Hello everyone,
I start with to introduce myself. My name is Paolo Maria Ruscitti and I'm an amateur astronomer from Italy. I hope to write in an understandable English. I hope without too many errors, you forgive me for that.
I have a good experience in the use of CCD for astrophotography, but now I want to devote myself to the study of variable stars. My observatory is equipped with a newton 200/1000 and a newton 130/650 on EQ6 goto. I use an Atik 314L + with LRGB filters and photometric filters BV and various software.
But now we come to my question: the correct calibration of the images. I'll explain. I was born some doubts reading the manual "CCD Photometry Guide" published by the AAVSO. It is clear to me what is written on getting a good Dark Master and a good Master Bias. The procedure explained in the manual is what I always use. But it is not clear to me, the procedure for obtaining the Master Flat. I have always done so: I take a number of flat frame, I take a number of dark-flat, I make the average of these to get a master-dark-flat, subtract from each flat frame the master-dark-flat and in the end I make the average flat calibrated. I get the master-flat. I will repeat this process for each filter used.
In the manual, however, I do not understand if this is the procedure to be used or another. Surely it depends on me, I will interpret incorrectly what is written.
I apologize if this question has already been asked, but I have not found anything about it.
Thank you all for the clarification.
Paolo
Hi Paolo,
What you describe is good if you are using a light box or a projection system. If you are taking twilight flats, you will need to median-filter images to get rid of stars (yes, they are always there!). Best of luck with your variable-star photometry. I would pick a few easy targets first and then work towards the harder ones. The more targets that you can follow with B&V, the better - try to avoid unfiltered photometry unless you are specifically asked by the researcher to image unfiltered. With the 200mm Newtonian, you will be able to work with stars from about 6th magnitude to about 16th magnitude, and there are a lot of variables in that magnitude range!
Arne
Hi Arne,
Thank you for your timely and accurate replay! The system that I'm trying now to make the flat is a hybrid. Basically I made a flatbox that is not illuminated by LEDs or lamps, but by daylight. In practice, point the telescope at the zenith at sunset, but the light is "filtered" by a thickness white translucent plexiglass. In this way, I avoid the problem of field stars.
I started doing some measure of RX And, unfortunately the weather now I was able to continue to take data. I hope as soon as possible to be able to upload my first observations.
Meanwhile, given the weather, I have more time to study!
hi Paolo,
That sounds like a good system. We've done similar things with AAVSOnet telescopes, using a piece of frosted plexiglass in front of the OTA. The "T-shirt" flats are also a similar concept of adding a diffuser in front of the telescope. The translucent plexiglass tends to be shiny. You should roughen both sides to eliminate reflections.
Arne
p.s. welcome to the AAVSO! I'm signing your membership letter as I write.
Thank you for the warm welcome in the AAVSO, Arne.
I used a wrong adjective, I use satin plexiglass, not glossy.
Thanks for the membership letter, I am very proud to be part of the AAVSO and I hope to bring my small contribution to the activities of the association.
Paolo