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Why is the left image – taken with a premimum APO refractor – blurred, while the right image – taken with a 9 ¼" SCT – is sharp?
The answer is differential flexure – bad news for anyone trying to guide a mount with a separate scope and camera. But there's good news too.
Differential flexure occurs when one part of the imaging system shifts slightly, while other parts do not. These parts may be the guide scope, the guide camera, the main OTA, the main focuser, and/or the main imaging camera. As the telescope tracks across the sky, the pull of gravity causes something to sag. It's unlikely the rest of the equipment sags exactly the same amount in the same direction, so the guide scope and main scope are no longer pointing precisely at the same spot in the sky. The autoguider valiantly keeps the guide star centered, but the image on the main camera moves relative to it, resulting in smeared stars and blurred details. Look at this example:
This is a stack of 17 five-minute sub-frames. They were not registered (stars aligned) prior to combining. Explanation: Sum-combining without registering brings each star's full brightness value into the final image, so any movement that occurred while the shutter was open for each sub-frame shows up as trailed stars. The movement is very small – visually, the stars in each of the sub-frames appear round, not trailed. But clearly they are not round. The trailing is too faint to see in the sub-frames, but summing the sub-frames causes the trailing to blend together, making the movement obvious. Close inspection of this composite image in Photoshop reveals the total displacement is roughly 11 pixels horizontally and vertically, or about 16 pixels diagonally. The image scale for my imaging equipment is two arc seconds per pixel, so something moved about 32 arc seconds over the total 85-minute exposure, or just under four arc seconds during each of the 17 five-minute sub-frames.
Just how much mechanical movement does four arc seconds (1/900 of a degree) represent? Very little! Assuming the guide scope or imaging train is 12" (304mm) long, simple trigonometry reveals that four arc seconds represents a movement of 0.0000232 inches (0.0059mm) – that's 23.2 micro inches, or about 1/8 the thickness of a human hair. Folks, precision machinist instruments can't easily measure this, and it certainly can't be detected by wiggling the equipment with your hand. You don't know the movement is there until it ruins an image.
So it does this all mean? It means it's impossible to pay too much attention to eliminating differential flexure. Even after making several modifications to the guide scope, and removing the Pyxis camera rotator to eliminate it as a suspect, a series of five-minute exposures still showed movement of about two pixels during a single sub-frame exposure. This represents the same four arc seconds described above – no improvement at all.
Here is a single sub-frame showing an elongated star with a circle (ideal star) superimposed over the brightest pixels. The elongation due to movement is obvious.
This is a single star enlarged from the NGC6992 image. Compare this 30-minute exposure with the five-minute elongated-star image above NGC6992. I'm impressed that no elongation is evident over this long exposure.
These are the modifications I made in my quest to eliminate differential flexure. I can't say for sure which one (or ones) did the trick, but I'm confident each was worth the effort.
| Modification | Why? |
| Installed a Robin Casady Triad Bar. |
Perching the guide scope on top of the main OTA rings means that any movement of the rings is amplified at the guide scope. The Triad Bar places the guide scope low on a very solid bar, so movement is eliminated. Furthermore, the Triad Bar is superior to the typical side-by-side (tandem) bar because the main OTA is mounted in the center of the bar, not out on one end where it must be balanced by the guide scope on the other end.
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| Added a second 2½” (63.5mm) threaded extension tube between the focuser and the camera. |
Originally I had a single 2½” extension tube, but this required the focuser drawtube to be racked-out nearly three inches. The extra extension tube means that 2½” more of the drawtube is supported inside the focuser housing, reducing droop.
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| Replaced the original Delrin spacer blocks under the main OTA rings with new ones made of aluminum. |
A couple of people suggested the Delrin might be expanding or contracting at a different rate than the aluminum in the rest of the system, or, being a slippery plastic, the blocks might be sliding. I don't know if these were factors, but I replaced the Delrin with aluminum anyway.
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| Milled-flat the bottoms of the main OTA rings. |
After machining the new aluminum spacer blocks, I set a ring on one and was astonished to see it rock back-and-forth. There was a 0.010" (0.254 mm) hump in the center of the ring base. The second ring had a similar hump. I milled both ring bases flat, and now they rest solidly on the spacer blocks.
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| Replaced the felt lining in the main OTA rings with solid styrene strips. |
With the extremely tiny movement needed to produce elongated stars, it seemed possible that the the felt lining in the rings might compress as the telescope weight shifts while tracking across the sky, as a friend suggested. This was reinforced when I measured the thickness of the rubber-backed felt – I could compress it by 0.005" (0.127mm) just by squeezing the dial caliper jaws together slightly. I removed the felt, cleaned off the adhesive residue, cut strips from 0.030" (0.762mm)-thick sheet styrene (available at hobby shops), and affixed them to the inside of the rings with super glue.
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| Added a metal strap to the guide scope mounting. |
Originally, the 300mm guide scope was mounted on two short aluminum posts. A friend commented he once had something similar but needed a metal strap, so I added a hose clamp over the scope, with the cut ends screwed to the dovetail mounting bracket. And, as described above, I moved the guide scope from atop the OTA to the Triad Bar.
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