ISPI Observing Notes
Updated on June 29, 2021, 12:11 pm
- ISPI data taking may be "scripted" and complete sequences executed through the ISPI GUI.
- Several examples are available (H, J, K). Each line of the text file corresponds to a single image written to disk. The key elements are filter changes (one filter per sequence in these examples), exposure time and coadds where exposure*coadds is the total time per image, and the resulting image is the sum of the number of coadds specified. For example, a 10 sec exposure of 3 coadds would produce one image with a total of 30 seconds. The image would be the sum of three 10 second frames. Currently Fowler samples, a type of read mode to reduce noise, are always set to 1. The offsets specify the dither pattern to use and can be either absolute or relative, the choice being made by selecting a button on the GUI itself.
See /home2/ispi/observers/ on the ISPI computer for many more example sequences. Any of these can be loaded directly into the instrument control GUI by clicking on the load button in the Sequence Control panel of the GUI. Use the dialog to navigate to the ascii text file of choice.
- Modify existing sequences by first making a copy. Each observer may create a sub-directory in /home2/ispi/observers/ to save their sequences. These will not be deleted.
- Each step of the sequence needs to have the same number of fields (see the GUI) even if nothing is changed during that step (e.g. the filter). It is best to copy existing sequences and modify them as needed rather than starting from scratch.
- Use care with the focus sequences as the f/8 focus mechanism has been known to go to the wrong position.
- Obtaining a good focus takes practice. The main cause of focus changes is due to systematic temperature changes of the telescope truss. The change in focus due to temperature is -1000 steps/C for the f/8 focus used by ISPI (go to higher focus numbers as the temperature gets colder).
- One can monitor the temperature by typing "temp4m" on a terminal on the observers console (not on the vnc session connected to ctioa9). Use one of the "upper truss" readouts.
- The focus can be changed by using the TCS focus control on the ISPI System Control GUI (lower left of main GUI). Click on the button to set the value entered in the box. Do not send a "0" value to the focus. Look on the TCS status monitor (an independent CRT usually to your left). Find the current focus (something like 185,500 at 10C), and enter that value to start.
- To find the best focus when you have no idea what it is, first use the last value from the previous night and estimate the change due to the difference in last night's temperature and the current temperature. Ask the night assisitant if you don't know these values. Next, begin a run of up to 10 images, each with a focus change of 1000 steps. The estimate for the best focus should be image five or six. The first image should be the farthest "in" and you should take successive images going "out" to higher numbers, each time entering a new focus in the System Control portion of the GUI. Analyze all the images, and set the final focus by first going "below" the target value and then moving the final small amount (say 200 steps) to the target by going out against gravity.
- Once a rough focus is set by the above procedure, a better focus can be found by repeating the procedure with a step of 200 units and 5-6 images.
- At the beginning of the night, the temperature can change significantly during the time it takes to set the focus. Beware.
- Always set the desired focus by moving out against gravity to avoid backlash.
- On the IRAF display, an out of focus image is typically elongated. For images which extend from upper left to lower right, focus should be adjusted to higher numbers. Go to lower numbers for images extended upper right to lower left. Use caution here since telescope motion or other problems could be confused with focus. At some point near focus, the image shape is not a good indicator of which way to adjust.
Detector Characteristics, Linearity, and Saturation
- Detailed linearity behavior is shown here.
- Keep ISPI counts below about 10000 ADU for data which are ~/< 1% non-linear, or 20000 ADU for data ~/< 2%.
- Hard saturation will occur at about 40000 ADU.
- The ISPI gain is 4.25e-/ADU.
- The ISPI dark is ~0.1e-/sec for long exposures.
- When changing filter or exposure time, the first one or few images will exhibit differing bias structure. This is a feature of the array. It means the first images will have poor background subtraction relative to others in the sequence. If this is important, plan to take one or more "junk" frames.
- Check here for information on how to take flats.
- The dome flat field lights are activated from control boxes in the computer room. Have the support astronomer or telops staff show you where the controls are and how to operate them. You will typically use only a single set of lamps controlled by the "bottom" box (of three).
- Typical exposures will be 3-10 sec with the lamp control set to 3 amps.
- Please write images to /home2/ispi/images/. You may make a subdirectory here of your choosing, e.g. 20040728/.
- It may be necessary to backup some of your data and then delete it to make room for the rest of your run.
- Your data are archived every night incase of a backup or disk failure. This archive is not user accessible. Contact telops or the support scientist if you have had a data disaster.
- You should have the entire set of data for your run backed up when you leave the mountain. After that, it may be deleted any time.
- For K-band images, the background will set the maximum observing time. Depending on the ambient temperature this could range from about 15 to 30 seconds.
- Use coadds to increase efficiency. Coadding images uses less time than writing the same number of separate images and dithering the telescope. I.e. for a given total exposure time, integrate as long as possible on each frame, take the minimum number of dither positions, and coadd as necessary.
- Experience shows ISPI is about 60% efficient for 60 second K-band images (20 seconds by 3 coadds) and 10 dithers typically less than about 60''.
February 3, 2005