Pics of what to do to avoid... When it comes to Flexture and twist. Kip. IMHO of course. Updated 02/25/08

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Beginning astrophotographers tend to really underestimate ‘FLEX’…. Flexure can be the biggest problems when it comes to calibrating and guiding when using a second scope for guiding…. I began using PHD and loved the fact that I could use a webcam for auto guiding… I simply mounted the camera on a C90 Mac and the C90 was mounted on a study piggy back adapter. Worked great.. but… I was limited to the guide star that I could pick because I needed to move it around and couldn’t. That’s when I decided to invest in some rings that would not only hold the 90 but would allow me to adjust the scope in order to find a guide star. Little did I know… That’s when the problems began…

The above shot is a 5 min Raw shot. Notice the trailing. With the new rings on.. all of the sudden.. problems. PHD appeared to have problems with giving me pinpoint stars. what happened? Surly the new version wouldn’t make this sudden change. What is happening? No matter what I did. PHD just wouldn’t give me pinpoint stars anymore.

Now here’s a close up shot (above) of the rings and adjusting screws that hold the 90. Notice the gap!! Not good!! Also.. the adjusting screws are nylon. Not very sturdy. Lots of flex there too. Metal is better.

There is one more factor that added to the problem. The C90 Mak is considered a spotting scope. Since it is considered a spotting scope, hunters and nature watchers will use it. So it has a rubber waterproof cover around the scope. This increases the flex problem as well. Don’t use scopes with rubber coatings. Lol

Since I was having these issues I thought that I would take down the 90 and mount my 5.5 inch Comet Catcher. I didn’t think I would have that much luck with it since the comet Catcher weighs over 15 lbs and the 90 was 3.1 lbs. No way the Comet Catcher was going to give me pinpoint stars if the 90 couldn’t.

What the??? Hmmm. Pinpoint stars on a 7min exposure. With a scope that weighs four times as much. Why?

Notice that the rings are wrapped around the scope. No screws. Big difference. So, In simple terms, Not only did it reduce the flex, but it stopped the Twisting too.

Here’s a graph (shown below) that was produced by Craig. In simple terms is shows two setups. Setup 1 has the flex issues. Setup 2 eliminated the flex problems as much as possible. Notice too how the flex gets worse the longer the guiding is… Notice that after 20 minutes that the flex really starts to take effect. Why?

Well, you can do a simple experiment yourself to see why without a telescope. Take a 5lb book or something that has some weight to it, and stretch out your arm directly over your head. Be careful not to drop the book on your head. Now, Move your arm to your side very slowly. Notice how heavy it becomes as you move it to your side. That’s the reason for the flex and why you need to minimize it as much as possible.

Here’s a simple way to rule out flex.

Let’s assume that PHD isn’t loosing the star and that it’s guiding somewhere between perfectly and off by, oh, 3-5 arcsec. So, your mount has some error it just can’t keep up with, it overshoots, etc (settings being off, mount being crud, etc.). If that is the case, over time, the star should wobble back and forth but *on average* be in the same place. We overshoot, we undershoot, we oscillate back and forth past the star - whatever. On average, the star is in the right place, but we have line segments instead of points.

Go out some nice, still night (please don’t attempt this with 40 MPH gusts...) and shoot, say an hour of anything at something less than say 1 min exposures. We want something short enough that your mount isn’t coughing up furballs during the exposure. Be guiding of course during this.

Now, take those images and do two stacks of them:

1) Do an Align and Combine without any aligning (i.e., fixed alignment). Do this for say 1 min worth of shots, 5 min worth of shots, and for the whole shebang. Does the 1 min stack look pretty clean? How much worse is the 5 min? Now, the big question - how much worse is the whole shebang? If the big stack is *a lot* worse than the 1-5 min stack, you’ve got flex. Why? The guide scope kept the guide star on target, plus or minus a few arcsec. That error may show up in the 1-5 min (which can show flex too) but if an hour is a lot longer trail, the only explanation is flex (assuming PHD kept a good lock). A 50 arcsec trail there isn’t PHD wobbling.

Note, you can use the Measure Distance tool in Nebulosity to see just how many pixels long your trail is. See how wide a star is in a single shot and see how long the trail is, subtract the two (e.g. 117 pixels - 5 pixels = 112 pixels per hour = 1.8 pixels per minute = you’ll not be exposing for 10 minutes with clean stars).

2) Do an Align and Combine with Translation (or Translation + Rotation) in Nebulosity. You’ll find in your directory an align.txt file with the dx and dy (shifts in x and y) needed to bring that frame into alignment. You can open this up in something like Excel and plot the total distance the stars moved. Ideally, dx and dy would always be 0. If you’re having issues, they won’t. Use good old Pythagorus to determine the total distance the stars moved: sqrt(dx*dx + dy*dy). If this is a horizontal line on average with some bumps up and down / some noise, you’ve got no flex. If there is a real drift component, you’ve got flex.

So what should you do to reduce flex and twist.

1) Use rings that will wrap around the scope and hold it snug. If you can’t do this .. Reduce the gap between the scope and rings as much as possible. This will help to reduce not only flexing but twisting too.

2) For sensitivity... Use a camera with a ½” mono chip for guiding. Since you won’t be able to adjust the scope, you will need a sensitive camera. Remember, the more the camera cost, doesn’t mean it’s better. Just more weight. The more gadgets and gizmo’s that are on the guide camera… The more weight there is… Keep it simple.

3) Use metal adjusting screws instead of nylon. Use rubber caps to protect the scope from marring.

4) Avoid using Maksutov and SCT type scopes. They really don’t work well as a guide scope. Extensive testing has been done with this. The main problem here is the moving-mirror design. So, avoid them if you can as the have an internal moving-mirror mechanism that can itself flex with nothing you can do about it.

5) Use the AutoGuide Port if possible, Connecting to the AutoGuide Port makes a big difference when guiding IMHO.. The Orion Autoguider is the best, and least expensive way to do this if your scope supports this. On a personal note, I have used both methods. I have used ascom for 12 months or so exclusively for guiding using PHD. The difference is like night and day. Go autoguide port if possible.

One more thing about nylon screws.... They tend to develope a memory... In other words... they start to bend over time and will favor a certain position and make it hard for future adjustment.. if you are one of those that doesn’t tear down the scope at the end of an imaging session.. you may encounter this problem. All the more reason to use metal adjusting screws..