[Aavso-photometry] Reality Check - Basic Concepts

[David Trowbridge]

I have a number of questions I'll need to resolve before I'll have much
confidence in my data processing techniques. 

 

I've been imaging the Nova V5558 Sgr with a V filter on an ST-7 through a
Tak 106 refractor. Before chart 1804-18 was released, I had been using 5
stars in area Landolt SA-110 for comparison. With the charts, I've been
having some luck with differential photometry but without them I had major
problems with all-sky.

 

Consider the AAVSO light curve for V5558 Sgr (http://tinyurl.com/3bh74o) 

 

Question #1: In general, do the data points in this light curve represent
the results of observers who have been doing all-sky photometry and using
standards such as Landolt, M67, etc? Are the values in this graph exactly as
submitted by various observers, or have they been normalized in some way
after they were submitted?

 

Say I examine the graph at the date of my last observation (May 30, JD
2454250.8965). I see that a "reasonable" observation of V magnitude would be
approximately 8.72 by interpolation.

 

When doing differential photometry using five individual comparison stars in
chart 1804-18, I do get "reasonable" values around 8.713 (std dev 0.105). 

 

After reducing and combining my images, I've been using the Single Image
Photometry Tool in AIP4Win (v. 2.1.10) to obtain a Raw Instrumental
Magnitude, v for V5558 and the Differential Magnitudes (V-C) for each
comparison star. I then get a predicted standard magnitude V by adding each
of the (V-C) values to the corresponding chart magnitudes Cn.

 

Question #2: How do I use the ensemble data generated in AIP4Win after
clicking on several comparison stars? I know that this would give a more
accurate result, but I don't know how to get it. There is a (V-Ens) value
reported, but what number (an ensemble "standard magnitude" from the chart)
do I add it to in order to obtain V? 

 

When I try to do all-sky photometry using the Landolt stars, my results are
way off.

 

Question #3: Are the published values for Landolt standard magnitudes and
color indexes normalized for air mass = 1 (at the Zenith), or are they air
mass = 0 (above the atmosphere)?

 

I have data for the Landolt field SA-110 with two different filters, R and V
at two different times, bracketing my exposures of V5558, so I can compute
an atmospheric extinction coefficient, K for each of those filters. Also,
using five stars in the Landolt field, I can compute transformation
coefficients and zero points. 

 

In equation (V-R) = Eta*(v-r) + zeta_vr, I get Eta = 1.056 and zeta_vr =
0.360.

 

In equation (V-v) = Eps*(V-R) + zeta_v, I get Eps = -0.129 and zeta_v =
17.114

 

I then go to my reduced and combined image of V5558 Sgr and use the first
equation to come up with a standard color index (V-R).

 

I get (V-R) = 0.895

 

Then I use the second equation to compute a standard magnitude for V5558 Sgr
and I get, V = 8.980. Also, R = 8.085

 

I know that I still need to take into account the different air masses of my
Landolt image and my V5558 image, but I'm confused as to how to do that.

 

Question #4: Does the number I have computed so far represent the magnitude
of V5558 AS IF IT WERE IN THE SAME FRAME as the Landolt image? 

 

If so, can I write Vactual = Vasif + Kv*(Xactual - Xasif) ?

 

Proceeding in blissful ignorance, I get:

Vasif = 8.980 [The standard V magnitude we found for Nova V5558]

Xactual = 2.56 [The actual air mass of V5558 at the time the V-filtered
images were taken]

Xasif = 1.49 [The air mass of the Landolt frame and the air mass of V5558 as
if it were in that frame]

Kv = 0.4204 [The slope of the graph of instrumental magnitude, v vs. air
mass, X]

 

Using the equation above, I get,

 

Vactual = 9.4310  which is obviously wrong!

 

Question #5: What am I doing wrong here???

 

Now is the time I could really use some guidance from someone with more
experience. Thank you in advance.

 

David Trowbridge

Tinyblue Observatory