Faulty flat frames with OSC camera and a solution!

[steppenwolf]

Recently, I discovered that the flat frames captured with my normally totally reliable SXVF-M25C one shot colour camera had developed a bad fault making them useless for calibrating my images.

Testing by the manufacturer (and in particular, Terry Platt whose technical judgement I trust implicitly ) indicated no fault with the camera!

However, I had a Eureka moment and have discovered the cause!! I am posting the details here in case it helps anyone else who experiences the same issue.

As can be seen from the image below, there are strong vertical lines throughout the fully illuminated sections of the image, indicating a vertical register problem which makes the flats unusable. The telescope in use had a particularly fast focal length - much faster than most telescopes - which meant that the flat frame exposures were shorter than usual (0.036 seconds instead of my usual (typical) 0.180 seconds).

When I first discovered the fault, I carried out some further test using a camera lens for convenience and got the same problem. The Eureka moment was realising that both the original flats and the 'control' test flats used very short exposures because of the fast focal ratios.

According to Starlight Xpress, the problem is caused by light penetrating the vertical register shielding during readout and the way to obviate this is to ensure that the flats are taken with longer exposures by dimming the light from the EL panel or twilight sky - this can be done with sheets of white paper.

This is of course, only likely to be an issue on very fast focal ratio instruments.

[Alien 13]

Its good that you found the route cause of this problem i once had similar issues with a DSLR taking daylight flats if the camera was left to its own devices (AV mode) it would sometimes run at a very fast shutter speed that would show a dark gradient on the frame from the shadow of the mirror still in motion.

Alan

[sloz1664]

Hi Steve,

You state fast focal lengths. I've the QHY8 Pro which has the same chip and shoot at f4.72  I usually take flats with my IDAS filter as luminence. What adu are you using I normally shoot at 15000.

Steve

[steppenwolf]

Hi Steve,

I'm talking about focal ratios of f3.8 for the telescope and f2.8 for the camera lens and I aim for an ADU of 25000.

There is a 'cut-off point' for the SXVF-M25C and I only know from testing (which I have now finished because I have resolved the issue!) that the cut-off for my own M25C is between 0.036 and 0.098 seconds in that it was all fine at a 'random' 0.098 and but an automated reading using a plugin for MaxIm DL showed the fault at 0.036 seconds.

It could well be that the same chip will respond differently with a different readout circuitry so what holds for the SX may not hold for a QHY.