james_screech

Have a look at these https://neilross758.wixsite.com/cygnus-astro-covers/breathable-covers should do the job at a reasonable price. I've used them outside and inside as dust covers. James

I've do idea, but as it happens on two consecutive maximums I don't think its an artifact due to cloud or something. James

Interesting light curve of RZ Cep, with double peak at maximum. Top graph is raw data, bottom a running average of observations to smooth out the noise. Two double peaks seen in raw observations, seen even more clearly in smoothed graph. ED70, ASI1600MM-C & photometric V filter. 10 sec exposures with a gain of 0.

I've tried flats with both an LED panel and an EL panel, both show a similar pattern, so it's likely to be real.

To take flats I need the tube to be vertical, so unfortunately no.

If the image train was stable that would be fine as it would be correct, however if there has been movement the flat will no longer "match" the image and so produce the problem. In fact the flats both side of the meridian will probably be incorrect as movement will have occurred between the flats (tube pointing vertically up) and the first side of the meridian images.

The flat is applied to the image files without any rotation as the image train has not changed, just the orientation of the train with respect to the stars. The star matching is done in software which can compensate for the flip, the flats are applied before this so should work, unless the image train changes (movement somewhere).

The images don't need rotating as CMuniWin matches stars without it. So the only thing must be something is moving. As I never had this problem with my refractor I think I will just use the newtonian for visual work and go back to the refractor for photometry.

I also had the discontinuity in different direction with different stars in the field, as said, I suspect movement of something just not sure what as everything is done up tight, but it probably doesn't need much movement to cause the problem.

OK, attached are a few files either side of the flip and the master flat. I know guiding isn't good on some of them, but that shouldn't effect photometry, am finder guider is pushing guiding a bit when the main scope has a focal length of 900mm. flip.zip

I've just analysed my images from last night and they still have the problem, so the issue isn't to do with the flats panel. If I can't find what is changing soon, I'll go back to using my ED70 for photometry, possibly on my SE4 mount in EQ mode so I won't even need a meridian flip. A nice small setup that is quicker to setup and much lighter to move around. But just lacks the dynamic range in images that I get with the SW200PDS scope.

Thanks for the offer, the problem would be that the images files are about 40GB. I have wondered if it might be the EL panel I used for flats, so last night I switched back to an old LED panel. I haven't processed those yet, but hope for an improvement.

Everything is done at the same time, the discontinuity isn't exactly the same for all stars. I think as andrew s suggested there was some movement in the optical train.

When you save the photomtric data (in my vase BAAVSS format) the software asks for various information including the magnitude(s) of the comparison stars.

It doesn't astrometrically link stars, it asks for the actual magnitude of the comparison star(s) and calculates it from that. Regards James