Now for something a little different.....

Taking images of the night sky definitely takes some time to learn how to do. There are lots of problems, besides the clouds, that need to be addressed. One of those is called Periodic Error. Periodic error is the result of inaccurately manufactured gears in the scope drive mechanism. Can the gears be manufactured to not have periodic error? No. Interestingly, the difference between a $500 mount and a $50,000 mount, as concerns periodic error, is that the $50,000 mount has a predictable, repeatable periodic error. Why is that important? Because the predictable periodic error can be mostly programed out. All new, computerized telescopes has PEC, or Periodic Error Correction. If we look at an image, how does this show up? If, in a perfect image, a star shows up as a dot (.), the star image of a mount with periodic error looks like a horizontal line (-). (That's not the only thing that can cause the horizontal line, but we will assume everything else is OK.) Periodic error causes the mount to speed up and slow down, above and below the tracking rate that it needs to be at to perfectly track the star. PEC in essence, records the correcting speed changes and applies them (plays them back) to correct the tracking speed as best it can.

Let's see what this looks like on my mount. (Maybe I should inject at this point that a “telescope” is actually made up of 3 parts; the base, the mount, and the optical tube.The base sits on the ground. The mount sits on top of the base and holds the optical tube. If the optical tube tracks the sky, the mount is the part that does the moving or tracking. The optical tube is the part that you look through to see the stars.)

Periodic error uncorrected

This graph shows the periodic error of my Meade LX200GPS (ca 2002), uncorrected. This is actually 3 “runs” of the gears, or 3 periods of the gear, susperimposed on each other. It's fairly consistently +10 to -22 arc-seconds. That's apparently fairly common for this type of scope and definitely would cause the star to look like a horizontal line. After analyzing and “curve fitting” a correction curve, the graph looks like the right pane of the image below.

Left, before. Right after. Both at same vertical scale

The left pane is the same before graph shown at the same scale as the after graph. Look at the vertical scale, on the far left of the graph: the after is about +2 to – 4 arc-seconds. That's a pretty good improvement; one which, hopefully, can be autoguided out.

By the way, the graph shows something called “RA worm phase.” What this refers to is the gear that moves the scope to track the stars. The gear is called a worm gear. For more information, and to see what one looks like, see : https://en.wikipedia.org/wiki/Worm_drive

Rain, rain, go away.....

Originally written November 9, 2015 published November 18, 2015, but I've been busy :)

I heard the weatherman on one of the local TV stations say tonight that it has rained 15 out of the past 16 days. Why he didn't say it has rained the past 15 days and left it at that I don't know, but... it has been rainy and cloudy for a while. Fortunately, I took advantage of the few dry days before hand to gather some images and do some experimenting, which I like to do. As you would expect, some efforts were better than others.

Let's start with the NGC7380, The Wizard Nebula. 

NGC7380

I originally thought I might publish this as a Halloween “present”, but things didn't work out that way. The images was taken on the night of October 8^th. Credit for discovery is given to Caroline Herschel who found it in 1787. It is said to be extremely difficult to see visually. I find it interesting that NGC 7380 refers to the open cluster of stars within the nebula. NGC 7380 is located in the constellation of Cephus and is about 8000 light years (ly) away. As with many objects in astronomy, I don't see the namesake. Do you see a wizard?

M31, The Andromeda Galaxy

M31 Parts 1 an 2

This galaxy is said to be very similar to the Milky Way. It's a giant, spiral galaxy. It's bright enough to be seen naked eye under only modestly dark skies; I've seen it from Starlight naked eye several times. It's an easy target for binoculars from almost anywhere. It's about 2 million ly away and headed this way (or we're headed that way, take your choice; it's all relative). The reason for two images is that it's so big, it won't fit in one. Actually, this is one of my experiments. I knew it wouldn't fit in one frame, so I wanted to see if I could “stitch” the two images together. One image was taken the night of October 18^th and the other October 19^th. I made them as nearly identical as I could, but the area where the images join is extremely obvious. I haven't found a solution to this problem as of yet, so I let the two images separate for now. By the way, that little "fuzzy dot" in the lower left hand corner of the bottom image is actually another galaxy, M32.

M57, The Ring Nebula

M57

This is a perennial favorite. M57 is the ionized gas that was ejected, it you will, from a red giant star as the star collapsed into a white dwarf star (in the center of the ring). It's about 2300 ly away in the constellation of Lyra. Unlike the Wizard Nebula, this one is easily visible in at least a 20cm (8”) telescope. As with almost all deep sky objects, it will most likely be gray vs. the color you see here. Some people report they can see some color (shades of green, I think) if they have a large scope and are viewing under very dark skies.

And finally Rx And

Rx And

Rx Andromeda is the name of a variable star in the constellation of Andromeda, not a prescription for a star pill. However it is a prescription for fun. The star shown below (with the arrow pointing at it, of course) is Rx And. So far the variation has been measurable, but slight. It would be best to measure the variation on a consistent basis, like every night. With the rain we have had lately, I'm lucky to measure it once a week, and so far I haven't been able to do even that. Oh, well. Hopefully, I will be able to image it more often soon, and be able to show the variation here. We'll see.