SharpCap Forums

Hi Tim,

Great question. I'll take a stab it. What you say may be true for Cepheids as long as you know with accuracy the QE curve of your sensor, however there may be enough variation in the radiation in the population of Cepheids that you can not achieve accuracy of 0.001 magnitudes which is the gold standard that all photometrists aspire to.

Beyond Cepheids there are other stars of interest that don't fall into that mold including eclipsing systems and exoplanets. Furthermore long term studies of variable stars can span multiple generations of technology breakthroughs, optics configurations, and individual photometrists. One must have a way of standardizing magnitude measurement. The best way (so far) of achieving that is to derive your equipment's transformation coefficients so as to transform instrumental magnitudes to standard magnitudes.

Doug and I use a single filter, instead of B-V or B-V-IR, because we are mostly interested in the shape of the light curve rather than the magnitude value. In the case of Doug's Qatar-1 light curve, the time of eclipse start and end is extremely valuable.

Way back when I was an undergrad in Astronomy 101, one of the first labs required us to use the school's 25-inch scope to measure the light curve of a variable star. CCDs were relatively new and expensive back then, so I used a photomultiplier tube. The technology was different but the mathematical data reduction was the same as today, if I remember correctly.

Brian

Also, I should have remembered that Cepheids undergo color changes as they pulsate:

"Cepheids are very luminous, massive variables with periods of 1 -70 days. They are named after the first-such pulsating variable, δ Cephei discovered by John Goodricke in 1784. Cepheid light curves are distinctive and show a rapid rise in brightness followed by a more gradual decline, shaped like a shark fin. Their amplitude range is typically 0.5 to 2 magnitudes. The spectral class of a Cepheid actually changes as it pulsates, being about an F at maximum luminosity and down to a G or K at minimum."

-- an excerpt from this site:
https://www.atnf.csiro.au/outreach/educ ... ating.html

So to accurately measure magnitude throughout the star's period, one must utilize two or more filters. Traditionally, the color shift is calculated as the difference between the B-mag and the V-mag. Therefore B-V greater than 0.6 are red stars, and less than 0.6 including negative values are blue stars. Knowing your instrument's transformation coefficients ensures that your data can be joined with other sources for scientific analysis.

Brian

oopfan wrote: Wed Nov 30, 2022 5:27 pm Hi Doug,

I've only owned refractors but I've toyed with the idea of purchasing an RC8. Others have said to stick with refractors as they are solid and reliable. I yearn for a larger aperture but prices for a 100mm+ APO are astronomical. What would you suggest?

Thanks,
Brian

Hi Brian,

The RC8 is a good scope, but being an open tube with 2 mirrors open to the sky for sometimes over 8 hours, those optics soon get dirty. Dismantling and cleaning then colimating an RC is not for the faint hearted! Also taking flats often is obviously a must. On the possitive side, it's the perfect optical system for variables. It's Fast, I used a focal reducer to take it from F8 to F5.5 and it has a flat field, and being open tube you have an advantage over SCT's with really quick cool down time.
I've lowered my sights on variables, there's plenty of stars in my Refractors range, and Catacysmic stars are great to image when in super outburst, here's a lightcurve of mine of a rare outburst of OV BOO, there's about 9 hours of data here, although I was using the RC for this, it's well within reach of a small refractor.

Hi Brian,

Thanks for your reply. Sorry to be slow to acknowledge it cos I hadn't noticed that there was a page 2 on this thread ..duh. Your answer makes perfect sense - the spectrum changes with the pulsations. Use of a 25 inch telescope on an undergrad course sounds fun. I was all at the other end of the image scale with electron microscopes and X Ray diffractometers.

Tim

Hi Tim,

Yes, a 25-inch Cassegranian on a massive fork mount under an observatory dome. According to my arithmetic, a 10000mm focal length.

Brian

Hi Doug,

Magnitude 14 is pushing the limit for my 71mm refractor for a 60-second integration. Do you recall the approximate SNR of the target variable? I like to reach SNR 100 if possible Also, what is the Full Well Depth of your camera? Mine is only about 15000e- so I have to be careful with the magnitude difference between the target and comparison stars. I could definitely use a camera upgrade!

Brian