Many factors are involved in the determination of the telescope accuracy for pointing and tracking, including mechanical and electrical precision and observing conditions such as wind strength and direction. However, useful numbers to keep in mind are:
- All sky blind pointing: 1.15″ RMS
- Offset pointing from a bright setup star for faint targets: 0.08″ if target is within 20′ of setup star
- Sidereal tracking error with no influence of the wind: 0.07″ RMS
The telescope is affected by the wind, and, although our elevation and azimuth servos include wind disturbance rejection, it is impossible to completely eliminate the impact of the wind. Once the wind hits certain limits it can help your observing to point away from the wind. Rough guidelines to keep in mind are:
- When using the f/9, point away from the wind when it is 30 mph or stronger.
- When using the f/5, point away from the wind when it is 25 mph or stronger.
- When using the f/15 we must point away from the wind in 30 mph winds and close the chamber at 35 mph winds to protect the adaptive secondary mirror.
If observing with the f/9 or f/5 the front shutters must be closed if the wind has sustained speeds of 45 mph or gusting above 50 mph. If the f/15 is mounted the wind constraints are tighter to protect the deformable secondary mirror. If the wind is sustained at 30 mph or gusting above 35 mph we have to shut the front shutters when using the f/15. Additionally, if the wind is coming out of the North-East or East then it arrives at the telescope over Mt Wrightson and the ridge. This causes major turbulence above the telescope and will degrade the seeing and make it highly variable. In this case there is nothing we can do to improve the conditions other than wait for the wind to change direction.
If your target star is too faint to see on the f/9 acquisition/guide camera you can set up ‘offset stars’ (field of view of guide camera is ~ 3.5′). The operator will point to a nearby brighter star, tune up the telescope and then blind offset to your target star. If your target star is fainter than V ~19th then you must have offset stars. Please list offset stars in the electronic catalog that you submit using our catalog tool. Be sure to include proper motions. Organize your catalog as follows:
|HD284419||04 21 59.43||+19 32 06.4||19.5|
|HD283319_offset||04 20 06.42||+19 09 00.0||15.2|
|GK Tau||04 33 34.56||+24 21 05.7||18.8|
|GK Tau_offset||04 15 00.20||+24 20 00.0||14.8|
For best results keep your offset stars within 20′ of your target star, and, of course, the closer the better.
It is the astronomer’s responsibility to bring a list of standard stars to calibrate the data for Blue Channel observations. Please enter them in your main catalog, and please supply many options of standards all over the sky for the duration of your night. Depending on weather conditions some standards might not be appropriate, e.g., the wind is coming from the same direction.
Due to the red sensitive nature of the CCD in the WFS camera for the NGS AO system there are limitations on the magnitudes of stars we can correct on. When running the WFS camera at full speed (and thus getting full wave front correction) we can usually keep the loop closed on a star of V~11th or 12th magnitude depending on conditions. If your target star is fainter we can slow the camera speed down and do fewer corrections but still improve the image quality over the natural seeing. The limiting magnitude for closing the loop on is V~13.5th or 14th, again depending on the conditions.
If your target star is fainter than V~14th we can do one of three things:
- Leave the mirror flat and do no corrections.
- Use a very nearby (less than 20-30 arcseconds away) brighter star to correct on.
- Close the loop on a nearby bright star and ‘fix’ the shape of the mirror then return to the target star, then keep the mirror stationary for the duration of the science observations.
Which scheme you wish to use will depend on the science requirements and/or target field. Talk with your AO operator to find the best solution. If you are planning to do offset guiding, you may wish to alert Joannah Hinz before your run. Overhead for setting up AO at the beginning of the night is estimated to be 30 minutes, with an additional 15 minutes for each new object during the night. Please include this in overhead estimates when planning your run.
Twilight Flat Calibrations
If you require twilight flats for the Blue Channel Spectrograph, please note this in your Observing Form and speak to the TO well before sunset. We recommend taking 1 second test exposures beginning at sunset or shortly thereafter. Once you are able to exposure for 5 seconds without saturating, 3-5 sky flats can be acquired with a peak flux of ~20,000 counts.