Data Taking Procedures


3.1. Introduction

The IRAF-based user interface allows observing commands to be sent to Arcon from within the IRAF cl. This results in a single uniform user interface for data taking and data reduction and allows the Arcon user to employ features of the cl such as the parameter mechanism and the history editor. It also allows advanced users to write cl scripts which freely mix data acquisition and data reduction operations. We have tried very hard to make the user interface look very similar to "ICE" as used at KPNO, and it should become more similar in the future. The underlying hardware is, of course, very different. So users should be aware that it is NOT identical and parameter lists, etc. will differ.

Below we describe recommended observing procedures. You are recommended to use IRAF for reducing your data, and for writing your frames to dat or exabyte. Information on the use of CCDRED, and IRAF in general, is given in the manuals and cookbooks kept near the SUN terminal. Observer Support personnel and the Data Reductions Specialists can assist you with IRAF. But you should not go to the telescope and expect to learn IRAF at the same time as you observe!

THE MANUAL "THE ARCON-IRAF INTERFACE - A PRELIMINARY USER'S GUIDE FOR DIRECT IMAGING" Describes observing software and procedures. You should refer to this manual for details on how to take data. Only a brief outline is given below.

3.2 Starting Up, and Some Miscellaneous Advice

3.2.1 Loading & Running the Software.

You should login on the data acquisition computer using your VISITOR username and password. When you login for the first time, you will be offered a choice of OPENWINDOWS or SUNTOOLS environments. On subsequent occasions your initial choice will be loaded automatically. Several windows will open automatically, including a (blue-colored) IRAF Arcon user-interface window via which you interact with the CCD. There is also a (brick red-colored) IRAF reduction window, an Arcon console window, an Arcon status window with countdown timer, and an SAO-image (or XIMTOOL) display window (in OPENWINDOWS). You will almost certainly want to re-size and move some the windows around to get a display that you are comfortable with.

The Arcon controller boot-up is automatic (watch the Arcon console window). See the Observing Software Manual for procedures. IRAF should automatically start in the two IRAF windows, load the arcon package and connect itself to the controller.

  1. observe this command controls data-taking via two parameter files obspars and detpars. You can also use bias, comp, dark, pflat, dflat, sflat, zero.
  2. focus a special form of the observe task, for focussing the telescope.
  3. abort, stop, pause, resume, tchange interupt an exposure (temporarily or permanently)
  4. preview, movie read out the CCD without storing the data, once (preview) or continuously (movie).
  5. doobs chain together a series of observations.
  6. more n do n more exposures the same as the previous one.

As with ICE, most of the details are hidden in four parameter files, obspars, instrpars, detpars, and telpars. You will need to set these up at the start of your run, afterwards they should need little or no modification. In particular (and if you need to re-load the software, you should run setdetector force=yes in order to synch the controller to the parameters you have set in detpars.

3.2.2 Data Storage.

You should check you know where your home directory is (show home) and where your pixel files are going (show imdir). Check that you have lots of disk space (disks). Make sure that when you do an observe that an IRAF image ends up where you expect it do be. Display the image in IRAF, using display. See below for instructions on how to check whether your CCD system is functioning correctly.

3.2.3 CCDs with Multiple Outputs.

Some of the CCDs now converted to Arcon operation have more than one (usually four) output amplifiers available. The CCD is read out through each amplifier in parallel, the data is then digitized through (1,2,4) 16-bit analog-to-digital converters and then transmitted on a single serial fiber-optic line to the SUN computer to a spool file, and then converted to an IRAF image file. A raw quad-amplifier picture looks a little unusual. Each quadrant has a slightly different electrical offset (ie overscan level), and the four overscan segments are in the center of the picture. A CCD read out through two amplifiers can either have two overscan strips at the right hand side of the picture (parallel split) or two parallel strips in the center of the picture (serial split, eg Tek 2048 #3)). The real time display automatically removes the overscan and applies an offset to each quadrant to normalize the DC level. In addition, the real time display colors any saturated (ie 65535) pixel in red.

3.2.4 Processing Quad Images.

"Quad" CCD pictures are trimmed and overscan-subtracted in a separate pre- processing task called quadproc (in the quad package). You will need to load this package into your IRAF DATA REDUCTION window. Run setinstrument to set up quadproc. Note that even after the pictures are trimmed and overscan subtracted the sky levels in each quadrant will not be identical (unless sky level is zero). This is because each of the four CCD amplifiers has a slightly different gain. The procedure of quadproc'ing the pictures, plus the usual zero (bias) subtract, flat-fielding etc is now all been integrated into quadproc.

3.2.5 Writing Tapes.

There should be an exabyte drive and a DAT drive (at the 0.9-m, ctio36!mtx) ctio36!mtd) available in your dome. The Arcon produces 16 bit data of type 'unsigned short integer' with valid data ranging from 0 to 65535. Thus the unprocessed frame from a 2K CCD requires about 8.8 Mbytes.

3.3 Instrument Setup and Checkout

3.3.1 Introduction.

At the start of your observing run, Observer Support personnel will check that the CCD is operating correctly, and that all the hardware functions. They will fit your selected filters into the filter bolts. They will also ask you which gain setting you require. The various options are listed by typing

ccdinfo and you change the gain by doing setdetector force=yes. For direct imaging gain = 1 (2 for Tek 2048 #3) is in almost all cases the best to choose. The gain and read noise are calculated using the gfind task which is in the contributed package. You will first need to do two low level (bias) exposures, and two high level exposures such as dome flats (both at the same light level).

You can also choose to bin the CCD if you wish. Given that the read-out times are short, even for the Tek 2048, this is not a very popular option in general. If you wish to read out a region-of-interest rather than the whole CCD, this is also an option, and you can change from reading a quad amplifier CCD to using only 1 or 2 amplifiers if need be. All these options are controlled by parameters in detpars. You should consult with Observer Support or your Support Astronomer if you wish to make changes, not all options are possible for all CCDs.

It is your responsibility to check that the instrument is operating to your satisfaction.

A few tests are suggested below which should aid you in this evaluation. You should also be on the lookout for any changes in the CCD characteristics during the run, which may signal the onset of CCD or dewar problems. Count rates from dome flat field (white spot) exposures should be the same from day to day, to better than 10 percent. You should also divide the night's flats by the previous night's and check to see that the division is quite flat, with perhaps only the addition of one or two "rings" due to specks of dust falling on the filters or CCD window (ask for these to be cleaned if the number of spots seems excessive, but don't try to use your old flats later on). Examine bias exposures, they should not show horizontal or diagonally slanting noise "stripes", which move position from frame to frame. Sometimes very low level noise features are seen on bias frames, before complaining check whether these are at a significant level to affect your data. Most of our CCDs have very low level fixed pattern noise (1 adu or less). Use of the IRAF Imhist command is also recommended. Histograms should exclude bad columns, edges etc., they should be smooth with no missing codes.

It is worthwhile making a few test exposures before proceeding further. These tests can be repeated as desired throughout your run. All can be done during the daytime.

3.3.2 Zero (Bias) exposures.

These zero illumination exposures should show no noise patterns down to about the 1 adu level. They should be approx. flat although bright columns will show up (Tek 2048 #3, STIS 2048). Now do an exposure (dome flat) that adds a small signal. Any exposure with some illumination (preflash, flats) should show the overscan strip on the right hand side of the picture (single channel readout, or down the center (quad readout). Don't forget that the Real Time Display automatically clips out the overscan so you will need to display the image within IRAF.

3.3.3 White Spot exposures.

Ask for the telescope to be pointed at the white spot on the dome, and turn on the quartz halogen lamps. Make sure the telescope focus is approximately correct for the filters you will be using during the night. Put the color balance filter in the beam. This filter makes the 3000 C quartz lamps look more like the 5500 C night sky. Take a trial exposure, typically a few seconds through a broadband (B,V,R,I) filter. Display the frame, and do a row plot across it. The response should be reasonably flat (say 10 percent in general). After (quadproc if you are using a quad or dual readout CCD) subtracting off the bias exposure you can try dividing frames at two different count levels whereupon all the structure seen in the dome exposures should disappear to better than a few 0.1%.

3.3.4 Dark Exposure.

A five minute dark exposure (lights off in dome) with no preflash should only show the hot columns and hot pixels and a few cosmic rays. The general background should be at the same level as the overscan. Cosmic rays will be slightly trailed column-wise with a TI CCD otherwise most events should not show trails. For all these CCDs, the dark rate is so low in comparison to sky background rates as not to warrant doing long dark exposures.

3.3.5 Shutter Timing.

The shutter unit has a 100mm clear aperture. The effective "open" time is always longer than the nominal number of seconds requested for your exposure time. This is a delay prior to opening and closing. The speed of the shutter blades is high and the corner-center exposure difference is only a few ms for the smaller CCDs. For the Tek 2048 the center-to-corner difference is some 50 ms (ie exposures are 50 ms longer than nominal at the center, and 0 ms longer at the corners). So beware when doing short exposures for flat fields and standards. Sometime during your run, it is a good idea to determine this correction. The easiest way of doing this is to compare (for example) a 20 second dome flat with the sum of 20 1s dome flats. The latter can be done extremely quickly using the focus procedure but not bothering to move the telescope, whereupon you can avoid reading the CCD out 20 times but the shutter will have opened and closed 20 times. Bracket this exposure with normal 20s exposures (just choose a filter and exposure time to give several thousand adu total counts), the bracketed exposures can be averaged to take out any drifts in the lamp output, but in any case it is wise not to begin this test until several minutes have elapsed after turning on the lamps. Subtract bias off the two frames, then if the counts are C1 for the sum of the 20 1s exposures, and C2 for the single 20s exposure, then to first order we have

shutter delay time (s) = (C1/C2) - 1.

You can easily do a contour plot which is probably the best way of displaying the result. You could also in principle correct all your data. Remember -- you are likely to compromise your photometry with the Tek 2048 if you take flat or object exposures shorter than 5 seconds and do NOT take account of the radial variation of the shutter timing.

Shutter times less than one second are possible, but below about 0.1s shutter operation is unreliable. Non integral shutter times are possible, and are correctly recorded in the image header. They will most likely be seen when using pause and resume.

3.4 Calibration Frames.

3.4.1 Introduction.

Most of the calibration frames can be taken during the afternoon. Almost all observers prefer to take calibration frames each day, and we also recommend this, but it is not absolutely essential. Observer Support personnel will help you to get set up if required. If you have special requirements (dome flats in the morning etc) please consult with Observer Support in advance so that arrangements can be made. Note that in order to ventilate the domes so as to minimize "dome seeing" it is normal for the shutters to be opened approximately one hour before sunset. Please try to finish your calibration frames prior to this time. This is particularly important for the 0.9-m where the focus is a strong function of temperature and very poor images can be expected unless the telescope is at a similar temperature to the outside air

3.4.2 Zero (bias) Exposures.

These zero exposure can be started using the commands zero or observe. You should obtain between 5 and 25 exposures, if you are doing really low S/N photometry through narrow band filters then you should aim for the higher number. These can be combined into a single frame with an "average sigma clipping" or "ccdclip" algorithm (IRAF), the idea being to produce a "zero level" frame from which any cosmic ray events have been removed. You will get plenty of cosmic rays on the Tek 2048 biases due to the relatively long readout time and large area, so make sure you have at least 5 biases for this CCD

3.4.3 Dome (white spot) Flat Fields.

These exposures map the sensitivity variations of the CCD, and since the response alters as a function of wavelength it is necessary to prepare a flat field for each filter. Ultimately, the quality of your data will depend on how well you prepare your flat fields. You can use the dflat or observe commands to take dome flats. Broad band (eg BVRI) dome flats require use of a color balance filter. Do not use the color balance filter for U, or for medium band (eg Stromgren, DDO) or narrow band filters. You should aim for flat levels of several thousand (say 5000 - 40000) counts. Take a sequence of exposures per filter

The resulting frames can then be averaged to produce a single dome flat field frame for each filter. Data frames should successfully be flattened with dome flats to 1-2 %, perhaps a little better for redder passbands such as V,R,I. To do better than this there are two alternatives -either to use twilight flats alone, or to correct the low spatial frequencies in the dome flats by using a dark sky exposure(s) or with the twilight flats. However you are advised to take dome flats anyway, just in case the twilight or dark sky flats are unsuccessful. You may find the dome flats are good enough for your purposes. NOTE - You can get excellent pixel to pixel statistics with dome flats, and in some cases this may be more important than low spatial frequency errors

3.4.4 Twilight Sky Flat Fields.

Broadband B and U (especially) twilight sky flat fields appear to flatten data frames better than do dome flats. Unfortunately, twilight is short and the number of flats you can take is limited, which can limit S/N. CCDs up to 1024x1024 can be read in just a few seconds, but with 2048's, particularly if you are using several filters, other techniques include using smoothed twilight sky flats to take the low spatial frequencies out of your dome flats, thus preserving the good high spatial frequency S/N of the dome flats, or consider taking 2x2 or 4x4 binned frames and later use them as smoothed corrections to your dome flats in order to correct the low spatial frequency errors in the latter

You will need to point the telescope towards the east (HA -3 hrs) in order to lessen polarization effects (in theory -- or just leave it at the zenith, polarization effects seem to be minimal). Do not use color balance filters. As the sky darkens after sunset, take one second exposures (with U if you are using it) until the sky is dark enough so that an exposure of a few seconds gives about 20000 counts. Use the preview command, and look at the counts in the Arcon STATUS window. You should have time to take several flats in each filter, and if so these can be combined later (IRAF combine in the ccdred package). To make it easier to remove star images from your flats, take the flats with the TELESCOPE DRIVE ON, and jog the telescope a little after each exposure. If you wait until it's too dark, then your exposures will be longer and the chances of getting significant contamination by stars is greater, particularly in R and I. Twilight flats should be done in the order UBVRI. Exposures should be at least 5 seconds long in order to reduce the center-corner variation due to the shutter to negligible (<0.1%) proportions. Start the test U exposures at sunset or just a few minutes after.

3.4.5 Dark Sky Flats.

Exposures of "star-less" high galactic latitude fields can be used to provide an "illumination correction" to flat fields. The technique is to take several stepped (e.g. 20 arc sec) exposures of the field then combine the exposures in such a way (e.g. median filtering) that faint stars are removed. Since the final frame will be heavily smoothed a sky level of 200-300 ADU over bias is sufficient. Be sure to remove any bad pixels before smoothing. The resulting smoothed frame can then be combined with the corresponding dome flat to form a master flat. That is, the sky flat removes low spatial frequency errors in the dome flat. The advantage of this procedure over using twilight sky flats is that you retain the good pixel to pixel statistics of the dome flats, and the color match is improved. In practice the latter advantage does not seem to be very important. The disadvantage of using a combination of dome flat + smoothed sky flat is that any high spatial frequency errors in the dome flats will not be removed, however in general these are few

Note that dark sky flats are necessary to remove any fringes due to night sky emission lines causing interference in the thinned CCD (Teks). None of these CCDs fringe excessively (<1% in the I band is typical) and so you may not need to de-fringe

It is also possible to prepare flat fields from combinations of your object frames. This is generally only possible if they contain few bright stars, and even then it may be necessary to "patch" out the brighter objects prior to combining the frames. If you are unfamiliar with these rather specialized techniques you are encouraged to consult with CTIO staff prior to your observing run.

3.5 Night Time Procedures.

3.5.1 Start of the night procedures.

About an hour before sunset the dome shutters will be opened unless calibration frames are still being taken. All doors should be closed and lights turned off. During twilight, your Observing Assistant will fill the dewar with liquid nitrogen and check the telescope pointing on a bright star near the zenith. If it is the first night that the CCD has been installed then the offset between the CCD and the acquisition TV camera will also be determined, and the focus of the TV camera checked. Your Observing Assistant is familiar with focussing methods, offset determinations, operation of the TV and Guider, and will help if required.

3.5.2 Focusing the Telescope.

On the first night set the telescope focus to the "nominal" value. Ask your Observing assistant or look amongst old Observing Record Sheets to ascertain the relevant value. If the focus is far from correct, read the CCD out using movie while you adjust the focus. Choose a focus star with V 10-11 (0.9- m, scale to other apertures) whereupon your exposure time will be a few seconds.

Accurate focus is found by taking several snapshots of the star, each at a different focus position. Use the focus command, with the relevant obspars parameters set. It is normal to choose shtype = detector (charge will be reverse-clocked rather than the telescope moved between exposures), and at the 0.9-m and 1.5-m focmode=manual, ie you need to change the focus manually between exposures. NOTE in all cases the double-spacing is made between the first and second exposures. And for shtype = detector, on quad readout CCDs the stars in upper and lower halves of the CCD will move in opposite directions.

Please save your focus frames and write a note on the night report sheet saying where they are, they will be written to tape and kept as a database for seeing and optics improvements. Offsets for the standard UBVRI sets are known (ask Observer Support) so you need only focus one filter.

When focusing, always focus against gravity: move out (to larger readings of the pedestal). The procedure is as follows:

Once rough focus is known, move focus in (lower numbers) about 500 units (0.9-m) to take up backlash. Exact focus is determined by doing 8 - 10 exposures at different focus settings, offsetting the telescope slightly between each one. The CCD will be read out after the last exposure. Vary the focus by 40 units (0.9-m, Futaba) between each exposure.

The quickest way to examine your images is to use imexam to plot profiles. Best focus will be that with highest peak intensity and minimum distortion in the wings. This last criterion is perhaps the most stringent one. The focus will change during the night, as it appears to be a function of temperature and possibly telescope position. In good seeing (FWHM <1.5") frequent focus checks are recommended especially if you move the telescope more than 10 or 20 degrees.

3.5.3 TV Camera.

A Peltier-cooled CCDTV camera serves both for field acquisition and for guiding. Field of view is approximately 4 by 3 arc minutes. On the CCDTV control panel integration time can be switch selected and gain/offset controls are available. Try integration time of 1-2 seconds, gain = 2 and offset so that you see star images.

Focus the TV after focusing the CCD. Watch for smallest images, preferably with the Leaky Guider switched to "digital". This focus is not that critical, as the guider will work fine on slightly out of focus images.

Normally the determination of the TV-CCD offset will be carried out entirely by Observer Support personnel on the first night only. In any case, in these days of large CCDs it is more efficient to find objects and center up using the CCD in preview mode. But if you want to determine the offset, see the Software Manual.

3.5.4 Guider Operation.

We are changing over to using a program written by Steve Shectman, which runs in a PC and displays the star-field on the VGA monitor. At the 1.5-m, the TV is feed by a moveable guide probe. Be careful not to get the probe vignetting the beam. For the Tek 2048, allowable guide probe co-ordinates are X = anything, Y > 50.

3.6 Observing.

3.6.1 Finding Your Object:

The quickest procedure is to point the telescope at the correct position and take a preview or movie. Alternatively the telescope can be positioned such that the object is on the TV, and then offset back to the CCD. Then find a guide star.

3.6.2 Exposure times:

All the CCDs bleed charge down columns, and to a lesser extent across rows, when an integration is long enough for a star image to become saturated. All recover well from over-exposure, however excessively bright stars should be avoided. All the CCDs have very low readout noise so there is an advantage in dynamic range by splitting a long exposure up into a number of shorter ones. You should also beware of bright stars just outside the CCD field. These can easily scatter light onto the CCD frame and make photometry difficult.

Cosmic rays tend to be either single or double hot pixel events. The standard way to remove them is to do at least 3 exposures (4 or more give better statistical results) of the same field and median (or use avsigclip option in the IRAF combine command) filter to remove transient events. The IRAF cosmicray and imedit commands can be effectively used to remove cosmic rays from single frames.

You should be careful not to over-exposure your object(s) of interest. Saturated pixels are colored RED on the real-time display. Beware that for 2048 CCDs the display averages four CCD pixels for each display pixel. It is not (yet) clever enough to color the display pixel red if one or more of the component CCD pixels is saturated.

3.6.3 Bad pixels:

there are two types: 1) hot (above local background) and 2) cold (less than local background). Sometimes hot pixels tend to be found at the beginning of a column of cold pixels - it appears that most of the charge finds its way into just one pixel. Neither type are transient events: they are both permanent defects in the CCD. However, the number of bad pixels is a function of intensity: there are many more at low light levels. The STIS 2048 and Tek 2048 #3 have both hot columns and blocked pixels. These are obvious, and should be avoided. The Thomson CCD has about 20 short traps visible only at low background levels; at higher backgrounds almost all of these shorten to a single low pixel.

3.6.4 Standard Stars:

In broadband UBVRI work excellent results have been achieved by using standards from Landolt (AJ 104, 340, 1992). There are fields (eg T Phe, SA 98, SA 110) which contain several stars in a 3-4 arcmin diameter field with a wide range in color, and so are invaluable for use with large CCDs. John Graham's uncrowded E region standards (PASP, 94, 244, 1982) in the magnitude range of 10 to 16, the older Landolt standards (AJ, 78, 959, 1973; AJ, 88, 439, 1983) and some of the fainter (10-11 mag) Cousin's stars (J.W. Menzies et al., SAAO Circ., 13, 1) are useful. Copies of these lists are kept in each of the console rooms. Look carefully at the quoted errors on the standards before observing them. Residuals in the photometric solutions are a few percent at worst, and standard stars (properly done) repeat to 0.005 mag or better from night to night. It is preferable not to mix Landolt standards with those of Cousins/Graham if you are covering a wide range in color and are interested in getting systematics down to the 0.01 mag level.

For narrow band imaging one can try using spectrophotometric standards (R. Stone, J. Baldwin, 1983. MNRAS, 204, 347 and 1984. MNRAS, 206, 241, D. Hayes, 1970. ApJ, 159, 165, M. Hamuy et al., PASP 104, 533, 1992).

The biggest mistake made in observing standards is to underexpose: try for a peak in the star of at least a few thousand ADU above background (exposure times of 5-20 seconds for V = 12-14 at the 4-m, with the Tek CCDs and gain at 10 e-/adu you can go a magnitude brighter than these figures). For such short exposures the dominant source of error is the readout noise in the large measurement aperture necessary to contain 99% of the stellar light. Shutter clocking is accurate enough to permit 1 second exposures, but be aware of the shutter delay discussed above. If necessary you can remove the shutter timing effect from the data (Stetson, 1988 IAU reports).

Use of faint photoelectric sequences (particularly those in crowded fields) requires caution; these often have proven to have systematic errors, and require long exposure times in each filter to reach the desired accuracy in a large measurement aperture.

The care with which one observes standards will be reflected in the accuracy of one's photometric calibration. It is common to observe several standards at the start of a photometric night, several more in the middle, and still more at the end. If you have fields with only one or two standards, try taking two exposures of a standard field per filter, stepping the telescope by 20 arcseconds between exposures. The extra frame takes very little time, and insures against your star landing on a bad pixel or cosmic ray. You could even use the observe command with focus mode enabled to give you several images of the standard but only one readout. For the highest accuracy one should definitely observe enough standards at a range in airmass to solve for the extinction coefficients. Tololo extinction can be as low as kV = 0.12, or > 0.2 if conditions are dusty, so one should not rely on the nominal values (in 1991 volcanic dust caused elevated extinctions). The CCD camera has shown itself capable of doing better than 1% photometry: it seems a shame to skimp on the external calibration necessary to achieve this. Furthermore, to make proper use of such an accurate calibration will require the existence in the object frames of uncrowded stars exposed to the same level as the standard.

Updated on April 17, 2024, 5:35 am