Chose the star from this list which is nearest to the zenith (also appears taped to side of PC tower).
RA (2000) |
Dec (2000) |
V |
GSC Number |
Tel# |
|
00 00 10.71 |
+41 19 29.2 |
9.18 |
2789 1682 |
488 |
|
01 59 28.58 |
+41 13 44.3 |
8.77 |
2820 1425 |
489 |
|
03 58 55.33 |
+41 15 4.1 |
9.16 |
2872 1615 |
490 |
|
06 01 2.65 |
+41 18 4.5 |
9.38 |
2933 1955 |
491 |
|
07 59 1.25 |
+41 28 43.2 |
9.29 |
2968 1276 |
492 |
|
09 57 30.04 |
+41 11 39.0 |
8.93 |
2996 350 |
493 |
|
11 57 24.44 |
+40 55 24.3 |
9.22 |
3014 1120 |
494 |
|
13 59 47.76 |
+41 20 59.9 |
9.58 |
3030 599 |
495 |
|
16 01 18.50 |
+41 24 30.7 |
9.47 |
3064 788 |
496 |
|
17 57 38.78 |
+41 13 10.7 |
9.01 |
3093 165 |
487 |
|
19 59 28.45 |
+41 12 11.7 |
8.98 |
3141 622 |
498 |
|
21 59 42.29 |
+41 34 24.9 |
9.35 |
3193 1694 |
499 |
Tel# used to be the Table number in the Telescope Control System (though they sometimes get changed). GSC is the HST Guide Star Catalog number (to remember where the coordinates came from).
Updated 1999 June 09 --ACL
These data were taken during focusing the telescope one night. Your numbers and choice of best focus will be different!
|
Focus Value |
FWHM X |
FWHM Y |
Max |
Comment |
|
2377 |
19.8 |
17.4 |
890 |
doughnut! |
|
2378 |
13.1 |
11.0 |
1832 |
smaller donut |
|
2379 |
8.3 |
7.4 |
6711 |
broad, low star |
|
2380 |
4.2 |
4.0 |
12491 |
getting good |
|
2381 |
3.7 |
3.6 |
14432 |
good |
|
2382 |
3.2 |
3.3 |
15323 |
best focus? <<-- YES, USE THIS! |
|
2383 |
3.6 |
3.9 |
13983 |
lower and broader, worse |
|
2384 |
3.9 |
4.2 |
12821 |
worse |
|
2385 |
7.1 |
8.3 |
6104 |
very broad, low ... way past best focus, stop! |
Warning: This is a challenging procedure to master, so I consider it an optional, advanced skill. Most pictures will look fine without applying a skyflat. Proceed only if you are really gung-ho!
Skyflats are images taken of the blank twilight sky that are used to remove variations in the sensitivity of different pixels on the CCD. Skyflats can be taken at dusk (evening) or dawn (morning). Doing them at BOTH is ideal, if you can manage to stay awake until 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 9000 ± 2000 counts in each image, though any sky flat with 2000 to 13000 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. 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 exposures shorter than 10 sec with our CCD. The shutter in front of the CCD takes a certain 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 35 mm 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!
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. Set the telescope focus using the handpaddle to the value used by the previous CCD observer (check their paper log sheet).
3) Which filters will you be observing with tonight? For example, V and I. 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 V filter -- hit filter button 3 (1=U 2=B 3=V 4=R 5=I 6=Clear).
4) Take a 0.1 sec exposure:
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 sky flat has between 7000 and 11,000 counts.
5) If the image is saturated (all pixels uniformly 16,383), 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.
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 (I in our example). If you are observing in just one filter, keep taking skyflats until the exposure times are longer than 250-300 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!
9) Take your first (I) flat with an exposure about 1/2 that of your last flat (in V), since the redder filters in general have a higher throughput. Adjust subsequent exposure times as in Step 5.
10) If you have taken 5 images in both filters, continue on, but switch back and forth between the filters (for example: I, V, V, I, I, V, ...) until the exposure times are about 250-300 sec. At this point, its getting dark enough to focus the telescope and begin observing variable stars.
1). Dawn comes on quickly and can surprise you. Check the time of morning twilight on the table pinned on the corkboard, 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 9000 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 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! Be sure the telescope tracking and autodome are ON.
3) Which filters did you observe with tonight? For example, V and I. 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.
4) From you last star image, calculate the exposure time needed to give you 7000 counts in your sky flat 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 300 sec. Take an 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 sky flat 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 16,383), 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 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 ~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. If you are working in one filter only, keep going until the exposure time reaches 10-12 sec (again, "the more sky flats, the better").
8) Take your first (V) flat with an exposure about equal that of your last (I) flat, since the sky is brightening. Adjust subsequent exposure times as in Step 5.
10) If you have taken 5 images in both filters, continue on, but switch back and forth between the filters (for example: V, I, I, V, ...) until the exposure times are 10-12 sec. At this point, its getting too bright to do anything. Put the telescope to bed.
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.
Updated 2000 July 06 -- ACL
I suggested that you select the Settings tab in the "MaxIm CCD" window, and click the Simple Auto-dark radio button in the "Auto Calibration" field.
When you request a 45 sec exposure, this causes the CCD to take a 45 sec long dark frame (shutter closed) and then a 45 sec long "light frame" (shutter open). The computer subtracts the dark image from the light one, automatically removing the bias level and dark count from the image that gets saved to disk. The resulting picture looks cleaner than a "light frame" alone.
However, this technique doubles how long it takes to get your picture (eg, 2x45 sec rather than simply 45 sec). This becomes a pain when your exposure times are long. It also loses effectiveness in long exposures, since the brightness of the background sky begins to swamp the bias/dark contributions. In general, if your exposure times are longer than about 100 sec, you probably want to do "light frame" alone (choose None instead of Simple Auto-dark in the "Auto Calibration" field) -- though feel free to play around with it!