by Albert Highe
Once again the satellite images looked promising for Saturday and Sunday nights, so I headed to Plettstone in the Sierra Foothills.
The temperature dropped to the high 30's. As the %RH reached the high 90's, dew formed on charts, table, and secondary. Occasional use of the dew zapper on the secondary allowed me to enjoy the excellent seeing. There was zero wind, so it didn't seem cold. The seeing deteriorated around 10PM, so we all abandoned the big dobs and enjoyed low power views through Michelle's 4" refractor.
The temperature was slightly warmer, dropping only to 40°F. The RH didn't get much higher than 80%. Dew wasn't a problem tonight, but the seeing was a tad softer. Clouds began to move in around 9:30PM, but the sky completely cleared again by 11:30PM.
| Scope | 17.5" f/4.5 ultralight |
|---|---|
| Eyepieces | mostly 5mm Takahashi LE. Also 7mm and 9mm Naglers. |
I've been systematically searching out galaxies within the Perseus Cluster, Abell 426, since September, 2002. This month was the last opportunity to expand my list. By next month, the Sun will be setting later while Perseus will be setting sooner, making observations of challenging objects nearly impossible. So I focused on finding a few more faint and obscure fuzzies in this cluster. The tally now stands at 217 galaxies within 3° of NGC1275. Nine of these galaxies are likely not gravitationally bound to the cluster. With 40 targets still on my list, I'll have to resume the hunt on my birthday later this year.
During this observing project, I discovered a few techniques that improved my ability to hunt down ultra small and ultra faint fuzzies. I thought I would share some of them here.
A large fraction of the galaxies in the survey cannot be seen at low powers. Many can only be seen at 300-400X. That means my f.o.v generally is less than 8 arc minutes. One might expect that it would be easier to locate the star field with an eyepiece providing a wide field, and then switch to a higher power, narrower field, eyepiece. However, more often than not, I would move the scope slightly when changing eyepieces. I wouldn't recognize the new field, and I would have to repeat the process. It also didn't help that my dob doesn't track. An 8' field moves by rather quickly.
I found it was best to leave my high power eyepiece in the scope during the entire observing session. Consequently, at the beginning of the session, it was important to precisely align the cross hairs of the finder scope with the center of the field of the main scope. When hunting down a new target, I would first find a relatively "bright" star within 0.5° of the target and locate it in the cross hairs of the finder. Then, while viewing through the main scope at 400X, I would star-hop from that reference star to the target.
Star hopping at 400X requires extraordinary star charts. No printed atlas exists. Some planetarium programs, with the appropriate data files, have the capability of displaying enough stars. However, I don't like using a computer in the field. Also, planetarium programs can display just about any field size. I find this flexibility confusing in the field. I like to work with charts with consistent field sizes and calibrate my senses to them. Consequently, for objects away from the center of the cluster, I used "The Sky" to make finder charts approximately 1°X1.25°. These would show the locations of perhaps a dozen objects. In addition, I downloaded a Digitized Sky Survey image of each object centered in a 15'X15' field. With this set of charts and images, I could quickly star hop to an object, compare the view to a real image of the area, and unambiguously identify the target. I became quite proficient at star hopping at 400X. The greatest difficulty was overcoming the rapid drift while consulting charts. However, because the charts were all consistent, the time I spent at them was minimal. I also quickly learned to move the scope about the right amount for the amount of time I was away from the eyepiece.
Objects are much denser at the center of the cluster. I didn't bother making a printed star chart for the central 1°X1° area. Instead, I downloaded one DSS image file for the 1°X1° area and labeled each galaxy in Photoshop. For use in the field, I divided the 1°X1° labeled image file into six sections, approximately 0.3°X0.5°, and printed them out. Each of these smaller charts has as many as 40 galaxies on it and was sufficient to identify most of the objects. Individual DSS images of each galaxy wasn't necessary. Often I spent an entire observing session at 400X on just one of these charts, never moving more than 0.5°. Once I got started, unless I bumped the scope, I never had to look through the finder again.
It sure would be nice to get a tracking scope for my next birthday.