marky1973]

Yeah, I wouldn't stretch them - been working through the PI book and following that through... curiosity often gets the better of me and I like to stretch things to "see what's there" - sometimes I'd be better of leaving well alone....would have made an even bigger hash of it than Pandora.....

[Jessun]

I have the same frame like pattern on my NB flats using Astrodon 1.25" filters regardless of illumination.

I decided not to let it bother me though and worst case I sort it out in post processing. You could try to change filter brand and see if you have better luck.

/Jesper

[Marky1973]

17 hours ago, Jessun said:

I have the same frame like pattern on my NB flats using Astrodon 1.25" filters regardless of illumination.

I decided not to let it bother me though and worst case I sort it out in post processing. You could try to change filter brand and see if you have better luck.

/Jesper

Not quite ready to upgrade to Astrodons yet, Jesper ;-) but thanks for reassurance about the patter! :-)