Looking for advice on Flats

[lardy]

dabbling my toes into the dark arts of astrophotography and looking for some advice.

Got myself a Gerd Neumann 100mm aurora panel and the battery box to allow me to carry out the Flats on site ...

 

Having a play with the panel and my 85mm lens this afternoon, I find taking a flats with my camera set on AV I am getting lines on the picture, the Histogram is to the left of the middle which is recommended, so is this the "normal" flat? 

If i up the exposure time then I get rid of the lines but wash out the image with the histogram being on the hard right. 

 

Hopefully somebody can point me in the right direction

 

Many thanks

 

Mark

 

 

 

[bottletopburly]

You could have a faulty panel  I would drop an email to manufacturer 

[david_taurus83]

What exposure were the flats?

[lardy]

My thoughts bottletopburly but never having taken a Flat before using the Gerd I dont have anything to compare.

 

1/500 david_taurus83 , a longer exposure gets rid of the lines but moves the histogram.

[andrew s]

Dim the panel with some sheets of white paper and use a longer exposure.

Regards Andrew 

[david_taurus83]

^^^This. Or dim the panel if it has that function. Aim for around 1s exposures.

[symmetal]

The manufacturers info does say that for fast systems and/or using luminance filters, sheets of paper need to be used to reduce the brightness. This implies that the panels light output is not continuous but switches or varies at a high frequency, due to the oscillator driving the inverter, used to generate the high voltages needed to drive the panel. Similar effect to many led panels using PWM to control brightness.

The panel inverter input voltage can be reduced to dim the panel to some degree but using sheets of paper is preferred.

At your AV exposure setting the exposure is very short and the camera is effictively 'freezing' these brightness variations. With longer exposures these fluctuations will even out over the duration of the longer exposure and you'll get a smooth flat.

Alan

Edited October 22, 2020 by symmetal

[lardy]

Thank you for all the great advice, having never taken a flat before I didn't know what to expect.

 

Gerd answered my email this morning and pretty much put what you have said above ..... onwards and upwards.

 

Regards

 

Mark 

[bottletopburly]

Asda sell their own brand of baking paper looks like tracing paper that’s good for dimming few  layers  as required 

[lardy]

I presume that you add or subtract these layers to achieve that recommended 1/3 -1/2 histogram? and the use of baking paper is a more controlled way rather than a sheet of A4?

SOooo a longer exposure to remove/smooth fluctuations then tone down the brightness with paper to get that perfect flat?   

So much to learn .... I can see why I've stayed visual these last 50 years😳

 

Yet again thank you to you all for all your help .... Bias and darks once i wrap my head around achieving a usable flat👍