Alpha Com eclipse observing campaign

The 0.8 mag drop is well within range of visual observing. Would the PI's accept visual, or just CCD for this campaign?

Mike LMK

IR observations may provide additional information, as noted in the announcement.  I encourage all PEP observers with SSP4 to participate if possible.  In general a good target for single channel PEP given its brightness.  If we coordinate well we may be able to get pretty good coverage due to our differences in longitude.

Jim

Hi John,

The 60/68 suggested comps work well for PEP observers too.  They have close companions, but those companions will be contained within the PEP aperture.  In fact, that is what the researchers are doing, with their long-term monitoring using PEP techiniques.  They are not the most optimal comp stars, but after including their nearby companions, their summed contribution is stable to within a millimag or so over two decades.  What we need to do is include this target in the PEP program. I'll talk to Matt about this on Monday.

Those comps are probably too faint for the SSP-4 folks.  Perhaps Jim has some suggestions as to bright nearby possible comps?

Arne

Since this is a very bright star, the three brightest comparison stars in the visual sequence won't plot on a VSP A-scale chart because they are too far away.
They can be plotted if we make a chart using a different position, since all of them lie to the NW.

Make a chart using coordinates.
Center the chart at 13 00 + 21 00 (easy to remember!) and use the largest possible FOV which is 1200 arc minutes.

13641BU is the chart I made. Check the Binocular chart option. Then identify alpha Comae in the chart (it is the bright star at the SE of the chart's center).

Cheers,
Sebastian

I don't know if everybody is already aware but Muterspaugh and Henry's paper on the up-coming eclipse appeared on arxiv.org on Thursday.  http://arxiv.org/abs/1412.1432

Richard Wagner

WCR

I'm adding alf_Com to the BSM monitoring queue, hopefully to be observed by all 6 BSM systems on a nightly basis.  This will be at BVRI, plus I'll get some monitoring grating images.

Does someone want to raise their hand to extract the photometry from these images?  I can have them sent to your VPHOT account, or to our anonymous ftp site.

Arne

This is a nice bright star. What about recording spectra at various stages in and out of eclipse? This may be too bright for many professional telescopes, but should be well suited for amateurs. 

Does anyone know the position angle of the orbital plane?

Another question: VSX lists both stars as F5V but Simbad lists α Com A as F5V and α Com B as F6V. Is this a typo in VSX or is there a difference in classification between sources?

Finally, how does one insert Greek letters in Drupal (this forum)? They aren't included in the special character list that displays and the list doesn't scroll to show additional characters.  Currently I compose in Word, copy and paste if I want to include Greek letters. 

Brad Walter, WBY

Hello everyone,

I'd like to take some measures next month of Alfa Com. With my Newton and Atik 314L+ I cover a field of about 30 "x20" and with another Newton a field of approximately 50 "x35". There are no stars of comparable magnitude in these fields. How to proceed in these cases?

Thank you, Paolo

Thanks Tonis,

I understood the method. This is a new procedure to me, from experience because I have never worked with a photoelectric photometer.
I would say that with a control script of the equatorial mount, plate solving and synchronization of CCD, the procedure will be more comfortable.
When the weather will be better, I will start the tests.
Thanks again.

Paolo

Hi Paolo,

What Tonis recommends is one good way to handle bright stars without nearby comparisons.  You still have to move a good distance with alpha Com in order to find something comparable, and the two drawbacks are that it takes more time to complete the measure and it requires photometric skies.

Another technique, which we are using with the Bright Star Monitors, is to offset the target far enough to pick up a handful of relatively bright stars (6-8mag), while keeping the target in the field of view.  I will have these calibrated shortly and then will use them as an ensemble to measure alpha Com.  Using multiple lower-signal stars as an ensemble effectively gives you one "master" comparison star that has enough signal/noise.  The other common technique is to take multiple short exposures, short enough to not saturate the target.  Then when you stack these exposures, you increase the dynamic range, and the fainter stars in the field now have enough signal/noise to be useful.

In general, though, this is a very bright star out of the galactic plane.  It is a good target for PEP (and we have a couple of SSP3 photometers that we are willing to loan), for DSLR, or for very wide field systems like a BSM.  Traditional 6-8-10 inch telescopes will have difficulty with this field, requiring very short exposures to not saturate alpha Com as well as having small fields of view.

Where there is a will, there is a way.   I am sure that you will find a method that works for your system.

Arne

Hi Phil,

Three of the sites are currently up and running: HQ, Hamren (HI), and South.  Two others will come on-line prior to the eclipse:  NM (most likely in a few days) and Berry (Perth, most likely in a few weeks).  There is an outside chance that Argentina will also be ready in January.  Remember that is is unlikely that all of these sites will be clear or functional on any given day; my guess is that we'll get daily coverage from one or two sites.

Thanks for raising your hand!

Arne

Having just completed the DSLR Photometry course, would this be a good project for DSLR Photometry?

Barbara

Hi Brad,

All AAVSOnet data is transferred from the telescope sites to AAVSO HQ, where it is dark subtracted and flatfielded.  The creation of the master calibration frames is done at HQ "behind the scenes."  Those processed images requested by researchers are then transferred to VPHOT and/or to our anonymous ftp server.  For alpha Comae Berenicis, we are looking at an average of two telescopes observing on any given night.  Each telescope will take a set of 33 images (5B,7V,9R,12I), each 1.5MB in size after compression.  Therefore, the transfer to the VPHOT server will typically be about 100MB, well within its capacity.  While it would be great to take time series data during the eclipse, each site will only get 4-5hrs of coverage during the expected eclipse date.  We'll do one dataset per night until near the eclipse date, then change to one dataset per hour, which will be a hit on VPHOT for only a few days.  It is possible to automate the stacking at HQ as part of the pipeline, before transferring the images to VPHOT, but the coding is complex enough that it won't happen before the expected eclipse.  Note that your DSL connection won't be impacted in any case, since all of the data before analysis is already on the VPHOT server.

A lot of the steps you have to take for your own images are taken care of for you with AAVSOnet!

Arne

Hi Barbara,

Yes, this will be an excellent target for DSLR observers.  The two real advantages are that it is bright, so even simple techniques such as a fixed tripod will be adequate; and the field is sparse, so the wider coverage by a typical DSLR telephoto will help in finding comparison stars.  The other advantage in my mind is what I mentioned in my reply to Brad:  each site will only get a few hours of coverage, as this is an early morning object.  Having a few dozen DSLR observers spaced around the world would be an excellent way to cover the whole light curve!

Arne

Please escuse me if the following is a statement of the obvious for many of you.

If you are planning to do CCD photometry for the campaign and won't be using a small aperture telescope (say less than 150 mm), you might think about experimenting with aperture masks ahead of time that will allow you to do 10 second or longer exposures without exceeding your CCDs linear region. If you have a set of master flats that you use for multiple night's data you will need new ones with the mask in place. Stacking multiple shorter images can overcome scintillation, but if you have an astronomical CCD camera that has an iris shutter (I think that is essentially all  other than SBIG or "shutterless" frame transfer types) you will have vignetting issues due to the shutter reaction time for really short exposures.

I don't have a feel for the size of scintillation effects for telescopes in the amateur size ranges and different exposure times.  In the Dravins papers the autocovariance seem to be negligible for any telescope in the amature range at times well below 1 second. Hwever, his measurments were done at relatively high altitude at La Palma. It may take longer for the correlations to die out at sea level but I think exposures longer than a couple of seconds would bring scintillation noise well below other noise sources. 

Brad Walter, WBY

Hi Brad,

A good source for scintillation information is

http://astro.corlan.net/gcx/scint.txt

which will give you the scintillation noise for typical amateur apertures.  Scintillation is why there are so many frames being taken by the BSM systems for alpha Com (you decrease scintillation by stacking).

Another trick with bright objects and "big" telescopes is to use a neutral density filter.  However, the biggest problem with typical telescopes is the small field of view.  This target is in a very sparse region of sky, and you won't have much to work with, no matter how you decrease the amount of light through your system.

Arne

Hi Arne,

you're right, you need to think of something for the observations. I'm preparing a setup consisting of CCD and lens at short focal length, and also I'm "dusting off" my Canon DSLR with telephoto. I will try a wide-field photometry really!

Thank you, 

Paolo

Alpha Comae, Field of View and Comp Stars

There has been a lot of discussion about the lack of availability of comp star and huge fields of view that would be necessary to do Photometry for this object. However there hasn't been much quantitative information to date.

So I went to SeqPlot and downloaded a table of all the BSM North stars noting which ones were shown as variable (yellow). There are 21 stars of v mag 10.0 or less in an 83 minute (2500 sec FOV centered on Alpha Comae. This is the widest field that can be plotted in SeqPlot. I didn’t plot adjacent fields because I was interested in potential comps stars available in a  field of view available to fairly common small refractors. A 4" f/5 refractor will give you about 3.5 arcsec per pixel for 9 µm pixels and about 2.34 arcsec per pixel for 6 µm pixels. So if you have a camera with at least 2000 pixels in each direction you will probably be able to cover the above FOV with most CCD cameras and many DSL cameras. The telescope also cannot vignette or distort drastically over this sized FOV. Many wide field refractors in this size range can do this. 

The remaining question is which of these stars will provide useful SNRs.  Doing a bit of analysis, I found that there were 10 stars in V and 17 in R that appear to provide SNR greater than 40 with single exposures with max single pixel net count  of 25000 in the Alpha Comae stellar image. Using these stars in ensemble should provide effective SNR close to 100, and stacking 5 or 6 images should provide precision well beyond the 100 SNR level.   A spreadsheet is attached for anyone interested in the Seqplot comps. 

To make the calculation I made some assumptions. I assumed a radially symmetric Gaussian distribution with 2 pixel FWHM (diameter). Given the telescope parameters above, you will probably have to de-focus slightly with 6 µm pixels and you will definitely have to de-focus with 9 µm pixels to avoid under sampling. Therefore, a two pixel FWHM does not assume overly optimistic seeing conditions and is reasonable to use for calculations.  The Gaussian distribution assumption and the symmetry assumptions may not give highly accurate total flux values but they seem to provide 10% guidance for total flux. I would expect the ratios to be more precise. 2 Pixel radius FWHM equates to σ of approximately 0.849 pixels. The formula for integrating the total flux for a given star is then 

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or even an image of it in Drupal>

A scaling coefficient, “C”, of 5.05 was derived to go from the max counts of the central pixel to the total net counts for the star image. This was determined by taking the ratio of the value from the unbounded integral to the value of the integral evaluated to a radius of 0.5642 pixels (which gives an area of one pixel). This ratio is then multiplied by the Max Net counts in the central pixel entered in Cell AE1 and converted to flux using the GAIN in cell AG1. The spreadsheet is designed to allow different values to be entered for Max Net Counts and GAIN. The FWHM assumption of 2 pixels is fixed. The SNRs of the comp stars are calculated by multiplying the values for Alpha Comae by the square root of the flux ratios.

At first, this method seemed to give low SNRs. However when I compared them to some actual values measured at my site at similar max pixel counts (17,000 to 25,000) and adjusted by the square of the ratio of the FWHM values (FWHM with the f/10 OTA I normally use for photometry are most frequently in the range of 3.5 – 4.0 pixels). The actuals were within 10% of these rough theoretical calculations and the theoretical values tended to be slightly higher than the actuals.

So I think you can work effectively the FOV of a small refractor with stars that have been monitored by BSM North.    I know Arne is planning to put together a list of additional comps in a reasonable FOV. That should be an improvement over this list.

Brad Walter, WBY

Dear all,

My english is not very good, please don't worry about.

That's the first time I post a mail on this forum. I observe from south-east of France, and the Alpha Com eclipse is a long awaited one. For the last couple of weeks I have tried to federate french observers on this rare event. Our group comprise now about 10 observers (visual+DSLR+CCD), hoping for a cooperative weather in january, which is not assured ; autumn was severe here, as I did not even see any star for at least one month.

Our observations will be sent to the AFOEV (Association française des observateurs d'étoiles variables) and then, sent to AAVSO.

As many other, our main trouble is about the lack of reference stars for photometry in a medium sized field (say : one degree).

Asked about this subject, Mr Muterspaugh and Mr Henry where kind enough to answer and to point out those stars :

*

Quote :

HD 114300 (V=8.27, Hipparcos scatter of 0.015 mag)
HD 114265 (9.77, 0.037 scatter)
TYC 1451-117-1 (V=9.98)
HD 114219 (9.45)
BD+18 2691 (9.67)
BD+17 2592 (9.49)
HD 114762 (7.29, 0.0011 scatter; historically, this may be the first star around which an exoplanet was discovered, in 1989)
BD+18 2692 (V=9.63)
GJ 9428 (V=11.8, but bright for anyone with infrared equipment)
HR 4962 (V=5.87, 0.014 scatter)
BD+18 2699 (V=9.63)

I'm not sure how faint you can use reference stars without saturating the detector for the bright target star---it somewhat depends on the well depth of your CCD and its noise properties.  While the V=9-10 stars may thus not be useful, HD 114762 and HR 4962 probably provide sufficient references.

Gregory Henry : Unfortunately, there are not bright comparison stars within a one degree field of alpha Com A.  The closest & brightest within the field is HD 114300.  It's about 7 or 8 arc minutes to the southwest of alpha Com.  As far as I can determine, it is not known to be variable.  This will be a tough observation for CCD users.

*

About HR4962, I'm a little bit amazed, since this star is repertoried in the VSX. (Hipparcos scatter : 0.014 hpmag, VSX : V5.87-5.94).
 

So, I have a question that I'll leave to your sagacity, if you don't mind. I think this will be at least unorthodox, but still, I hope this not to be too severly wrong.

My mount is a steady one, but with no GoTo capabilities, so I will not be able for nights to move back and forth between Alpha Com and the comparison stars. Humm .. I can do that for a night in case of special alert, but as I have a day job, this will not be a practical solution for several nights.

I've tought about having two optics working conjointly : a telescope or a refractor, with a field ranging from 1 to 2 degrees for high precision photometry, with a 200 mm lens piggyback : field 5°+ for lower precision photometry.

The lens will survey a larger area that will comprise the two recommanded stars (HD114520 V=6.82 and HD 113848 V=5.99), and HR4962, and Alpha Com. The goal here is to be sure there is no significant decrease in the sky quality by passing clouds or other light gradient all night long.

Once I'm sure about the sky quality in the global area, I can check the star HR4962 for variability both in the lens field and the telescope field with other reference star in the field. If no variablity overnight of HR4962 is found, now I can compare Alpha Com with HR4962 in the telescope field for higher precision photometry, which could be better than comparing Alf Com against a 8.3 mag star, I mean. If any significant variability is found on the light curve of HR4962, I can still compare Alf Com with another star in the one degree filed, with a lower expected precision.

Can you advice on this, please ?

*

Finally, maybe of interest here : a mailing-list has been created on yahoogroups for last minute news, live alert and preliminary reports during the different phase of the eclipse.

This list can be found here :

https://groups.yahoo.com/neo/groups/Alf-Com-Eclipse/info

As this campaign will be an international one, the main language will be english. Some of our team members have already subscribed, and Mr Muterspaugh honored us by subscribing to this list too.

New subscribers are welcome.

Best regards,

Christophe.

I kept wondering why Brad was finding BSM data in Seqplot for Alpha Com.  I had done a search for the star under its two primary names (alf Com and NSV 6116) and found no data.  Then I checked by coordinates, and see that I had entered the star in the BSM queue as "NSV_6166".  Typical typo from me.  Anyway, we have 2928 images from BSM over the period 2011-01-13 through 2012-06-20, comprising BVRI datasets on about 180 nights.  This dataset should be useful to look for variation of alf Com outside of eclipse, and for studying the 5.8mag HR 4962 (NSV 19637).  So I've done the differential photometry for both targets and submitted the BVRI observations (only rudimentary culling of bad data).  Anyone using VStar or similar can now have a dataset to play with.

Arne

Hi Christophe,

I don't think that you gain anything by your use of two different fields of view.  Why not just use the 200mm system with its 5degree field of view?  I guess the advantage would be if the 200mm system was unfiltered or used a single filter, and the telescope system had a filter wheel so that you could take exposures with several filters.  Otherwise, you are always limited by the precision of the 200mm system in knowing the stability of HR 4962, so I'd also use it for measuring alf Com.

My Bright Star Monitor observing in 2011/2012 used an ensemble of nearby stars: 6 stars of 9th magnitude and one star of magnitude 7.3.  Combined, these give photometric errors in the 5-10mmag range per exposure, with better results when stacking images.  So using the brighter, but more distant, recommended comp stars may not be necessary.  If you offset alf Com in the field of view, you can pick up slightly brighter comp stars.

I would experiment.  You have a month before the critical exposures start, so spend a night or two with your two systems and see how well they perform and with what kind of photometric error on repeat measures.  That will tell you the right set of observations to make when the eclipse begins.

Arne

The observing campaign on alpha Com has been cancelled at the request of PI Dr. Matthew Muterspaugh. Please see AAVSO Special Notice #395 for details.

Many thanks, and good observing,

Elizabeth Waagen, AAVSO HQ