[Aavso-photometry] SSP-4 photometer available for loan

[Jeff Hopkins]

Hi Arne,

If someone finds a suitable JH band comparison star for epsilon 
Aurigae, I would be interested in knowing about it and how it works 
out. It would be good to standard what comparison star is used. For 
now I recommend staying with lambda Aurigae and using a 12" or larger 
scope.

I made hundreds of observations of epsilon/lambda Aurigae during 
2006/2007 so I have a good feel for what can and cannot be done. 
First, I used an integration time of 10 seconds and gain of 100X. The 
big problem is with dark current and drift. Typical data is as 
follows using the 12" LX200 GPS:

EPSILON AURIGAE
RAW OUTPUT DATA FROM SSP4 DATA ACQUISITION PROGRAM
UT DATE= 03/06/2007   TELESCOPE= 12" LX200 GPS   OBSERVER= JLH HPO
CONDITIONS= CLR NW NM -40.0

MO-DY-YEAR    UT          OBJECT     F  ----------COUNTS---------- 
INT SCLE
03-06-2007 01:59:50      SKY        D  04846      0      0      0 
10 100
03-06-2007 02:00:05      SKY        H  05080      0      0      0 
10 100
03-06-2007 02:00:52     COMP       H  05977  05971  05962      0  10 100
03-06-2007 02:01:36     COMP       J  05507  05494  05476      0  10 100
03-06-2007 02:02:05      SKY        J  04828      0      0      0 
10 100
03-06-2007 02:02:24      SKY        D  04555      0      0      0 
10 100

03-06-2007 02:03:11      SKY        H  04916      0      0      0 
10 100
03-06-2007 02:03:53     VAR        H  09595  09578  09555      0  10 100
03-06-2007 02:04:36     VAR        J  07977  07949  07944      0   10 100
03-06-2007 02:05:15      SKY        J  04634      0      0      0 
10 100
03-06-2007 02:05:33      SKY        D  04513      0      0      0 
10 100

As can be seen the dark counts are nearly 5,000 (Optec says to set 
the offset to between 4 and 6 counts with gain = 1X and 1 sec 
integration. This relates to 4,000 counts for 10 sec @ 100X). The BIG 
problem is that dark count drifts significantly. With the comparison 
star at 5,970 counts and sky at 5,000 counts, that is not a good 
signal to noise and will not produce data that is in the 0.01 
magnitude range. And this is with a 12' scope. A 10" will have even a 
poorer SNR. If a suitable comparison star can be found (within a 
reasonable distance, approximately the same magnitude and color) that 
would be great, but I could not find one. With bright stars, finding 
suitable comparison stars is no easy task.

SSP-4 photometry of epsilon Aurigae can certainly be done with a 12" 
and probably to a limited degree a 10". A larger scope will be much 
better. With an 8" scope lambda was in the noise (sky/dark counts) 
and could not be used. As I mentioned in a previous message i 
experimented with a 16" scope and the data was much better. Brian 
McCandless is using a 14" with his SSP-4 with success. Compared to my 
photon counting unit and CCD photometry, the IR band detectors are 
not very sensitive. It is amazing that they do work, however.

Jeff

At 06:24 -0700 02/18/2008, arne wrote:
>Note that there will be an epsilon Aurigae campaign as part of IYA2009,
>conducted by the AAVSO for Bob Stencil (with whom Jeff is working).
>
>The NIR is quite different than optical photometry.  If lambda Aurigae,
>the optical comparison, is too faint, then choose another compstar that
>is better suited for NIR measures.  You do not have to stay locked into
>the optical comparison star.  Epsilon Aurigae is J=1.9, H=1.7.  It dropped
>about a magnitude visually during the 1983 eclipse, so it should still
>be easy with a 10" telescope and the SSP-4.  Certainly more aperture
>is always nice, but not essential.
>Arne
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-- 
Jeff Hopkins
HPO SOFT
Counting Photons
http://www.hposoft.com/Astro/astro.html
Hopkins Phoenix Observatory
7812 West Clayton Drive
Phoenix, Arizona 85033-2439 U.S.A.
(623)849-5889
(623) 247-1190 (Fax)
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