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Questions and Asnwers

Frequently asked questions


1. Preparation of the MSBs
2. Exposure times and overheads
3. Data handling
4. Guide stars
5. General

1. Preparation of the MSBs.

Q. I have a few MSBs that are 1h 30m long. Is this okay?
A. An MSB of 1.5 hours can be accepted if it is the best way to obtain your science goal. Yet, it is recommended to keep the duration of the MSBs below 1h.

Q. I need to include J,H, and K sky observations. Can I use a survey container to do that?
A. Yes, a survey container is a good way to link your main target to sky observations.

Q. In taking sky observations in multiple filters, which way is more efficient:       
    1) J target, J sky, H target, H sky, K target, Ksky     or...

    2) J target, H target, K target, J sky, H sky, K sky ?

A. It depends on the science. Closely alternating target and sky will give you finer calibration if you are really worried about sky levels, but will increase your overheads.

Q. Is there a quick way in the OT to examine the coverage of the array in a survey container?
Unfortunately, there's no elegant way to do this yet. The reason for this is that the instrument configuration is usually specified below the Survey Container folder row in the OT, so for each given position, the JSky plot window doesn't know which instrument configuration it should be overlaying. One possible workaround for this is if you were
to temporarily copy the WFCAM component from inside your Survey Container folder block up to between the parent Site Quality component & parent Survey Container Folder, so that the Survey Container positions all then inherit the instrument setup. You should then be able to successfully plot out each of your Survey Container's fields in
turn, overlay the science fields, & blink between them. Just remember to delete the temporarily inserted extra WFCAM component afterwards, once your positions are all set up, so as to avoid possible unforeseen (& unwanted) inheritances. The other possibility is that (for a 2x2 tile) you could just specify the tile central position, & use a combination of x-offsets & y-offsets of +/-395 arcseconds for each position.

Q. Is it ok to overfill the database (going over the allocation) to increase chances of getting data in different sky conditions?
A. Yes. It is actually a good strategy provided you don't use sky constraints that are more relaxed than the one specified in your proposal. If you overfill the database, please be very clear in specifying your priorities, especially toward the end of your allocation.

Q. Can I relax the sky conditions constraints specified in the proposal?
A. No, unless specifically authorized by UKIRT Associate Director and TAG.

Q. Can I use the OT "skyflat" element to obtain a sky flat?
A. Yes, you can use the skyflat element to obtain a sky flat. Note that this is not normally needed, since sky flats are routinely taken by the Observatory staff, and made available for data reduction.

Q. If I decide to obtain a skyflat observations, do I need to set adequate constraints so that it is executed at the beginning of the night, when the sky is still bright?
A. If your run is classically scheduled, you will be the one deciding when to run your MSBs, so you'll be able to run it at the beginning of the night. If your run is flexibly scheduled, then there is no real way to do this. Specifying a very narrow set of sky constraints in order to get high enough sky counts will probably just wipe out the possibilities of getting your MSB done. In any case, we do not see the need of taking your own set of skyflats. We do routinely take flats in every filter, and it is out experience that flat field variations are negligible compared to sky effects.

Q. How much lead time would you recommend to prepare the MSBs.
A. We do not enforce any hard deadline, but it is of course recommended to upload your MSBs as soon as possible. In practice, if you can upload a first attempt about 3/4 weeks before your run, that gives us enough time to vet and iterate with you on any fixes/improvements that might be implemented.


Q. If I want to increase the total exposure time, is it more efficient to increase the number of coadds or do additional exposures/readouts?
A. Increasing the number of coadds is the best way in terms of efficiency (smaller overheads) but it is not the recommended method. Rather, you should consider increasing the number of repeats. The reason for doing this are 2: 1) if you use multiple coadds and have one bad exposure for some reason, it will affect all the coadded exposures, and 2) you will have longer time between the individual raw frames being written, so you'll have less frequent visual feedback on the data acquisition while observing. Finally, if you combine a long sequence of coadds with a long sequence of jittering, you might end up not sampling the sky frequently enough.

Q. How can I retrieve my OT program?
A. Use the Fetch Program function within UKIRTOT software. It will ask you for your password. You can have a new password issued by visiting the OMP web pages.

Q. My MSB validate ok, but when I try to validate the whole progam, I get many "SCHEMA VALIDATION" errors.
A. Unfortunately, this is a known bug of the OT. Either validate your MSB individually, or ignore the Schema Validation errors.


3. Exposure times and overheads

Q. Can I use any exposure time of my choice for WFCAM observations? 
A. Because of the overheads with filter changes, and due to a large number of stars that will be saturating and leaving latency on the arrays during the observations, we take all the darks for the usually used exposure times at the start of each night (the darks are stable). We currenly obtain the daks for the following exposure times and coadds:

CDS: (1s x 1, 1s x 2, 1s x 5, 2s x 1, 2s x 2, 2s x 3, 5s x 1, 5s x 2, 5s x 3, 5s x 4, 10s x 1, 10s x 2, 10s x 3, 15s x 1, 20s x 1);
NDR: (10s x 1, 15s x 1, 15s x 3, 20s x 1, 30s x 1, 40s x 1, 60s x 1)

If you have a pressing need for any other exposure time, please discuss with your support astronomer before defining it in the OT.

Q. What are the overhads that I need to take into account before applying for telescope time? 
A. Depends very much upon the exposure time and the coadds that you use. Please see the section on WFCAM overheads for a list of overheads associated with different exposure times.


2. Data handling

Q. What sort of data volume is to be expected per night? 
A. We roughly estimate a data rate of 250 Gb per night, but this figure highly depends on the typical exposure times. A long sequence of 1 or 2 seconds exposures can easily lead to huge amount of data, while narrow-band 30 seconds exposures won't probably generate more than 100 Gb.

Q. Is there a way to bring home a copy of the raw data? Can I connect my external hard disk? How should it be formatted? 
A. We do NOT officially support this but your support astronomer will try to help you on a best effort basis. Copying data to your external disk might take several days, so be prepared to leave with us a proper shipping package. We do not assume responsibility for the loss of the disk.  The disk needs to have a USB-2 interface. We do not support Firewire nor eSATA. The ext2 filesystem is the easiest for us. Note that another population option is the use of LTO tapes. We can support LTO-1 and LTO-2 tapes. The data provided will be compressed using the Rice-tile lossless algorithm. In any case, please make sure you contact your support astronomer well in advance and that the procedure has been discussed with her/him.

For additional instructions on how to access raw and reduced data at the summit, please see this section.

Q. I have an observation with the object observed in two separate cameras (e.g. using JITTER_FLIP). When I combine the images from the two cameras, I get double images on point sources towards the edges of the field. 
See the page below for a patch in IRAF for this:
http://casu.ast.cam.ac.uk/surveys-projects/wfcam/data-processing/iraf-zpn-patch.

Q. What is the reason we can't save the data to the drive immediately?
A. We can probably try to save immediately if your run is classically scheduled (UH, Japan). In other cases, we need to carefully separate data related to different projects and their corresponding calibrations.


3. Guide stars

Q. How can I tell if a guide star is too faint to guide on? 
A. There's unfortunately no definitive hard & fast rule on what's guidable & what isn't. as it depends on observing conditions (cloud, seeing, etc.). As a rule of thumb, we generally try to pick guide stars with a USNO catalogue R mag of 14.5-15 or brighter whenever possible, although one may get away with slightly fainter than that if the source is very red.

Q. Do I need to worry about finding a guide star for all four positions of a tile?
A. Absolutely. Each position in a tile should be regarded as an independent pointing. There is no way to use the same guide star for different positions. 

4. General

Q. The moon will be withing 20 degrees from our field. Is this a problem?
A. Moon ghosts are very unpredictable and the effect obviously depends also on the phase of the moon. We do have a constraint on the QT that does not allow the telescope to point withing 30 degrees from the moon, but that can be overridden.

Q. Which are the two best chips of WFCAM ?
For the time being, we suggest the use of Camera 3 or Camera 2. Camera 1 has a QE valley, and Camera 4 has a dead column.

Q. What are the offsets to move the target to different cameras?
Chip Xoffs Yoffs
1 795 795
2 -795 795
3 -795 -795
4 795 -795


Q. How many observers can we have at the summit?
A. There is enough room for at least 3 observers.

Contact: Watson P. Varricatt. Updated: Wed May 27 09:47:31 HST 2015

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