«1. Introduction The aim of this project is to gain experience using the CCD camera by obtaining images of planetary nebulae or H II regions. ...»
CCD Imaging of Planetary Nebulae or H II Regions
The aim of this project is to gain experience using the CCD camera by obtaining
images of planetary nebulae or H II regions. Planetary nebulae are ionized clouds of gas
surrounding very hot stars. The nebula is formed when a star of moderate mass reaches
the end of its life as a red giant star, and makes the transition to its ﬁnal state as a white
dwarf. In this process the outer layers of the star are ejected to form the nebula, while the newly exposed core appears as a very hot central star, which emits most of its radiation in the far ultraviolet, thus ionizing and heating the nebula. Most of the radiation emitted by the nebular gas, which has a temperature of ∼10,000 K, is concentrated in the emission lines of a few elements such as oxygen and nitrogen, as well as the most abundant element, hydrogen. An H II region is an interstellar gas cloud with newly formed stars in or near it which are hot enough to ionize some of the gas. While the physical conditions in the gas and the emitted spectrum are similar to planetary nebulae, H II regions are much less regular in appearance and contain much more gas and dust.
We will take the images of the nebulae through two narrow-band interference ﬁlters which transmit the light of the strongest emission lines in the nebular spectrum. These are the Hα line of hydrogen at 656.3 µm and the line of doubly ionized oxygen (O++ or [O III]) at 500.7 µm. The great advantage of such narrow ﬁlters is that they only transmit light with wavelengths near that of the nebular emission line you are interested in observing.
Thus you get all the photons of the nebular line, but greatly reduce the photons from the bright College Park sky, with its reﬂected city lights, since these are either due to incandescent lights (which emit continuum emission) or mercury lamps (which emit line emission at wavelengths not transmitted by the ﬁlters). Once you have the images, you can then compare them with published pictures, observe the diﬀerences in the distribution of H+ compared to O++, and perhaps measure the ﬂux in the nebula compared to that in the star.
An alternative project would be to image a bright H II region. In this case, we would observe the nebula through the Hα ﬁlter and also through a ﬁlter centered on the Hβ line at 486.1 µm. These lines are always emitted with a constant intensity ratio, Hα/Hβ = 2.8.
However, the ratio we observe may be diﬀerent because the Hβ line suﬀers more absorption by interstellar dust than the Hα line. By studying the variations in this line ratio, we can map out the distribution of dust in the nebula. The problem with this project is that the Orion Nebula, which is by far the best H II region, can only be observed in the early morning hours at this time of year!
We want to image planetary nebulae of suﬃcient size to be able to study their shapes.
Since nebulae of this size are faint, all groups should start with the Ring Nebula (NGC 6720, M 57), which is probably the easiest object to ﬁnd. It is in the constellation of Lyra, and the nebula is located almost exactly between two naked-eye stars. The nebula itself looks like a faint “smoke ring” in the main telescope. Look up the RA and Dec and ﬁnd the object on a star chart before you go out to observe.
You will observe a second planetary nebula in addition to the Ring Nebula. You will want to pick a bright planetary nebula which has as large a size as possible without exceeding the ﬁeld of view covered by the CCD camera. You will ﬁnd the ﬁeld of view of the CCD in the hand-out on the 14” telescope. You also want to pick an object that is high in the sky sometime during the evening (or early morning) hours. Look at the illustrations in the catalog to see what sort of structure the nebula has and if the central star is visible.
It would be nice if you could observe the central star. (In principle, such observations would allow you to calculate the temperature of the star.) To save time, look at a list of nebulae suitable for observation by amateur astronomers;
that way you will not waste time on objects which are too faint. But at some point you should turn to the Strasbourg-ESO Catalogue to obtain basic information such as the size, the ﬂux in Hβ (Hα is always about 3 times brighter) and in [O III] λ5007. You will want nebulae that have [O III] as bright or brighter than Hα. The larger the value of Hβ the better – note that the number listed is the negative logarithm of the ﬂux, so a smaller number is a brighter nebula. Traditionally, observers would prepare a ﬁnding chart to locate the object, but our telescopes should point accurately enough to get it into the ﬁeld of view by just entering the coordinates or its name. If you want to locate it on a chart, we have found that the atlas “URANOMETRIA 2000.0” by Tirion, Rappaport, and Lovi (in the Astronomy Library and at the Observatory) has a convenient scale. You should also have at least one extra back-up object.
With regard to the H II regions, it is unfortunate that the only really good target is the Orion Nebula, which does not rise until the early morning hours (i.e., after 3:00 AM!) at this time of year. If you wish to do the H II region, we can arrange to try it, but we will have to make arrangements for a mutually acceptable date(s).
You should dress very warmly. You are likely to get much colder than you might expect, since you’ll be motionless outside at night for several hours. Dress in layers. Be sure to have a wind-proof shell. Especially later in the fall, gloves and a warm hat are a necessity. The concrete ﬂoor gets very cold, so it’s best to have warm, thick socks and thick rubber-soled shoes. A thermos of a warm drink is also a good idea. You may wish to bring a ﬂashlight.
You should bring a notebook and pen for your notes.
Because of problems with the 20” telescope in the West Bay, we will have to use the Celestron 14” telescope in the East Bay. (The 20” telescope uses the (relatively) new Apogee CCD camera. Unfortunately, we don’t have operating instructions for the Apogee written up yet.) This handout is written, for the most part, as if you were using the 14” telescope.
Before coming to the observatory, you should read through the instructions for operating the telescope.
You should bring the relevant pages to the observatory with you when you observe.
3.2. Dark Frames
Thermal electrons (ie electrons which get excited due to the non-zero temperature of the CCD chip) get collected by the CCD just like electrons liberated by photons striking the CCD, and dark and light electrons obviously cannot be distinguished. The number of thermal electrons collected increases linearly with the exposure time. Consequently, one needs a dark frame with an exposure identical to the exposure on the object (called the light frame). This must be subtracted from the exposure of your object. Note that since any exposure has a bias oﬀset, subtraction of the dark frame from the light frame also automatically does the bias subtraction.
Note that you must keep a record of all the exposure times for useful exposures, and then be sure that you have obtained and saved dark frames of identical exposure time. By the way, since no light enters the telescope for a dark frame, obviously the ﬁlter does not matter for a dark frame — all that matters is the exposure time. So a dark frame can be subtracted from any frame of identical exposure time, regardless of which ﬁlter was used.
Actually, the 14” CCD observing software will usually take both light and dark frames and automatically subtract the dark frame from the light frame. If this is done, you do not subtract a dark frame during the later reduction stages, since it has already been done.
However, since telescope time in good weather is hard to get, we might prefer to obtain the dark frames after astronomical observations in order to complete the astronomical observations as quickly as possible, in case the weather turns bad or the nebula sets. If this method is chosen, this means that you must be sure to record the exposure times for all useful exposures and then be sure to obtain dark frames for each of those exposure times.
The risk with this method is that the rate at which dark electron accumulate with time may vary, for example if the temperature of the CCD varies; if this happens, then you will not get a good subtraction of the dark current.
To recap, if object (i.e. light) frames are taken with automatic dark-frame subtraction, then do not subtract dark frames during later data reduction. If dark frames are not acquired and subtracted automatically, you must be sure to obtain manually dark frames of identical exposure time to your object frames, and subtract the dark frames from the object frames during your data reduction.
3.3. Telescope Focus: Observing a Bright Star
The optimum telescope focus position varies from night to night due to thermal expansion of the telescope tube. It can also vary with the elevation of the telescope. You need to ﬁnd a relatively bright star (say magnitude 5 or 6), preferably not too far from your target. You want a star which is bright enough so that the exposure time is 2 to 4 seconds so that guiding is not needed, yet does not saturate. Take a series of exposures that range from a clearly out-of-focus setting, through good focus and on to out-of-focus on the other side of the focal point. Write down the focus setting from the micrometer and include it in your write-up. Do this for each of the ﬁlters you will use, since the optimum focus can vary between ﬁlters. Make sure the star is well exposed (more than 1000 counts), but it is essential to avoid saturation of the central pixel(s). While the CCD can record numbers up to 216 = 65536, the response becomes non-linear much before this, at around 16000 counts (it’s hard to give an exact number since the bias has been subtracted, and thus you don’t see the total counts).
The best focus setting is the one which gives the sharpest stellar images. Normally, astronomers ﬁnd the stellar image which has the smallest full width at half maximum intensity (FWHM). To do this, zoom in on the star to a very high zoom. Then using the cursor, ﬁnd the brightest pixel (again, make SURE it is not saturated!), and subtract the background brightness. Next, moving along a row, ﬁnd the pixel with the brightness (after subtracting background) closest to half of the brightest pixel; and write down its column number (x number); then go to the other side and again ﬁnd the pixel at half intensity; the diﬀerence in column number is the FWHM. Check to see that you get a similar number (to within a pixel) in the opposite direction. Keep in mind that as the focus improves, more and more light will be concentrated in fewer and fewer pixels, which may consequently saturate; keep checking, and adjust exposure time accordingly.
Keep in mind that you must adjust the stretch of the image displayed in order to see the essential brighter part of the stellar image. In other words, the maximum in the display stretch should be set maybe 10% above the value of the brightest pixel; the minimum should be around zero.
You do not need to save the focus images.
Although if you know the optical layout of the telescope you can determine how the optics inverts the image, it is generally unclear how the CCD is oriented and read out.
Relative to north, the observed image may be rotated by an unknown angle; in addition, it is possible for the image to be inverted (ie top to bottom or left to right). The simplest way to determine the image orientation is to (1) set the guide speed to ”5 = B:1” (2) begin an exposure of a bright star centered in the CCD frame. (3) Wait about 30 seconds. (4) Then guide the telescope to the North (by pushing the North guide button for about 10 seconds).
(5) Next, drive the telescope to the east for about 30 seconds and stop the exposure. The resulting image should show an L-shaped pattern, with a knob at one end of the L. The knob is where the star was when you started the exposure; the side of the L starting FROM the knob heads due South (lower declination) (since the telescope moved north); then, heading from the 90 degree bend in the L to the non-knob end points due West (lower right ascension). Save this image as a.ﬁt ﬁle. If you do not obtain this image, get it from the common area (but so note in your report).
3.5. Focus and Plate Scale: Observing a Double Star
As a preliminary observation, you are to take an image of a well known double star.
You will use the separation of the stars to determine the plate scale of the CCD. Choose either Lyr (separation 208 ), or β Cyg (Albireo; separation 34.4 ). Albireo is a beautiful double star, with a striking color contrast between the components. You can use the double star as the focus star discussed in section 3.3. But note that the components of Lyr are themselves doubles with ∼2.5 separations, so don’t mistake this for lack of focus! Save one of the best in-focus exposures to disk (as a.ﬁt ﬁle) for measurement of the stars’ separation (and also to get the exact orientation of your images).
3.6. Observing the Nebulae
Locate the Ring Nebula and center it in the main telescope. Take an image with the Hα ﬁlter ﬁrst, since you will get more counts with this ﬁlter than with [O III]. Try 60 sec to start. Look at the stars in the ﬁeld to see if they are sharp. You may get some drift in even this short exposure. You will need an exposure of at least 5 min (300 sec) to get a reasonable number of counts in the [O III] ﬁlter. (This ﬁlter is very narrow in ˙ wavelength, and its peak transmission is only 30%) You should also take a 5 min (or longer) Hα exposure. Don’t bother with the Hβ ﬁlter; the image will look just like the Hα image, but it will be much fainter.
Now locate your second target. Take at least one long Hα and one long [O III] exposure of it.
Save all your “good” images as.ﬁt ﬁles.
3.7. Taking Your Flat Field Images