CGS4 Object acquisition Procedures

Peak-up procedures with CGS4 seem to be a large source of confusion, especially for observers determining before hand what their requirements are in terms of astrometry, co-ordinates and reference stars. This page aims to clear things up a bit.

The complication arises in that there are many ways to attempt to ensure that you have accurately hit your target with the slit, all of which apply to different situations.

Concepts and Introduction

The aperture defines the position of the slit within the telescope focal plane. The default aperture is re-determined during engineering time whenever the instrument has been off the telescope, and at regular intervals inbetween. The aperture varies slightly with flexure etc. Fine adjustments to the aperture definition are made with the peak-up procedure.

The actual peak-up procedure is the process whereby the aperture definition is finely adjusted to give the maximum signal on the CGS4 array, whilst pointing at some (bright) target. This involves taking several (typically 15 or so) CGS4 integrations. Each of these must convincingly detect the object being peaked up on. It is possible to automatically subtract sky frames from the images used for peak-up. The integration time needed depends on the instrument configuration in use and on how bright your target is.

With the 40l/mm grating, peaking up on a target fainter than 14th magnitude takes a long time. If you need to estimate peak-up times for fainter targets or other configurations, refer to the CGS4 sensitivity page, and use the following: Calculate the exposure time for a 3 sigma detection of a point source at your target's magnitude. Multiply this by a factor 2 (to account for the fact that for most of the frames, you're not peaked up, so you're only seeing a fraction of the light). Multiply this by 15 (say 15 observations needed).

The Fast Guider is mounted on the cross-head. The cross-head can position the fast guider very accurately (~0.1") within the telescope focal plane. The crosshead limit is roughly a circle, 180" from the pointing centre (ie centre of the slit).

The Fast Guider brightness thresholds also vary with conditions. Because the fast guider works in the optical is is affected by scattered moon-light. In exceptional conditions - clear skies, exceptionally good seeing and no moon, we can guide on stars down to V magnitude of 18.7. A V mag limit of 16 or 17 is more realistic for average conditions. On fainter targets, we increase the integration time of the autoguider CCD, which reduces the Tip-Tilt frequency. On bright targets, we guide at 100Hz. We can go down to ~20Hz for faint sources. Bright sources are more likely to give you really good Tip-Tilt performance.

UKIRT employs a dichroic tertiary mirror, feeding the IR light to the instrumentation, and the optical light to the fast guider. This means that it is possible for the fast-guider and the IR instrument (eg CGS4) to observe the same target.

Trivial case - bright (~14th Mag or brighter in the IR) guidable targets.

In this case you guide on the target, and you peak up on it before you start taking data. The peak-up takes a few minutes.

No special co-ordinate, astrometry or guide star requirements.

Bright (14th Mag or brighter in the IR) non-guidable targets.

Reasons they might be non-guidable include: Target not point-like in the optical, or target is so red that despite being bright in the IR, it's too faint for the fast guider to see (see notes above).

In this case, you guide on an offset guide star (needs to be guidable and within 180" of target), and you peak up on the target.

Requirements: A guide star. You can pick one using the ORAC-OT preparation tool when you get to Hilo. You don't need any special astrometry. If your co-ordinates are off by more than an arc-sec or two, peak-up will take longer.

Intermediate brightness targets.

OK, the definition of "intermediate brightness" is non trivial. By this, I mean something you can guide on, but something that would take too long to peak up on. See the notes above about the fast guider sensitivity and peak-up times.

Procedure: The TSS will slew the telescope to a nearby (within a few degrees) bright CMC star, and will peak up on that whilst guiding on it. This ensures that the slit and fast guider are at the same place within the telescope focal plane. Then they'll slew back to your target and guide on it. Because peaking up on the CMC star aligned the guider and the slit, the slit will now be accurately placed on the target.

Note: The guider and slit will not move relative to each other over a small slew to the CMC and back. If it were a long slew, there would be concerns about flexure and differential refraction between the optical and IR beams.

Requirements: The TSS will locate a suitable CMC star as needed. You need reasonably accurate co-ordinates (ie within a few arc-sec). If the target is towards the fainter edge of the guider's capability, better co-ordinates will help. If the source is not point-like enough to guide on, treat this as a faint target.

Faint targets.

By "Faint", I mean too faint to guide on. This obviously implies also too faint to peak up on. There are several options in this case.

Blind offset

This is the most reliable way of acquiring very faint targets. You need an accurate astrometric offset between your target and a nearby (within 180") guide star (see notes above on fast guider for what qualifies as a guide star). You would usually measure such offsets from a deep image of your target field. By "accurate", I mean to within a fraction of the slit width you will be using. Say 0.2" for the 0.6" slit. The more accurate your offset, the more light you'll get down the slit.

Proceedure: Simply enter the RA and Dec co-ordinates for the target and guide star in the OT. Ensure that the co-ordinates you enter have the correct offset between them, taking into account the factor 15 between seconds of RA time and seconds of arc. When you slew to the field, inform the TSS that you have an accurate crosshead offset set, and thus to adjust the telescope position rather than the crosshead position to bring the guide star into the fast guider if necessary.

The TSS will ensure that the telescope pointing model is good prior to you starting to take data. This will involve at least going to a nearby CMC star, and probably doing a peak-up on it. If necessary they will collimate the telescope pointing model to the CMC star co-ordinates at this point.

Requirements: Accurate astrometric offset to a nearby guide star.

Blind Pointing

This is the "last resort" method, though the UKIRT Telescope Control System and Pointing model is sufficiently good that this method usually works well.

You would use this method if you don't have the astrometric offset to a guide star necessary to carry out the previous method. You do, however, need accurate absolute co-ordinates. You shouldn't rely on this method with the 0.6" slit. Many observers have had great success using this method to observe radio sources, with accurate co-ordinates from high resolution radio interferometers, using the 1.2" slit.

Proceedure: Type your accurate co-ordinates into the OT. Use the OT to find a nearby offset guide star. The TSS will go to a CMC star close to your field and collimate the telescope pointing model. When they slew back to your field, inform the TSS that the target co-ordinates are accurate, and to move the crosshead to bring the guide star into the guider if necessary.

Threats: You need accurate, co-ordinates. You might need to be aware of the co-ordinate frame your co-ordinates are referenced to (how well is this frame tied to the Hipparcos or Radio reference frames?). This method becomes less reliable in high wind conditions - during the time the TSS is adjusting the guider position to acquire the guide star, the telescope is sat on it's main drives, as opposed to being locked onto a guide star. If wind gusts are knocking the telescope around at this point, the drives are unlikely to be able to hold it in position to great accuracy.

Requirements: Accurate absolute co-ordinates.

Absolute Desperation

If your targets are too faint to guide on, and you don't have either sufficiently accurate co-ordinates or astrometric offsets, then your best option is probably to use some time at the start of your observing run to get UFTI images of your fields, from which you can measure offsets in order to do a blind offset. You should discuss this with your support scientist and Paul Hirst, the CGS4 instrument scientist, well in advance of your arrival in Hilo.