Reducing flexure

[KathyNS]

Recently, I have noticed the odd frame that has movement other than slightly oblong stars. This sort of thing:

For comparison, here's a normal frame from the same shoot:

I went into the PHD2 logs to see what, if anything happened at the time of that frame. Nothing at all happened! The guiding was good, < 1" RMS. Good guiding + movement = flexure. My guidescope is a finder-guider (50mm x 162mm) in a standard Synta finder holder. They are known for flexure.

So, I have ordered a set of rings from ADM:

I have a spare dovetail rail that I will mount inverted on top of the scope rings. It will have several advantages. It should reduce the flexure. It will get the guider onto the centreline of the OTA, finally allowing me to balance Dec laterally. It will allow me to adjust its position fore-and-aft, allowing me to fine-tune the longitudinal Dec balance. The rail will allow me to add an auxilliary counterweight in case the mount won't balance in Dec with just the guider. (I have a Newt with the camera load hanging under the OTA in the home position. In order for the camera to clear the counterweight hub of the CEM60, I can't slide the OTA back far enough to balance, hence the need for auxilliary counterweights.) And, finally, I can mount a DSLR on the upper rail for piggyback imaging.

I will post before and after pics when the rings arrive.

[JayTee]

Hi Kathy,

I only use dovetail rails on all of my Scopes (Imaging and guide). No rings. That way I have a solid clamping surface to essentially eliminate flexure.

Cheers,
JT

[pmwolsley]

Kathy,
My guess is that your sequencing software performed a dither during an exposure. If this was differential flexure then, for each star, there should be one streaked star instead of two distinct stars. To create two distinct stars you need an abrupt movement which would be easily seen on your guiding graphs. I think one of two events occurred here...
1)Your sequencing software issued an additional dither during the exposure. The key here would be that the sky background is the same illumination as the other photos. Also you would notice the additional dither in the guiding graph.
2)The sequencing logic didn't tell the imaging camera to close it's shutter between exposures. The sky background would be significantly brighter due to the double length exposure. Also the timestamps of the images would reveal that there was a gap between images. The exif data stored with the image might also reveal a significantly different exposure length.

If you still have your PHD2 guiding log from that session...I can take a look at it for you.

Just my two cents...

Peter

[JimMinCT]

I agree with the above statements...
Flexure does not look like what your image shows. Flexure shows itself as a nearly in focus image, that marches across the screen as each image is collected because the guide scope and main OTA are not tracking parallel with other.
The two distinct images you show, mean you moved the whole apparatus a few pixels halfway through the shot, (which is why they are about the same brightness), if it happened close to one end of the exposure or the other, one star image would be a lot dimmer than the other.

[KathyNS]

Peter and Jim, thanks for the input. I have uploaded both the PHD2 guide log and the debug log from the session to Dropbox.
Guide log: https://www.dropbox.com/s/nppodizic01mg ... 0.txt?dl=0
Debug log: https://www.dropbox.com/s/g86amqvy7c08w ... 9.txt?dl=0

The SGP log from the session is at: https://www.dropbox.com/s/pp9zchducat6l ... 7.txt?dl=0

The frame that I showed in my first post in the thread was taken between 2019-10-05 23:34:25 (previous frame saved to disk) and 2019-10-05 23:36:36 (subject frame saved to disk). It looks from the SGP log like the actual exposure time was from 23:34:31 to 23:36:31.

[Stuart]

I used to have flexure when I mounted my guide scope outside my rings. Then I moved my guidescope and mounted it directly on the focuser and my flexure problem went away.

[pmwolsley]

Kathy,
I tried posting 3 times...they all disappeared. PHD2 and SGP don't show anything unusual...very frustrating. Because you don't use an OAG there always is the possibility that the imaging camera shifted position suddenly during the exposure. I would suggest checking the imaging train for tightness. Also check your cabling to ensure there is no possibility of snagging a cable. In this case it may be that a cable is snagging and then suddenly breaks free...causing the sudden shift in imaging camera position. This one is a real "head-scratcher".

Peter

[KathyNS]

Thanks, Peter!

I had assumed that the problem was in the guide scope mounting. But I suppose if it had moved, I would see PHD compensating for the movement, at least for a few seconds. Thanks for confirming my assessment that PHD2 showed nothing abnormal.

So that leaves the focuser as the likely culprit. I wonder if there is some lateral movement in it. It seems pretty tight. But I have had problems with the focuser (a GSO LF Crayford) suddenly letting go in the latter part of a session, with the camera dropping to an unrecoverable, severely out of focus position. If there was lateral movement, that might be the cause of (or at least related to) the sudden loss of tension.

It is looking like I now have two indications that I need a new focuser. That's gonna cost a bundle!

Meanwhile, my guide scope mounting rings have already shipped. They can't hurt, and they will help address some unrelated balance issues.

[JayTee]

The cheap telescopes I use for guide scopes all have slop in the focuser tube. That is why for both of them I have installed a locking screw so that once focus is set, I can clamp down on that tube so it will not move.

Cheers,
JT

[KathyNS]

The cheap telescopes I use for guide scopes all have slop in the focuser tube. That is why for both of them I have installed a locking screw so that once focus is set, I can clamp down on that tube so it will not move.

Cheers,
JT

There is no focuser on the guide scope. It's a modified finder with just the helical focuser on the objective lens. The suspect focuser is on the imaging Newt, not the guide scope.

[JayTee]

Hi Kathy,

Sorry, you have my full attention now.

I closely inspected your double star image and found the telltale sign (at least to me) that this is focuser flexure. If you look at the stars that have red circles around them (you'll have to zoom way in), you will notice a very faint light bar connecting those two stellar images. This shows that there was a "transit" between those two positions. It only occurs on stars of the correct brightness. If the star is too dim, you get no bar. If the star is too bright, the pixel saturation hides that bar. At some point in the middle of your image, the focuser moved a fraction of an mm and this is the result.

I think you're on the right track with trying to get a new focuser that can handle the weight you are asking it bear.

Cheers,
JT

[KathyNS]

Good catch, JT! Yes, I zoomed in on the original FIT file, and the connecting line was quite clear. Thanks for catching that. At least now I know what the problem is.

[goldstar]

It may not apply but I had a sloppy focuser on my guide scope. It did not produce any problem except I was aware that as I moved in and out of focus the draw tube sag shifted the image by about 1/2 degree!
Now since I had a guide camera attached to it with a different focus for the camera as opposed to the visual it gave me problems switching between the two after finding a good guide star. I could have used a flip mirror but I already had one on the imaging scope and could not afford another!
My solution was to grease up the rack and pinion draw tube and pack around it with an epoxy cement filling the whole assembly where the draw tube moved, avoiding the rack!
I had created a barrel that the tube could slide in with no deflection whatsoever.
After allowing to dry it was satisfying to crank the focuser, hear the click as the greased tube separated from the epoxy and continue on its way in and out with no deviation from the axis.
This may not be a method you might want to use on your expensive main OTA focuser (Crayford?) but hey it worked perfectly for me on a cheapo RP focuser.

[KathyNS]

Meanwhile, my guide scope mounting rings have already shipped. They can't hurt, and they will help address some unrelated balance issues.

My ADM guide scope rings arrived today. They look really sturdy: I'm certainly not going to get any flex there! Here are some before-and-after pics.

Before: Synta stalk mount. Note the off-centre, out-of-balance position.

Pieces on the workbench, ready to assemble.

Guide scope mounted on the dovetail.

"Almost" installed on the scope. (I need some longer M5 screws to complete the installation.) This will centre the guide scope, simplifying balance.

[bladekeeper]

Nice! That looks wonderfully solid, Kathy!