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Go Back   IceInSpace > Equipment > Astrophotography and Imaging Equipment and Discussions

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  #21  
Old 25-07-2018, 06:48 AM
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PRejto (Peter)
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Thanks Eric and Josh!


OK, Enough about the idea of a flat helping anything.

As far as the laser, I don't think aiming at a centered dot on a paper mask is going to get me very accurately centered but I suppose it is better than nothing. The Glatter laser I have projects concentric circles but these circles do not reach the edge of the objective over the path length. Now, if they could somehow be spread out more they might prove useful in centering.

1. I will take the rotator to my local machine shop. I'm hopeful the tech there will help me measure the runout as Josh suggests. If not I'll need to wait until the tool I've ordered arrives.
2. I will post a photo of the setup.



Peter

Last edited by PRejto; 26-07-2018 at 10:32 AM.
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  #22  
Old 26-07-2018, 10:38 AM
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PRejto (Peter)
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No run-out test yet.


The distance from the back of the rotator to the CCD is ca 190mm.


Parts shown, in order left to right.


PreciseParts adapter sitting on TEC locking collar
Gemini Rotator
Gemini to DSI adapter ring
DSI Extension ring(Optec)
Precise Parts adapter
TEC180 Flattener
PreciseParts adapter
SX Large Filter Wheel
SX-46 (KAF 16200)
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  #23  
Old 27-07-2018, 12:17 PM
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PRejto (Peter)
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I put a dial indicator on my rotator and received the following results:

1 tic = 10 microns


180 = 0
225 = 1
270 = 5
315 = 8.5
360 = 8
45 = 7
90 = 3
135 = 1.5
179 = 1


Repeated and got same values pretty much going the opposite direction (at 360 and 315 measured 9), but ended up at 180 = 1.5, so the measurement is probably only accurate to ca 10 microns or so. But, I think I can safely say the thing is tilting by 80-90 microns.


I put the dial indicator on the side to measure concentricity:


360 = 0
45 = 2.5
90 = 0
135 = -5
180 = -6
225 = -6
270 = -5
315 = -3.5
360 = 0


So, maybe 85 microns max movement side to side.



Eric, might you help me with the math here? How would 80 microns of tilt and 80 microns of non-concentricity display on the CCD ca 190mm further back?



Peter

Last edited by PRejto; 27-07-2018 at 03:39 PM.
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  #24  
Old 27-07-2018, 09:45 PM
ericwbenson (Eric)
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Hey Peter,

Those angles, are they the rotator PA while being moved by the motor, and the dial gauge is fixed? And what is being used as the reference surface, the rotator body I presume?

So I see two possibilities:
-the moving tube of the rotator is rotating true (no eccentricity) and the end, or mounting surface, is cut crooked
-the moving tube is cut square but rotating with eccentricity

In either case the other end of a 'perfect' adapter attached to the moving tube will draw a circle around the optical axis.

I assume you measured the surface at a diameter of 100mm

The angle is 80 um / 100 mm = 0.8 mrad
and the orbit radius will be 0.8 mrad x 190 mm = 152 um

The side to side motion will add/cancel to the above with an amplitude of 80ish um.

Not too bad really. A 0.5 mrad tilt of the image plane gives 0.5 mrad x 35 mm = 17 um of defocus in each corner of your KAF16200 chip (plus in one corner and minus in the other). No big problem at f/7.

EB
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  #25  
Old 27-07-2018, 10:49 PM
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PRejto (Peter)
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Hello Eric,


I appreciate your calculations! This is reassuring and confirmation of your assessment of the video of the laser. This, at least, represents the best I can hope for as long as I can get the rotator mounted properly.


Yes, the dial indicator was sitting on a table with the probe on the moving part. The angles are just arbitrary degree readings from the software during rotation. I did take care to keep the angles the same for both tests.


Thanks again!


Peter
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  #26  
Old 28-07-2018, 05:47 PM
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I must be cursed!



My Gemini now fails to rotate at all. I was doing a simple experiment with essentially no weight on the rotator. It rotated about 1.5 times without any issue but suddenly stopped and indicated "not homed." I tried to home it. Motors spin up but rotator doesn't rotate. After awhile it announced "homing failed." Hoping it was software I disconnected power and USB. Reconnected power and motors start spinning but no movement. After a while they stopped so I connected the software. It connected and indicated homed at 0.00 degrees. Tried to rotate and nothing. Strangely it counts up or down as it tries to rotate and the numbers stop on the assigned degree of rotation even though there is no physical rotation. I tried the hand controller too. My guess is that something is rotating internally to activate sensors of position and that something has broken that links that mechanism to the actual rotation.


Anyone here have any experience opening or servicing a Gemini? I fear I will be packing it up for a trip to the USA. At this rate I will never get to image.


Peter
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  #27  
Old 28-07-2018, 07:50 PM
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Peter that is really! Unfortunate! I feel for you there.

As you know, I use a different brand,so I'm not much help with this particular issue.


Josh
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