Hello,
I'm trying to get the right magnitud (with high accuracy) for stars with a french software called Prism. I use a Johnson V filter. But I don't understand how I can calibrate the fact that quantum efficiency is different from a wavelenght to an other into this pretty broadband filter.
Let's take an extreme example :
if the filter transmission is from 500 to 600nm, equal transmission for each wavelenght.
Let's take 5% quantum efficiency at 520nm and 95% at 580nm.
Let's take 2 stars in this example : one is strongly emitting light around 520nm and the other one at 580nm. The fluxes are equal for these 2 stars between 500 and 600nm.
But, I will find very different magnitud in V Johnson band because quantum efficiency are very different. I will find that the the star which emits light at 580nm is brighter.
I'm french, I hope you understand what my problem is. Maybe this is a parameter we can't controlate when doing absolute photometry and we accept the the quantum efficiency is pretty constant along the spectrum of my filter.
Thanks by advance
Johnson / Cushion photometric filters have a certain tolerance and we do differential photometry, so we compare a stream of a star located at thousands of km with the flux of another star also at thousands of km.
This flow is influenced as much by our atmosphere as by our optical train, the exposure times, our camera, so many factors can influence the result, this is why there is a tolerance in our data. The goal is to limit this tolerance and have the best result. It's a quest!
Are the "Sloan" photometric filters more precise and perhaps more suitable for you?
Hello Henri,
This adjustment is done by applying color "transforms" to your magnitude measurements. Search for Transforms in the Search box at the upper left of the AAVSO home page . You will find lots of sources of information about how to calculate and apply transforms.
You must first determine the transform coefficents for your system (camera, filters, telescope). The AAVSO software tool TG will calculate your transform coefficients if you feed it images from your system of a "standard field". Search the website for Standard Fields for more information on how to access the AAVSO's standard fields.
In the AAVSO CCD Photometry Guide, Chapter 6, you will find a description of how to calculate your transforms with the M67 standard field using the spreadsheet method. I think using the spreadsheet method, at least once, gives a better understanding of transforms than starting right off with TG.
The transforms can be applied by software or using a spreadsheet (or just using a calculator). Perhaps Prism has a tool that applies the transforms. The AAVSO's TA and VPhot software can apply transforms.
Phil
Thank you very much ! I don't think Prism have this function, Miniwin neither... but I think yes the only solution this coefficient is your website
Henri,
Here is the link to French version of the CCD Photometry Guide:
https://www.aavso.org/ccd-photometry-guide-french
Jean-Bruno (JBD), who was the first to respond to your question, was a member of the team that did the translation.
This document is the textbook for the two CCD photometry courses offered by the AAVSO. Here is a link to the on-line courses:
https://www.aavso.org/choice-course-descriptions
Calculating and applying transforms is covered in CCD Photometry Part 2 (aka CCD2).
Phil
You are absolutely right Philip,
I think I should reread my book a bit ...:0)
JBD
Wonderful ! Your review is very accurate and complete. Congratulations. I've read it to the end. I didn't know about the courses, I could be interested ! I started last week doing differential photometry at the Pic du Midi Observatory in France, the next step for me will be doing the same with my own equipment. I want to experiment three different type of stars : variable star, exoplanet transiting in front of its star, and asteroid photometry. Then, thanks to you, for the next step and all-sky photometry, I will come back to these courses.
Thank you again and see you.