BGSU Variable Star Project:

CCD Observing Procedure

Procedure Outline

1) Plan on arriving at the observatory 15-30 minutes before sunset. There should be a graphical observing plan for the night and a list of the RA/Dec values of each star (if not, phone ACL).

2) Ensure the CCD PC in the warmroom is shutdown. Check that the CCD cable is connected to the CCD unit on the telescope. If not, gently attach the connector and snug the screws. Never plug the cable in when the PC is powered up -- you will damage the CCD!

3) Boot up the CCD PC. Check the clock (lower-right corner of screen) to ensure that it is within a minute or so of the correct local time (the UT is computed from this clock and is stored in the image headers). If it is not correct, double click on the clock and set it correctly.

4) Enter MaxIm_DL by clicking on the "Shortcut to MaxIm_DL" icon on the left side of the screen. Initialize the CCD camera. >>> New: extra steps in Astr 309 is in session <<<

5) Set the camera temperature to keep the CCD cooled to a constant temp -- it may take a few minutes to stabilize.

6) Start a paper log of the night's observing (pages available in the STScI folder). Be sure to document the sky conditions and dome temperature. Also record the Universal Time (UT), Local Sidereal Time (ST), and Local Time (LT) at the beginning of the night. A sample log sheet is included in the back of the STScI folder. Note which filter is being used -- see the bulletin board on the wall of the Control Room.

7) Before opening the dome, with the dome lights OFF, take 8-10 bias frames.

8) Open the telescope as usual, following the "Telescope Observing Procedures for CCD" sheet kept in the warmroom. Check that the UT is correct. If not, set it by choosing "Set Date and Time" from the "Initialization Menu".

9) Slide the visual eyepiece all the way to the right and tighten the thumbscrew (lest it obscure the CCD!). Set the telescope focus to the default CCD value (2382 units).

10) If there are no/few clouds outside, take twilight sky flats. Each flat must have:

11) Zero-Point and Focus the telescope.

12) Pick a star from your observing list, locate the finder chart, and observe the star. Remember to save the images using the proper naming convention.

13) Repeat 12 until dawn, or you are tired, or it clouds up. Refocus if the FWHM gets large (stars get blurry).

14) If it is dawn, clear, and you can stand it, take more sky flats. : )

15) Slide the eyepiece back to the center (7.5 on the scale). Set the telescope focus back to 2420 (visual). Close up the telescope as usual.

16) With the dome lights OFF, take 8-10 more bias frames (see step 18).

17) Warm the CCD to dome temperature and shut down the CCD (from setup tab). You can probably take the bias frames (see Step 16) while the CCD is warming, if you are quick! You can also transfer the night's data to baade using FTP.

18) Exit MaxIm_DL (File/Close), and shutdown the CCD PC (fans in the dome should turn off). Unplug the CCD cable from the CCD unit (this minimizes danger of damage from lightning strikes). Never unplug the cable when the PC is powered (CCD fans running) -- you will damage the CCD!

19) Peak back in the dome before you leave: are the lights and CCD fans off? Lock up and get some sleep!

Note: Here is a page for troubleshooting CCD problems, and instructions for dealing with major problems and emergencies. Here is a page for troubleshooting problems with the telescope or observatory.

Updated 2006 Aug 08 -- ACL

1. Planning your Night

1) On the RRLyr Shelf on the cabinets to the right of the door, there should be a current Observing Plan plot, which should look something like this. If there is not, call Andy Layden to get one.

2) Hint: While observing, after you finish the observation set for a star, place a mark on the plot at the ST. In the event that you deviate from your plan (lose time to clouds or equipment failure, or even if you observe faster or slower than the default 15 min per object) you can revise your plan "on the fly". Just revert to the Rule in Red ("In general, ...") above.

Updated 2000 July 06 -- ACL

4. Link to the Camera and Filter Wheel

Note: In order to send and receive information from the camera, or to make the filter wheel turn, there must be links established to these devices. [Currently, we are not operating the filter wheel -- May'02]

1) If it did not appear already, bring up the CCD control window "MaxIm CCD" by clicking on the button icon on the toolbar (8th from the left). Select the Setup tab.

2) The three boxes along the left of the window should read:

If these do not appear, choose them correctly from the drag-down menus (see Troubleshooting). When all is OK, click Restart to initialize the camera.

>>> New: if Astr 309 is in session <<< Students may have left the filter wheel in a position other than the one you want (e.g., I-band filter). To ensure it is where you want it to be:

(a) Plug the small connector bound with the CCD cable into the filter wheel unit. Then get the power transformer (usually on a tall back shelf in the control room), plug it into the 110V power near the reticle light control knobs, and the small end into the filter wheel unit.

(b) In the Setup tab (see #2 above), enable the filter wheel:

(c) Take a 1-sec exposure setting the filter to the one you will use for RR Lyraes (e.g., I). The filter wheel does not move until you take an exposure.

(d) In the Setup tab, put "Filter" back to "No Filter" and disconnect the filterwheel power cords.

(e) Select the Settings tab. In the "Auto Calibration" field, click the None radio button (A309 students usually use Simple Auto-dark mode).

Updated 2007 Jan 19 -- ACL

5. Setting the CCD Temperature

Note: In general, CCD's perform best, producing the lowest noise, when they are cooled well below freezing (0C). We also want to keep the CCD at a constant temperature all night, so its noise and sensitivity characteristics stay constant (otherwise the bias and flat frames you take at the beginning of the night won't properly correct the images you take later at a different temperature).

Our CCD camera has an automatic temperature control which makes it pretty easy to set and forget, but it is important to set it correctly at the beginning of the night. The camera assembly contains a thermoelectric cooler that can drive the CCD temperature down to about 30C below the ambient air temperature in the dome. The CCD performance seems best if we can keep it at -15C or below.

1) In the dome, check the ambient air temperature in Celsius (let's call this Tdome) using the thermometer on the desk. We will set the CCD temperature 28C below this value:

Tset = Tdome -- 28C.

2) Select the Setup tab in the "MaxIm CCD" window, and click the Cooler On box. Enter your value for Tset into the box labeled New (C) and click the Set button.

3) The message "Cooling Down" should appear in the box at the bottom of the window, and the temperature in the Actual box should change in the direction of Setpoint. It may take a few minutes for Actual and Setpoint to match. Don't take any pictures until they match. Note: only rarely will the values in Actual and Setpoint match exactly -- usually Actual shifts around within 1-2C of the Setpoint value -- this is normal.

4) At the end of the night, you should allow the CCD to warm up to dome temperature before turning off the CCD. Do this by selecting the Setup tab in the "MaxIm CCD" window, and clicking the Go to Ambient box. You will see the Actual value change away from Setpoint, and the message in the box will say "Cooler Regulating". It may take a few minutes to warm up, so be patient. When the temperature stabilizes, click the Shutdown button to turn off the CCD. While it is warming, you can:

Updated 2002 Jun 07 -- ACL

7. Taking Bias (Zero) Frames

Definition: A "bias" or "zero" frame measures the amount of signal, or charge, on the frame due simply to the read-out. It has zero exposure time, and received no light. In addition to determining and correcting for the mean level of this signal, the bias can also be used to correct the object images for any pattern in this signal which is constant in time (often referred to as "fixed-pattern noise").

Why a bias? The CCD automatically puts a background level of counts on the chip. Imagine if there was no bias applied. If you took a frame with a very short exposure time or in very dark conditions, the counts due to the sky background would be about 0. However, there is noise associated with reading out the chip, so some pixels would want to have negative counts. Since the chip only records integer counts between 0 and 16384 (2^14 -- we have a 14-bit chip), these pixels would get the value 0, and the statistics of the sky noise would be corrupted. Putting a bias on the chip ensures that all pixels will have a positive value, and the statistics will be computed correctly.

1) Be sure all light sources are OFF in the dome, including the eyepiece reticle lights. The dome slit should be closed, and the dome as dark as possible. The covers should be on the telescope.

2) In the "MaxIm CCD" control window, select the Expose tab and

3) The image will read out and appear on the screen. For the first bias image you take, check the number of counts -- slide the pointer around the image and note the (X,Y,Counts) readout just below the image. You should see around 500 counts, depending on CCD temperature. This is the usual bias level for our chip. If it is much different, please report it to Andy Layden.

4) Save the image to disk by clicking the 3rd button from left on the toolbar (diskette icon). You will want to create a new directory for each night's data.

5) Take a second bias frame using the Expose button. Save it.

4) Repeat Step 5 six more times, giving 8 bias frames in the sequence.

Updated 2002 Mar 07 -- ACL

10. Taking Twilight Sky Flats

Twilight Sky Flats can be taken at dusk (evening) or dawn (morning). Doing them at BOTH is preferable, if you can manage to stay awake in the morning! The procedures for the two are subtly different, so explicit instructions for dusk flats and dawn flats are given separately. After that, there is some wisdom on when to start taking your sky flats.

Note: Sky flats are best taken when the sky is perfectly clear, so the sky is uniformly bright. Uniform clouds are ok, but you should try to get more images per filter (10 or more, if possible) to help average out any non-uniformities due to the clouds.

Goals: ideally, we would like 5 or more images in each filter you will be using that night. An ideal exposure would have 7000 ± 1000 counts in each image, though any sky flat with 2000 to 10000 is worth saving. All of the images must have exposure times longer than 10 sec. It is also important to move the telescope between images, so any stars change position on the chip from one image to the next.

Definition: Sky flats measure the relative sensitivity of each pixel. Assuming the sky is uniformly bright, the pixels with lower sensitivities will have lower numbers of counts. As part of the "image processing" procedure, we will later correct for the pixel-to-pixel sensitivity variations by DIVIDING each object (star) frame by a combined sky flat frame, thus increasing the measured counts in the pixels we know to be under-sensitive to where it should be. The pixel sensitivities may depend on the color of the light they are seeing, so we must take flats through each filter we will use that night. We need to get 5 or more flats per filter to reject "cosmic rays" and star images that appear on the individual frames. We move the telescope between images so the stars do not appear in the same (X,Y) location on the chip, and thus create an erroneous "divot" in the combined flat field image.

Why the 10 second Rule? One should never take sky flat or object (star) exposures shorter than 10 sec with our CCD. The shutter in front of the CCD takes a finite amount of time to open and close. Tests show that for exposure times less than 10 sec, this "shutter delay time" results in a true exposure time that is different from the requested time by more than ~1%. What's more, the shutter is an iris (like in your 35mm camera), so the center opens before and closes after the corners, resulting in extra exposure in the center relative to the corners. In theory, one can map out this effect (create a "shutter correction image") and apply the correction to all your images, but it is a pain. Easier to just avoid the short exposures altogether!

10a. Dusk (Evening) Sky Flats

1) If you are taking dusk flats, point the telescope about 1.5 hours East of the Zenith. The dome should rotate so the slit is facing East, away from the setting Sun (if there is a bright moon nearby, shift the telescope North so there is >60 degrees angle between the moon and where the telescope is pointed ... >90 deg is better still).

2) Be sure the telescope tracking and autodome are ON. Be sure the telescope focus is set to the default value for CCD observing (2382 units) and the eyepiece is all the way to the right.

3) Take a 0.1 sec test exposure: select the Expose tab on the "MaxIm CCD" window,

Once the image has read out and appeared on the screen, move the cursor around near the image center (brightest region) and see roughly how many counts there are using the (X,Y,Counts) readout at lower-left. A well-exposed skyflat has between 6000 and 8000 counts.

4) If the image is saturated (all pixels uniformly 16383), you will have to wait a while and try again when the sky is darker (maybe 5 minutes). If this image is not saturated, but has more or less than the optimal number of counts, estimate your next exposure time with

though remember that while you are doing this, the sky is getting fainter outside. Experience will enable you to guess a "seat of the pants" correction to your computed exposure time.

5) Take another test image with your improved exposure time estimate. When your exposure times finally get longer than 10 sec, start saving the images to disk.

6) After each good sky flat, move the telescope East with the handpaddle by an arcmin or more. This is to ensure that star images don't land atop each other.

7) Keep taking and saving skyflats until the exposure times are longer than 200 sec. The rule of skyflats is, "the more the better." Besides, you can't begin taking good star images until the sky has darkened, so you might as well be taking sky flats!

10b. Dawn (Morning) Sky Flats

1). Dawn comes on quickly and can surprise you. Check the time of morning twilight on the RRLyr shelf, and set an alarm (mental or mechanical) to alert you. You will probably want to start morning sky flats about 30 min after astronomical twilight. Another warning is that the sky level in your star images will start rising. You want sky flats with about 7000 counts, so when the sky background in your images reaches the following value (where T_star is the exposure time of your star image), you would switch over to sky flats:

2) Point the telescope about 1.5 hours West of the Zenith. The dome should rotate so the slit is facing West, away from the rising Sun). The telescope tracking and autodome should still be ON, and the eyepiece and focus as you had them while observing stars. If there is a bright moon nearby, shift the telescope North so the angle between the moon and where the telescope is pointing is >60 degrees ... >90 deg if possible).

3) From you last star image, calculate the exposure time needed to give you 7000 counts in your skyflat using the following equation, where Counts_star is the number of sky counts in your star image:

Let's say you estimate it to be 150 sec. Take an exposure (Expose tab on "MaxIm CCD" window):

Once the image has read out and appeared on the screen, move the cursor around near the image center (brightest region) and see roughly how many counts there are using the (X,Y,Counts) readout at lower-left. A well-exposed skyflat has between 7000 and 11,000 counts. If the image is good, save it to disk.

5) If the image was saturated (all pixels uniformly 16383), you cut your exposure time a lot (maybe a factor of 5). If the image was not saturated, compute your next exposure length as in Step 3, though remember that while you are doing this, the sky is getting brighter outside. Experience will enable you to guess a "seat of the pants" correction to your computed exposure time.

6) After each good sky flat, move the telescope East with the handpaddle (about an arcmin is good). This is to ensure that star images don't land atop each other.

7) Take another image with your updated exposure time. When your exposure times finally get shorter than 10 sec, you can stop (again, "the more sky flats, the better").

10c. When to Start Taking Sky Flats

This depends on whether you are taking dawn or dusk sky flats, and what filters you are using. The following are vary rough estimates for when you should start taking 1 sec test exposures. Of course, only flats with exposure times longer than 10 sec should be used, but starting with 1 sec test exposures allows you to increase exposure times gradually.

Dusk Sky Flats:

Dawn Sky Flats:

Updated 2006 Aug 08 -- ACL

11. Zero-Point and Focus the Telescope

General: We will determine the best telescope focus by obtaining a sequence of short exposures of a bright star, stepping the focus slightly from one exposure to the next. MaxIm has a handy tool which allows us to determine which focus setting gives the narrowest, roundest images, with the highest peak intensity, i.e., the best focus. We then set the telescope focus to that value. You can read more about this procedure in the MaxIm CCD Manual (pp.4-3 to 4-5).

NOTE: the focus is set using the buttons on the handpaddle (not with the keyboard).

1) Zero-point the telescope (RA, Dec) coordinates:

2) Point the telescope at a Focus Star near the zenith. Pick a star from this list of candidates.

3) Set a starting focus -- generally, we start 5 units below the default focus: 2382-5=2377. Drive the focus 10-15 units below this value, then approach the starting value from below. There may be some "backlash" in the focus screws, so always approach focus from below.

4) Using a piece of scrap paper, scrawl a "focus table" on it. For each focus image you take, record the focus value, the Full-Width Half-Maximum (FWHM) values in X and Y, the Maximum Counts, and comments on appearance.

5) Select the Focus tab on the "MaxIm CCD" window, and

this will take one 12 sec exposure and write it to the screen, then pause.

6) Find your focus star on the image (see Focus Star Finder Charts if needed).

7) Select the Inspect tab on the "MaxIm CCD" window. You see a 3-D picture of what the star's "profile" looks like, along with measurements of the FWHM in X and Y (the width of the star image in the X and Y direction), and Maximum (the number of counts at the peak of the profile -- if it is 16383, the star is saturated and you need to use a shorter exposure time, or chose a fainter star). Write these in your focus table.

8) Increase the telescope focus by about one unit (to 2378 in our example) and hit Start Focus to take a new exposure at this focus setting. Carefully record the focus value and results associated with each image in your focus table -- it is easy to forget!

9) Repeat Step 8. If all goes well, you will see that the first exposures in your sequence had low, broad profiles and few counts at peak, but as the focus increased, the images got taller and narrower. At some point, the images began to broaden and flatten again. This means that you passed through best focus, and it is time to stop your sequence. Look back to decide at which focus setting you got the tallest/narrowest profiles. This is your final focus value -- record it and the dome temperature on your observing log.

10) Set the telescope focus to the final value, but drive the focus 10-15 units below this value and approach it from below to avoid "backlash" effects.

11) Update the telescope position so stars are centered on the CCD:

Updated 2002 Jun 10 -- ACL

13. Observing A Star

Note: This part is specifically written for the RR Lyrae variable star observing program. If you are observing something else, your procedure may differ.

Note: In most cases, we will obtain several frames of the same star, at slightly different positions on the chip and in different filters, to create an "observation set". For example, if your observing plan calls for observing the star "SW And" at 10:30pm, you might obtain 3 V images of the star in close succession, and we would refer to these as a "set".

1) Chose a star to observe (see your Observation Plan plot) and get out its finder chart.

2) Point the telescope to the star's coordinates. They may be preset in the telescopes coordinate table -- if not, feel free to place them there.

3) On the finder chart, there are instructions on how to take an observation set for this particular star. For example, you might see

4) If you are observing with the V filter, take the first observation -- Expose tab in "MaxIm CCD" window:

5) Check the sky level; if it is bright (>6000 counts), you may want to shorten the exposure times and take more exposures through each filter (or, dawn may be coming on, or you may be pointed near a bright moon... check in the dome if the sky level seems odd).

6) Save the image to disk (floppy disk icon). Here is a description of our naming convention -- please follow it carefully, as it avoids accidentally overwriting your data, and facilitates data reduction later!

7) Move the telescope 9" E and 9" N using the OFFSET command on the telescope PC.

8) Go to Step 4 and take the next frame in the sequence (110 sec), save it, offset the telescope, and then take the final (80 sec) exposure in the sequence.

9) Record comments in your paper log : Starname, UT/Local Time, Sidereal Time, CCD Temperature, Exposure/Filters, Comments (e.g., moon & sky conditions, problems, errors in file naming, etc). Put a cross on your Observing Plan plot for this star at the ST of the observation, so you don't accidentally re-observe it. Go to the next star in your Observing Plan.

Image Anomalies: sometimes something goes wrong, and your images look funny. Here is a nice web page devoted to describing anomalous images, identifying what went wrong, and how it can be corrected. Please inform Andy Layden if you cannot correct the problem.

Updated 2000 July 06 -- ACL

Appendix 1. Naming Convention

Note: It is important to follow this naming convention. Otherwise, it is easy to accidentally give your new image the same name as an older one -- the older one will be written over and lost forever. Also, type carefully -- typos can have the same result.

1) Nightly directories: Each night should have a separate directory with the format YYYYMMMDD. If you start observing on July 27, 20002, you would call your directory "2002jul27". Each directory should be placed in "C: CCD Images\".


2) Bias frames: Say you take 10 in the evening and 10 more in the morning. Call the evening ones:,, ...,,

and call the morning ones,, ...,

The ".fit" stands for Flexible Image Transport and defines the format the image was saved with (kind of like .gif or .jpeg but different).


3) Sky Flats: The name should contain the prefix "sflat", it should contain the filter (V or I), and a unique number (so it doesn't overwrite an existing file). For example:

sflatV01, sflatV02, ..., sflatV12, sflatI01, ..., sflatI08.


4) Stars: The first part of each starname will be the 6-character name on the finder chart, with blank spaces replaced with underscores ("_"). For example, the star "SW And" would be "SW_And". Keep the capitalization (or make it all lower case).

Things get a little complicated, since during the course of the night you are likely to come back to the same star several times. Each time you return to observe a star, you are creating a new "observation set". For example, if you are observing with the V filter, you would take 3 observations of the same star back to back, each with a different exposure time. We need to make up different names for each V frame within a set, and have different names from one set to the next. Here is convention:


Another example shows the file names for 3 consecutive I frames in the second observing set for the star "RU Lup":,,

A few hours earlier, we had created the first observation set for RU Lup:,,

Updated 2002 Mar 07 -- ACL

Appendix 2. List of Focus Stars

Chose the star from this list which is nearest to the zenith (ie, has the RA closest to the current ST).


RA (2000)

Dec (2000)



00 00 10.71

+41 19 29.2



01 59 28.58

+41 13 44.3



03 58 55.33

+41 15 4.1



06 01 2.65

+41 18 4.5



07 59 1.25

+41 28 43.2



09 57 30.04

+41 11 39.0



11 57 24.44

+40 55 24.3



13 59 47.76

+41 20 59.9



16 01 18.50

+41 24 30.7



17 57 38.78

+41 13 10.7



19 59 28.45

+41 12 11.7



21 59 42.29

+41 34 24.9


A Finder Chart for each Focus Star is available in the "CCD Photometry" binder in the control room.

Updated 1999 June 09 --ACL

Appendix 3. Example of a "Focus Table"

These data were taken during focusing the telescope one night. Your numbers and choice of best focus will be different!

Focus Value














smaller donut





broad, low star





getting good










best focus? <<-- YES, USE THIS!





lower and broader, worse










very broad, low ... way past best focus, stop!