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. Here 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. There should be 9000 ± 2000 counts in each image, and all of the images should have exposure times longer than 10 sec.

 

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 "data processing" procedure, the we will later correct for the pixel-to-pixel sensitivity variations by DIVIDING each object (star) frame by a combined sky flat frame. 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.

 

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 camera takes a finite amount of time to open and close. Tests have shown 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 corner, 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 its a pain. Easier to just avoid the short exposures altogether!

 

Dusk (Evening) Sky Flats

 

1) If you are taking dusk flats, point the telescope about 1 hour East of the Zenith, and rotate the dome so the slit is facing East (away from the setting Sun). Of course, be sure the dome is not occulting the telescope!

 

2) Be sure the telescope tracking and autodome are ON.

 

3) Which filters will you be observing with tonight? For example, BVI. Do the bluest filter first, when they sky is brightest, and the reddest filter last, when the sky is darkest. In our example, we are taking evening sky flats, so we begin with the B filter -- hit filter button 2 (1=U 2=B 3=V 4=R 5=I 6=Clear).

 

4) Take a 1 sec exposure (flash camera icon),

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.

 

5) 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 changing brightness outside. Experience will enable you to guess a "seat of the pants" correction to your computed exposure time.

6) 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.

7) 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.

8) When you have 5 good flats in this filter, switch to the next filter (V in our example -- hit filter button 3).

9) Take your first flat with an exposure about 1/2 that of your last flat (in B), since the redder filters in general have a higher throughput. Adjust subsequent exposure times as in Step5.

10) You are finished when you have run out of filters to do (you may want to go back and get more good images per filter) or the exposure times in your reddest filter are longer than 200-300 sec.

Dawn (Morning) Sky Flats

 

1). If you are taking dawn flats, point the telescope about 1 hour West of the Zenith, and rotate the dome so the slit is facing West (away from the rising Sun). Of course, be sure the dome is not occulting the telescope!

 

2) Be sure the telescope tracking and autodome are ON.

 

3) Which filters did you observe with tonight? For example, BVI. Do the reddest filter first, when they sky is darkest, and the bluest filter last, when the sky is brightest. In our example, we are taking dawn sky flats, so we begin with the I filter -- hit filter button 5 (1=U 2=B 3=V 4=R 5=I 6=Clear).

 

4) About 30 min after morning twilight, take a 300 sec exposure (flash camera icon),

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 and compute your next exposure length as in Step5.

 

5) If the image was saturated (all pixels uniformly 16383), you cut your exposure time a lot (you waited too long to start sky flats): try 50 sec. If the image was not saturated, but had 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 changing brightness 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 ~50 sec, or when you have obtained 5 or more good sky flats, switch to your next-bluest filter. In our example, this is V, so hit filter button 3.

8) Take your first flat with an exposure about equal that of your last flat (in I), since the sky is brightening. Adjust subsequent exposure times as in Step5.

10) You are finished when you have run out of filters to do (you may want to go back and get more good images per filter) or your exposure times in your bluest filter are shorter than 10 sec.

 

When to Start Taking Sky Flats

 

This depends on whether you are taking dawn or dusk sky flats, and what filters you are using.

 

Dusk Sky Flats:

Dawn Sky Flats:

 

 

Updated 1999 June 14