SSP-4

Observation Techniques

Introduction
To be successful with your observations it is very important to develop a good technique.

While one can argue for a particular technique, it is whatever works best for you and produces good data.

The following techniques have been developed at the Hopkins Phoenix Observatoryy and have proven to produce good results..

Stabilization
With the SSP-4 the first thing that is very important is to make sure the system has stabilized. This means turning on the equipment and setting the temperature (ideally at -40 degrees) for at least 30 minutes before starting an observing session.

Offset Adjustment
Because the SSP-4 amplifier tends to drift, it is very important to set the offset once the temperature and unit has stabilized. Optec recommends this be set between 4 and 6 counts per second with a gain of 1.

I normally set the gain at X100 and integration/gate time at 1 and adjust the offset for 600 counts. This relates to 6 counts with a gain of X1. The amplifier will drift up and down a couple of counts around 6 (for a gain of X1 and integration time of 1 second).

Settings
The SSP-4 can be used by itself with the 4 digit readout or with a computer. Using the computer has the big advantage of increasing the dynamic range significantly. With the units readout the maximum count is 9,999. With the computer the readout can be us to 65,000. So it is my recommendation to always use the computer interface. That also has the ability to log the data which makes data reduction and archiving much easier.

Integration or Gate Time:
I recommend an integration time of 10 seconds. This is close to optimal to take care of atmospheric scintillation. With gate times of less than 10 seconds the reading will vary considerably more.

Gain Limits:
For gain, use the maximum amount you can without exceeding the 65,000 counts. The 65,000 count limit is due to the 16 bit analog to digital converter. There is another limit that must be considered. When using a gain of less than X100, the maximum count is around 15,000. This is due to the voltage-to-frequency converter having a maximum frequency of 15 kHz. This is not a problem when using the X100 gain because the 15 kHz maximum would relate to 150,000 counts, well over the 65,000 Analog-to-digital converter limit.

What I do is set the integration time for 10 seconds, then for a new bright star use a X10 gain. If the counts are under 6,500 I then switch the gain to X100 for the observations.

If possible do not change gain or integration settings during a set of observations. If one star is very bright and exceeds one of the limits, then the gain must be reduced, otherwise try to keep everything constant.

Observation Sequence
This can be debated, but from years of doing single channel UBV photometry I have found for long period variables systems the following works well:

Long period variables:
1 - Ch, Chj, Sj, Sh, D (move scope)
2 - Vh, Vj, Sj, Sh, D (move scope)
3 - Ch, Chj, Sj, Sh, D (move scope)
4 - Vh, Vj, Sj, Sh, D (move scope)
5 - Ch, Chj, Sj, Sh, D (move scope)
6 - Vh, Vj, Sj, Sh, D (move scope)
7 - Ch, Chj, Sj, Sh, D (move scope)
8 - Kh, Kj, Sj, Sh, D

Note:
These sequences should be done as quickly as possible. It takes a bit of coordination, but a set of Comparison star or Program star readings should take 3 to 4 minutes and the total time less a Check star set should take 25 to 30 minutes. It takes practice.

Where:
Ch is a set of 3 Comparison Star H filter readings
Cj is a set of 3 Comparison Star J filter readings
Sj is a set of 3 Sky J filter readings
Sh is a set of 3 Sky H filter readings
D is a set of 3 Dark readings
Vh is a set of 3 Variable Star H filter readings
Vj is a set of 3 Variable Star J filter readings
Kh is a set of 3 Check Star H filter readings
Kj is a set of 3 Check Star J filter readings

Note:
The sky readings are taken by moving the telescope about 2 to 3 reticule diameters away from the star, but not on any other star.

This produces three sets of variable star readings each bracketed by comparison star reading. The data is reduced and magnitudes calculated. Three Variable star magnitudes are then produced for each filter. These are averaged and a standard deviation produced as an indication of data spread.

Because the H reading is typically greater than the J reading the Sky J is taken right next to the lower Star J readings to minimize the effect of the dark count drift.

The Dark reading is a freebie while the telescope is being moved and allows a check on how the dark counts are drifting.

Short period variables:
For short period variable stars a slightly different procedure is recommended and one that will minimize the dark count drift and improve the accuracy.
1 - Ch, Sh
2 - Sj, Chj, D (move scope)
3 - Vh, Sh
4 - Sj, Vj, D (move scope)
5 - Ch, Sh
6 - Sj, Chj, D (move scope) . . . . .

This procedure takes longer, but sky readings (and thus dark counts) are measured close to the star measurement for a given filter. Here each Variable measurement is treated by itself and not as a set. The previous and next corresponding comparison star's measurements for each filter are used to determine the observation magnitude. There are no averaging of variable data sets and thus no standard deviations. There is just not enough time.

Observation Time and Air Mass
What time do you use for the observation. The computer data log records the start time of each set, but what time should you use?

Long Period Variables:
For long period variable systems the exact time is not critical. The first sequence is used for this project. Typically I use the middle time (Variable J measurement) for calculating the star's observation time and air mass. For the time of each set I use the Star's J measurement time. This is used to determine the air mass and is factored in for the extinction.

Short Period Variables:
For short period variables I use the second sequence and the individual times of each filter measurement. Steps 1 and 2 will produce J and H magnitudes for the Comparison Star. Steps 3 and 4 will produce J and H magnitudes for the Variable Star. Steps 5 and 6 will produce J and H magnitudes for the Comparison Star. The bracketing Comparison Star magnitudes are then used to calculate the Variable Star's magnitude (after the data reductions). Each Variable J and H measurement will have a time and air mass. This minimizes the observation times while still providing good accuracy. This can be repeated throughout the evening as many times as desired.

 

Present Page Version as of 6 November 2006

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