Not sure about this flat!

[Steve Cuthbert]

Hi folks

I don`t use a calibration module with my Alpy for taking flats instead I use a white cover over the end of the scope and then a halogen torch in front of that. The other night I took some flats but not quite what I was expecting when compared to the flats in Francois`s workshop file and wondered what other Alpy users flats look like?

This is a 12 second flat taken like described with a halogen torch behind a white cover-

Thinking this might be some anomaly with my setup I made an adaptor comprised of a ring of led`s which seem to work ok giving a satisfactory ADU count on this 2 second shot again with a white cover over the end of the scope –

giving an identical image?. Can I assume this is ok please? Ccd is an Atik 314L. If its different to others could it be because of where their calibration modules are positioned?

thanks

Steve

Attachments:

1. 

   torch.png
   

2. 

   ledadaptor.png
   

[Andy Wilson]

Hi Steve,

I’m not really an Alpy expert but I do agree that looks a bit odd. Was it taken in darkness or daylight? Just in case stray light might have been getting in. Also, do you know if that bright “line” is oversaturated and do you have an approximate idea of its wavelength from your wavelength calibration.

Best wishes,

Andy

[Steve Cuthbert]

Good point Andy the second flat with the led adaptor was taken in a closed dome during daylight hours which was really dark but the first flat was taken under dark skies. That bright line in the flat peaks around 3700 angst, not saturated about 18000 adu.

regards

Steve

[Robin Leadbeater]

Hi Steve,

Those do indeed look strange and very similar considering they were taken using very different light sources which should have produced very different looking flats. Are these images raw or are they the master flat after processing in ISIS? If so do the raws show the same effect?  Here is a flat from my setup last night (ATK428) using the halogen lamp in the calibration module. (reduced in size)

As far as I can recall flats taken with a halogen  lamp plus white sheet over front of the scope as a diffuser before I had the calibration module produced a similar flat.

Robin

[Robin Leadbeater]

Hi Steve,

Looking again they both look rather like typical blue white LED spectra to me eg

https://upload.wikimedia.org/wikipedia/commons/thumb/4/48/White_LED.png/350px-White_LED.png

Are you sure your torch is a halogen incandescent ?

Cheers

Robin

[Steve Cuthbert]

Hi Robin

Yes that is the master flat and I`ve just tried the others and they are all similar. The torch I thought was Halogen turns out to be a Xenon bulb would that account for the spectrum?. So the second spectrum taken with a ring of led`s giving a similar spectrum, does that imply that led`s are not an ideal light source for flats?.

regards

Steve

[Robin Leadbeater]

Hi Steve,

You are effectively taking a spectrum of the flat lamp when you take a spectroscopic flat so you need a light source with a smooth spectrum with good output and no sharp features in the region you are interested in, which for low resolution spectra effectively means incandescent lamps only, running as hot as possible to give maximum uv, hence halogen are favoured. LEDs definitely do not meet these criteria. (At higher resolution some EL panels are ok over at some wavelengths but not from ~3600-7500A that we need)   As far as I know Xenon torch bulbs (as opposed to Xenon car headlights) are incandescent filament lamps so should work ok so I dont understand why the flat using the torch looks essentially identical to your LED flat. I know it sounds silly but could you possibly have mixed up the files somehow? eg could ISIS be recalculating using the same raw files. I suggest checking back to the original raw flat images. Other than that, I have no idea what is going on !

Robin

[Robin Leadbeater]

What is the difuser made of? Could it have a strange absorption spectrum?  (some plastics do) 

Robin

[Steve Cuthbert]

Hi Robin

Bit weird really as I looked back on some old flats I did using a different halogen torch but with the same white cover and they looked just like yours so I guess the light source is the fault!. I’m just sussing out a new Halogen setup.

Cheers

Steve

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