Test Images with New Rings for RH200

[avandonk]

Last night after rain the clouds cleared at about midnight. I have fitted new rings to the RH200. Managed to align the camera close to ideal.

Collected two images.

M16 10x16 minutes in 3nm NII.

Large image x1.5 native pixel size 8MB

http://d1355990.i49.quadrahosting.com.au/2013_06/M16_NII.jpg


SMC 10x16 minutes in 3nm NII.

Large image x1.5 native pixel size 13MB

http://d1355990.i49.quadrahosting.com.au/2013_06/SMC_NII.jpg


The remaining flexure I had is now gone due to the new rings. I can now do far longer exposures. Will try for one hour exposures on something very dim.


Bert

[Joshua Bunn]

Great result Bert.
I see some elongation on the LHS of M16 thats not in the SMC. What do you put this down to?

Josh

[dvj]

Ah ha! See those rings do indeed help which is why I immediately put them on my scope. And some dealers who shall remain nameless were poo pooing the idea of rings on the RH200 OTA because it would pinch the optics.

[avandonk]

If the rings are installed with any static stress on the tube you may affect the optics depending on severity. The Officina Stellare rings were not available earlier on and any third party rings that did not fit the tube accurately would have affected the optics.

The trick is to fit the rings to a very flat dovetail or plate with the 6mm bolts holding the rings to the dovetail only done up finger tight. Then install the RH200 into the rings and progressively tighten the rings fully. Only then can you now tighten the 6mm bolts again progressively. This will minimise any static stress on the tube.

I still have slight differential flexure between the RH200 and the camera frame due to the double dovetail twisting under lateral load due to gravity.

I can get rid of this by fitting dovetail bars as in the images below.

Bert

[avandonk]

Would you believe the dovetail bar is twisting under gravity. The M16 data was collected with the optic train tilted and the SMC data close to vertical. My trusty dial indicator tells me there is NO flexure. The optics tell me there is. I have to stabilize even the double dove tail with outriggers. See images in previous post. My best guess is that the camera frame is twisting the rear of the single dovetail more than the RH200 on the double dovetail.

In this Universe there is no such thing as a rigid body.

The RH200 has a 42 mm fully corrected image circle. The PL16803 camera I am using has a 52 mm diagonal sensor. The RH200 was not designed to take the load I have put on it with the ten position filter wheel being way too heavy with the PL16803. It is up to me solve the flexure problems. This is all worth it as I am getting a very fast system with a 3.5 X 3.5 degree field.

Bert

[Joshua Bunn]

Bert,
Are you able to use thicker dovetails? Im quite impressed with the way you've gone about eliminating flexure on a fast system.

Josh

[avandonk]

It is the old problem the Saturn Five rocket faced, more weight more fuel and your problems escalate geometrically.

It is all about compromise and balance. There is no correct answer each situation has its own solution.

Bert

[RobF]

Or a filterwheel solution with interchangeable carousels? I imagine you've done all the research Bert and there aint no such beast you're happy with.

[avandonk]

After aligning the optics exquisitely you want me to change filters or wheels?!! I have 50 mm square filters to minimise vignetting. The nine filters I have cost over $8K.

Bert

[gregbradley]

I know you are limited to backfocus distance but ultimately the lack of an OAG or self guiding with an SBIG style chip on the filter wheel (perhaps you could make one yourself) means flexure is likely to come about.

I think ideally the guide scope if you have to use one should be connected to the OTA of the scope it is guiding not a dovetail that can move slightly independently of the OTA - hence the difference.

OAG eliminates all this. I wonder if OS can make a corrector to extend the backfocus somehow so all this can fit off the back. Or a QSI style camera with a built in OAG or one of the SBIG varieties.

Another alternative would be the SBIG STi and lens kit autoguider that is light, comes with rings and could be fixed to the OTA easily as it weighs nothing and should cope with a short focal length scope.

A guide scope off to one side on a different dovetail may be hard to pin down to no flex at all.
Greg.

[dvj]

Looks like this is turneing into a bit of an equipment discussion, but here is a photo of my RH200 with the STI mounted on top. It's solid. No moving parts. 1 hour sub exposures no probelm. This was not the case before I added the rings, replacing the single arm base plate that came with the original unit.

The need for the rings was necessary to eliminate flexure.

The STL also has internal guiding which I have tested with no difference between the STI guidescope or internal off-axis guiding.

[multiweb]

On a related note I've seen similar issues on a C11 with an hyperstar at F/2. The weight of the whole assembly was bending the bottom dovetail bolted in the mirror cell and the corrector cell and as a result the field shifted over time. That went away when I used parallax rings with a top and bottom dovetail.

[gregbradley]

Yes that looks very good. The only problem here is the STL having half the FOV of the 16803. A Microline 16803 would be nice for this scope having half the weight.

Greg.

[RobF]

Do you lose out on anything with a microline Greg? It seems almost too good to be true - same performance, half the weight?
(sorry if this is hijacking Bert, but you're way out on the tip of the performance curve here and its fascinating to learn more about the options)

[dvj]

Regarding the Microline 16803, you give up the USB hub and aux power connector. Those are great for connecting the filter wheel without an extra power adapter to manage. You also have a fixed download speed as I recall. Anyone I know who has used one, loves it. Those may be small features to give up to save weight on the optical train. I would still add the FLI Atlas focuser to the RH200. You will not find a more solid, flex free digital focusing device. I have one on the FSQ and the 16803 and sware by it.

j

[gregbradley]

You lose a few degrees C of cooling power on the 16803 chip. Not many, from reports about 4C. No big deal as these chips are clean anyway on a FLI after -15C. You should be able to get -25C all year round and -30C in cooler months.

Greg.

[avandonk]

I was most probably to close to my problems with all of this and could not see it all clearly. Thank you all for your ideas thoughts comments. We should always welcome constructive comment or even constructive criticism. The thread title did have the word test in it.

The rings have made all the difference. I added the bar to hold the rear of the dovetail at the camera or ITS end. It has completely stopped the camera from moving. See the DSS screen capture below. The columns dX, dY and Angle are a measure of what is going on.

I managed to get 15x4 min green frames of an area just below the Coal Sack and then the cloud really got worse. I did not use dither so all images should have been identical.


Image here at native pixel sie 6MB

http://d1355990.i49.quadrahosting.com.au/2013_06/TEST_green_NATPS.jpg

Here is the image x1.5 native pixel size. 9MB

http://d1355990.i49.quadrahosting.com.au/2013_06/TEST_green.jpg


We are just about there. I may need to realign the mounts PA as there is a tiny amount of drift.

Bert

[Joshua Bunn]

that's an incredibly fine result Bert. is that dx and dy a measure of the small movement due to flex as the mount tracked?