Is the object in the night sky at the IRTF?

In order to plan an observation, the first task is to determine whether the objects of interest are visible at night. The telescope scheduler assumes that 'night' means from astronomical dusk to dawn, i.e. Sun's elevation -18 degrees (below the horizon). Use a sky display program such as TheSky or Megastar to see what the rising and setting times are. Make sure you have chosen the IRO location [latitude 31.66, longitude 110.61] and the approximate date of observation.

How much of the sky can the telescopes see?

The telescope horizons are quite low (less than 5 degrees) except to the north, where the roll-off roof blocks the sky at elevations below 20 degrees. The minimum observable declination is about -53 deg (5 deg elevation at transit), although the image quality and sensitivity will be compromised since the telescope is looking through 10 air masses! There are no hour angle limits.

Do I have to specify celestial coordinates for an object?

In most cases, no. When an object is named in a schedule, the scheduler looks at a large number of catalogs for a corresponding entry. The only trick is to use the same name as the catalog. For example the name M13, Jupiter, NG682, and UX Ari are valid names form the Messier, Planet, NGC, and GCVS catalogs. Asteroids are specified by number only, e.g. 4 for Vesta. Click on the Catalog hyperlink (also on the observing request form) to see a listing of all on-line catalogs.

What observing time should I request?

Normally the telescope scheduler will choose times near transit automatically. This corresponds to the highest elevation, which minimizes atmospheric extinction. If you have a particular need for observations at another time, use the start time option, specifying either hour angle (HA) or local sidereal time (LST).

What filters should I use?

That depends entirely on the goal of the observation. For the best sensitivity, always use the clear (C) filter. Use the B, V, R, or I filters for photometry. Use the narrow band filters (X,Y,Z,O) only for special projects such as HI regions or planetary nebula, since they have a very narrow bandwidth. Here's a list of the available filters. Note that the BVRI filters conform to the Johnson-Cousins photometric standard.

What exposure times should I use?

The optimal exposure time depends on the magnitude of the object being observed, the filter used, and the sky brightness. The minimum recommended exposure time is 0.5 second, dictated by the time required to operate the shutter (about 100 msec). The maximum exposure time is in theory unlimited, but the mount pointing model limits this to about 60 seconds before tracking errors affect the image. For very deep exposures, use multiple/ 60 second images and average.

The following table gives the maximum recommended exposure times (sec) as a function of apparent magnitude and filter to avoid saturation (ADU counts > 40,000). The table assumes high elevation angle and dark skies. for low elevations, the maximum times can be somewhat longer.

What are the faintest detectable magnitudes for the IRO and Rigel telescopes?

The expected signal-to-noise ratio for faint stars as a function of exposure time is shown for the IRO telescope in the following plot. For the Rigel telescope, subtract one magnitude. The plot assumes a clear filter, dark (moonless) skies, high elevation, and good seeing. For objects fainter than V=19, multiple exposures will be needed.

What is the sky brightness and seeing at the Winer observatory?

Under moonless conditions, the sky brightness is typically 18.5-19.0 magnitudes per square arcsec. The seeing is often 2.5-3.0 arcsec (FWHM), although at vey low elevations it is somewhat poorer. The IRO optics have some defects that cause the point spread function (PSF) to vary somewhat over the image, so that the seeing increases up to 1 arcsec near the edges.

What about observing planets?

In order to observe the 'naked-eye' planets (Venus, Mars, Jupiter, Saturn), you will need to use a narrow band filter, since the CBVRI filters will all saturate even at 0.25 sec exposure. Try the O (green) or Y (blue) filters for best contrast on surface features and use the table above to estimate appropriate exposure times. Note that a short (0.25-0.5 sec) exposure is preferred since blurring due to atmospheric seeing is often 'frozen' for short exposures.

What software can I use to display and analyze images?

The format of IRO images is FITS (Flexible Image Transport System), the standard image format in most astronomical Observatories. There are numerous FITS viewers available. Here's a selected list that we have some familiarity with. Click on each name to see the web site for that product.