AAVSO Alert Notice 791 announces an observing campaign on the eclipsing triple-system b Per. Please see the notice for details and observing instructions.
There are threads for this campaign under the following forums:
Please subscribe to these threads if you are participating in the campaign so you can be updated. Join in the discussion or ask questions there!
Many thanks, and Good observing,
Elizabeth O. Waagen, AAVSO HQ
The Alert Notice asks for "several long time-series observations".
As a novice DSLR observer I would like to understand how long and how many are reasonable for a campaign like this one.
Could someone please provide a little more detail? Thank you!
For b Per I use a normal tripod with Sony alpha 6300 camera and 85 mm lens. In one session I take always 27 pictures of 8 sec at f/4 (RAW, ISO 100, without tracking).
After flat correction these pictures are split in R, G and B channels (for Sony alpha no dark correction is necessary). The 27 (fits) images in G are stacked with median function (Fitswork) to one single file. Simple differential photometry (b Per vs "55" comp, 5,456 mag V) on this file delivers one single observation. Lambda Per (HD: 25642, 4,287mag V) is used as Check star.
How long a single image can be exposed depends on your lens/telescope and the aperture ratio. None of the 3 stars (b Pro, Comp, Check) should saturate. Always take RAW images.
For more information see https://www.aavso.org/dslr-camera-photometry-guide (also in Spanish)
Hans-G. Purucker (PHG)
Thank you Hans! Although now I have more questions :)
I understand the general process you describe because I do photometry with my Canon 600D, but I usually take 20 lights of 12 seconds each, and then stack them in groups of 5 (after bias, dark and flat corrections) to make 4 measurements. With those measurements I use Blackford's Excel sheet to average the numbers and transform them into a V magnitude.
Do you think I should use my standard procedure, or stack all the 20 lights for a single number? How often do you take your 27 lights? Once a night? Once a week?
Thank you in advance for your support!
20 lights at 12 seconds each sounds good to me. It is up to you whether you generate 4 measurements (with larger scatter and close time) or one measurement. But with b Per it is not necessary to transform to V magnitude. You have to do that for Mira stars etc. and there I also do that. For b Per, the most important thing is to use a simple workflow that should not be changed during all observing sessions of an eclipse. It is not necessary to perform a transformation process – simply use the green channel (TG) of your camera; the most important thing is to produce a light curve in which the relative magnitude differences between the individual points are as accurate as possible!
See Alert Notice 791: It is important that observers observe several long time-series observations both before the eclipse and after the eclipse as well as all parts of the eclipse.
When you report all your measurements (always use the same workflow!), your out-of-eclipse points (before and after the eclipse) are used to calculate a value for the magnitude shift (="your" offset) of all your points, so that these points outside the eclipse "sit" on the known sinusoidal light curve of b Per. This is done separately for each observer (they all have individual workflows and different offsets). To see this directly, compare the reported points (without individual offsets - use Enhanced LGC!) with the analysed curve in Alert Notice 791 (e.g. for the last eclipse of b Per).
During the eclipse you can in principle take 20 lights of 12 seconds every half hour - if you can manage that :-). During the last eclipse I tried to generate 8 measurements per night, but I always had to go into the garden and use a tripod because Perseus cannot be observed with my telescope mount from the balcony.
Thank you so much!
The weather forecast tomorrow is pretty good here in Tudela (Spain), so tomorrow night I will try to take my first serie of TG lights before the eclipse.
The eclipse is nearing. Predicted JD:2459852.4 = UT Sept. 29,41
The beginnings of eclipse typically comes 1.5 to 2 days prior to the midpoint. That is ~Sept 26 UT.
There is a possibility that a partial grazing secondary eclipse of b Persei may begin tonight Sept 26-25 (JD 2459859). Remember we expect the center of the 3-4 day transit to be Sept. 29. All observers with clear weather are requested to observe at least one observation per hour (or continuously) throughout the night. If observing intermittently throughout the night, it is important to observe a burst of images especially exposures are short (less than 60 sec). A solution is to expose for many exposures of short duration to prevent saturation of detectors and include enough exposures so that the total integration time (exposure time multiplied by the number of images in a burst of images) be larger than about 60 sec. This reduces the fluctions inherent in scintillations from Earth's atmosphere. I can co-add the multiple photometry results from burst from many rapid observations after downloading the results from the AAVSO database.
Please observe as long as possible each night between now and October 2.
Many Thanks in advance to all observers who are braving the post-midnight observation windows for this fall's event!!
Javier, Your data for those three nights are very good! I hope you have good weather tonight - I expect the first dips to be seen in Europe and maybe the first dip finishes before the viewing gets to N. America. Keep looking!!
Thanks for your work!