Davey Lab 24" Teaching Telescope

Preparing for Your Observations

-MAB Spring 1997, Draft 1.0


Contents

Overview

I. S/N and observing time calculations
II. Calibration data
III. Observing techniques

Appendix 1: Telescope Characteristics
Appendix 2: CCD Camera and Filter Characteristics
Appendix 3: Telescope Limitations



Overview


This document describes the characteristics of the Davey Lab 24" Teaching Telescope and CCD Camera and provides information on how to prepare for, and conduct your observations. This telescope is located on the roof (6th floor) of Davey Lab, in University Park, in the dome located closest to the elevator stack. In addition to this document, users should be aware of two seperate manuals (hardcopy only) with explicit operating instruction for the telescope and CCD camera. A short summary of the basic operating instructions can be found here.



I. S/N and Observing Time Calculations

(See also Appendices 1 and 2)


How long will you need to expose in order to get sufficient signal? If you are looking at a very bright source, you will not have to be very concerned with this issue, but otherwise you will want to calculate the observing time (t) to a given signal-to-noise (S/N) given the following observing parameters:

fs = source flux incident on telescope (photons/s-cm2)
fb = background flux incident on telescope (photons/s-cm2-arcsec2)
A = aperture used for measuring fs (arcsec2)
AT = telescope aperture (cm2)
e = effeciency of telescope * instrument * detector (a fraction)
Rn = read-noise (rms) in electrons per pixel of your detector
p = plate scale of detector (arcsec/pix)

In this case, where t is the exposure time in seconds,

S/N = fs AT t e / sqrt[ (fs + fb A) AT t e + A (Rn/p)2 ].

This equation can be inverted to solve (quadratically) for the exposure time, t, in terms of all of the other parameters, including S/N.

Now all that you need to know are values for the parameters listed above. Appendices 1 and 2 contains values for AT, e, Rn, and P respectively. Note that in general e is wavelength dependent. It is conventient to think of fs and fb in terms of magnitudes. At this time, however, the sky brightness in down-town State College is unknown. One this, however, if for sure: The sky is MUCH brighter in the blue when the moon is up.

By the way, there are several regimes in which the above equation simplifies. This can be very handy for quick scaling to other setups.

- source-limited regime: fs >> fb and fs AT t e >> A (Rn/p)2
- background-limited regime: fs << fb and fb A AT t e >> A (Rn/p)2
- readnoise-limited regime: A (Rn/p)2 >= (fs + fb A) AT t e

It is currently left as an excercise for the reader to derive the simplified versions of the above equation in these limiting cases.

Just to get stared, you can estimate exposure times to reach a given S/N in any regime using the fortran (f77) program expose.f. After compiling the source code (in Unix) with the command:

% f77 -o expose expose.f

you can run it with the command

% expose < expose.param

where expose.param is a parameter input file, of which here are two examples:

a) background limited, minimal S/N for reliable detection
b) source limited, high S/N for precision photometry

NOTE that expose.f uses two input lists, one for the detector quantum efficiency and the second for the sky brightness, each listing values for each broad band U, V, R, I . In the example parameter file, thes are called qe.TEK24 and sky.SC, respectively. These currently contain VERY rough guesses for the detector QE and sky brightness. The column headed 't(rn)[sec]' gives the time to reach background or source limit on a given exposure. In general, your exposures should be several times this value, all else being equal (e.g. if guiding/tracking is not an issue). The column headed 't(snr)[sec]' gives the total time to reach the specified S/N.



II. Calibration data


bias frames - needed for all data

dome flats - needed for all data, with a few exceptions.

flux calibration - do you need to know a magnitude or just a difference in magnitudes? If the former, then you need to observe standard stars. If the latter, and you are doing time-series analysis (e.g. the light curve of a variable star), you will need to at least monitor the apparent flux of an intrinsically non-varying source.

references for standard stars - for most purposes it will be sufficient to use the UBRI standard stars listed in the Ephemeris in the telescope dome. You will find that you will need to use the fainter stars in this list or else the CCD images of the stars will be saturated (make sure you check for this). You do not want exposure times shorter than a few seconds in order that exposure times are long compared to the time it takes to open and close the shutter.

Other things to consider:

  • what time and airmass (or range of times and airmasses) are your target observations?
  • what times and airmasses will you observe the standards?
The most accurate flux calibration would be done on a clear and stable night over a range of airmass in several bands for a set of standard stars that span a range of colors that bracket the colors of your program targets. However, in most cases your will not need to go to such lengths.



III. Observing Techniques

(See also Appendix 3)


Setting the astrometric zeropoint for the telescope:
- acquire a known, bright star
- center in finder telescope
- center in camera eyepeice
- set telescope to these coordinates
- note new position in finder for later reference

Focusing the telescope:
- acquire a star with V>=5 mag
- center in camera eyepeice and focus.
- note position of screw with a peice of masking tape.
- run focus sequence with CCD and adjust focus screw until focused in CCD.
- note this new position with a second peice of masking tape.

Acquiring your program field:
- how bright is your object?
- if it is faint, approach it from a nearby star that is sufficently bright to identify in the finder. But see Appendix 3 on telescope problems. In short, plan ahead!
- re-zero telescope on this star
- offset from the star to your object

Appendix 1: Telescope Characteristics




Appendix 2: CCD Camera and Filter Characteristics


CCD Camera:

The detector is a Tektronix 512 x 512 CCD with 27 micron pixels, thermo-electrically cooled.

THE FOLLOWING INFORMATION IS A WAG AND WILL BE UPDATED.
In fact, you will help measure these quantities:

  • pixels are roughly 0.5 arcsec - TO BE UPDATED
  • area is roughly 5 x 5 arcmin - TO BE UPDATED
  • peak quantum efficiency is roughly 50% - TO BE UPDATED
  • read noise: 15 e- per pixel (rms) - TO BE UPDATED
  • dark current: ??? - TO BE UPDATED
  • electrons per digital number (DN): ??? - TO BE UPDATED
Filters:
There are two sets of 4 filters currently available. Filters are pre-loaded into two filter slides, with a 5th slot left open.

broad band : B, V, R, I

narrow band :

central wavelength width
[OIII] 5007 A 61 A FWHM
H-alpha 6563 A 8.5 A FWHMH
H-alpha 6563 A 70.0 A FWHM
[N II] 6548 A 123 A FWHM

Transmission curves for the narrow band filters are available.


Appendix 3: Telescope Limitations


Acquisition and tracking can be problematic with this telescope. While the CCD system is capable of relatively detecting faint sources, the brighter your object, the easier it will be to conduct your observations. None the less, with careful planning and a little practice (and clear weather!), you should be able to push the system to do work are relatively faint limits.

Here are some specific problems and solutions:

Last updated: Mar 28, 1997 Matthew A. Bershady