# Subject: using 3.5-m to measure QSO colors

## Message number: 5 (previous: 4, next: 6 up: Index)

```  Michael Strauss suggests that we use the 3.5-m, plus DSC, to
measure the colors of about 100 QSOs in the SDSS passbands.
Let me try to work out the amount of time we'd need to do that...

One problem is that the filters must be changed by hand, at least,
until we get a filterwheel.  In that case, let's say that the best
way to work is to take images of a set of, say, 10 QSOs in the same
filter, then switch filters and go back over the same set of 10,
then switch filters, etc.  How long would this take?

Let's assume we can:

- slew from one object to another in 1 minute
- don't need to take acquisition pictures
- use the DSC in "stare mode", to avoid ramp-up and -down
- take 120-second exposures, and can transfer the image to
the computer while slewing to the next object
- switch filters in 5 minutes

Then, in order to acquire ugriz images of a set of 10 QSOs, we
must allow

10 x 2 minutes for exposures in each band
10 x 1 minute  for slewing to next object, each band
-----------------
30     minutes for set of 10 images in each band
x 5         passbands
-----------------
150    minutes for ugriz measurements of 10 QSOs

In addition, we really must take ugriz measurements of, let us
say, one field of standards in each passband at the start and
end of each passband; that is, we would follow a sequence like this:

switch to next filter
take image of standard field
take images of 10 QSOs
take image of standard field
switch to next filter
...

That adds 6 minutes per passband to the above numbers:

150    minutes for ugriz measurements of 10 QSOs
30    minutes for ugriz measurements of  1 standard field
----------------
180    minutes for complete set of observations

At 3 hours for a set of 10 QSOs, it would take 30 hours to complete
a study of 100 QSOs.  I suspect that this is a larger amount of time
than people would be willing to devote to the project.

Or is this important enough that we want to do it?

Michael Richmond

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