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paracorr star elongation

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#1 turnerjs085

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Posted 06 August 2014 - 08:29 AM

Hi all,

 

Im having some issues getting a paracorr2 + camera up and running on my 10" f5.8. The stars are equally elongated across the frame, out of focus donuts are also elongated but with the secondary shadow perfectly centered and equal brightness all around. The direction of elongation doesn't coincide with ra so I don't think that it is due to tracking, besides it can be seen in a 10 sec or shorter exposures.

 

I had to rack the focuser out much farther than when using a mpcc, can focuser axial error (due to sag) cause this sort of distortion?

 

Thanks for your help

Jeremy

 

 



#2 gatorengineer

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Posted 06 August 2014 - 12:03 PM

Yes

#3 Thomas Karpf

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Posted 06 August 2014 - 12:55 PM

Are all of the stars stretched in the same direction?  If so, that's probably astigmatism. 

Is the primary astigmatic?  How about the secondary? 

 

What does a star test without the camera show?



#4 turnerjs085

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Posted 06 August 2014 - 01:15 PM

Stars are all the same direction but don't switch directions through focus. Star test w/o camera or paracorr shows no trace of astigmatism. Primary is a Gordon waite ~1/10 wave and my own focault tests agree within the margin of operator error :) secondary is quartz 1/30 wave, shows dead straight fringes to the edge on a 1/8 wave flat. The sec holder is barely touching the glass (I lined it with thin felt).

 

I haven't done a star test with paracorr yet due to weather, I am trying to figure out if it is more likley a mechanical or optical issue with the paracorr. Maybe the compression ring in the drawtube was too tight?

 

Can astigmatism (possibly in the paracorr) make the stars all evenly elongated in the same direction in focus? I thought it would present as a cross shape, elongating in one direction and then the other when out of focus.

 

Jeremy



#5 Starman1

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Posted 06 August 2014 - 01:28 PM

If the focal plane is tipped, the stars can all elongate in one direction.

Alternately, the sweet spot of best coma correction may be moved outside the field of the camera.

That would be severe sag.

 

What focuser is being used?

How much does your camera weigh?

What tools do you use to collimate?

Are you locking the focuser down after focusing?  This may drive the drawtube sideways a bit.

How are you obtaining the exact distance from Paracorr to chip?

 

It's unlikely to be a problem with the Paracorr.  The answers to the questions might help assign cause.



#6 turnerjs085

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Posted 06 August 2014 - 02:31 PM

Thanks to everyone for your replys!

 

1. The focuser is a moonlight

2. Camera is a qsi 583wsg  4lb 2oz(with paracorr)

3. I use catseye AC + cheshire for collimation

4. Focuser isnt locked, it has a stepper attached to the shaft.l

5. Paracorr is spaced from the scope side to the camera: televue is thread .25" (6.5mm) spacer, 2 mm spacer, televue is thread to stl thread adapter (1mm), astrodon filter (-1mm), 53.3mm camera body depth. This adds up to about 62mm. 

 

I did have to pull the paracorr out of the drawtube slightly to reach focus, the camera was about 4in from the focuser body. Today i moved the primary down so it should now focus fully seated with the drawtube out about 1in. How much sag counts as severe?

 

Jeremy



#7 Starman1

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Posted 06 August 2014 - 03:02 PM

So:

--focuser sag is probably not the issue

--camera is not unusually heavy (of course if the focuser were way out from the stop.....

--good collimation, so collimation not a problem

--so no lateral movement when stopped

 

That is about 5mm too much distance between Paracorr lens and camera chip (optimum is 57mm)

 

Ah, the focuser is all the way out with the weight pulled even farther out.  OK, that nails it--it's sage in the focuser or even movement of the focuser by buckling/bending the tube it's attached to.

I suspect moving the primary down will help.  Every millimeter will count, here, in reducing the lever arm that weight has on the focuser.

I hope you don't have a thin steel tube.



#8 turnerjs085

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Posted 06 August 2014 - 04:40 PM

The tube is .25 carbon fiber with reinforcement inside the top and bottom 1.5ft.

 

I set the spacing longer than stock due to discussions with an optical engineer friend who uses the paracorr. He uses a 10in f6.5 and has found best correction to be at 65mm spacing. I dont understand anywhere near enough about optical design to argue with him but aparently the better correction for slower than design focal ratios is somewhat expected if you assume the paracorr design is close to that presented in "telescope optics".

 

I found someone else who had done similar testing: http://www.pbase.com...mage/124811410

 

(Sorry,  I'm not sure how to make a short link from my phone)

 

I hope that having the drawtube extended less will fix things. the more I think about it, the less likely anything other than the camera being tipped relative to the optical axis seems. Too bad there aren't any practical daytime tests, then I could mess with telescopes 24/7! ;)

 

Jeremy

 

 



#9 MitchAlsup

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Posted 06 August 2014 - 08:10 PM

You can test Don's idea by attaching a weight to your laser collimation tool such that it weights the same as the camera.

 

If the laser indicated collimated with the weight not attached

BUT

indicates miscollimation with the weight attached

THEN

either the focuser is not stiff enough

OR

the tube it is attached to is not stiff enough,

OR

a bit of both.



#10 turnerjs085

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Posted 06 August 2014 - 08:57 PM

Thats a good idea.  I'll try a bag of shot hung off the drawtube tomorrow.

 

I've always wanted to build a focuser that does away with the drawtube and bearing arangement. Something like a box built into the side of the tube that encloses the camera. It would have a track like a kinematic stage to move the camera body towards and away from the secondary and a bellows from the camera to the inside of the box for light shielding. The idea being to a. Spread the torque from the cameras weight over a very large area of the tube and b. Constrain the camera to one degree of freedom. I haven't quite figured out how to make it a real thing yet, it would probably require building another carbon fiber tube with the camera box molded in.



#11 Starman1

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Posted 07 August 2014 - 12:38 AM

The tube is .25 carbon fiber with reinforcement inside the top and bottom 1.5ft.

 

I set the spacing longer than stock due to discussions with an optical engineer friend who uses the paracorr. He uses a 10in f6.5 and has found best correction to be at 65mm spacing. I dont understand anywhere near enough about optical design to argue with him but aparently the better correction for slower than design focal ratios is somewhat expected if you assume the paracorr design is close to that presented in "telescope optics".

 

I found someone else who had done similar testing: http://www.pbase.com...mage/124811410

 

(Sorry,  I'm not sure how to make a short link from my phone)

 

I hope that having the drawtube extended less will fix things. the more I think about it, the less likely anything other than the camera being tipped relative to the optical axis seems. Too bad there aren't any practical daytime tests, then I could mess with telescopes 24/7! ;)

 

Jeremy

TeleVue's site says 57mm.  If there's a doubt that what they posted is correct, call them and report back.

They're usually dead on.



#12 turnerjs085

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Posted 08 August 2014 - 11:05 AM

Hi all, 

 

I moved the mirror down temporarily so that the camera reaches focus almost fully racked in so as to eliminate drawtube sag as a variable. I'll set it to about .5in from racked in once I get the flexure sorted out. The star shapes are much imprived but still elongated about 20%. This leads me to believe that the tube is flexing slightly at the focuser or spider bolts.

 

My next step is to cut plywood rings/baffles that press fit inside the tube, above and below the focuser and just above the spider. Hopefully these will eliminate the buckle/flex. The tube is 12in inside diameter with a 10in f5.8 mirror.

 

How can I figure out what the minimum inside diameter of these baffles can be before vignetting becomes an issue?

 

Thanks for all your help,

Jeremy

 

Don: I really would like to talk to Televue about spacing with slower optics, but I feel somewhat obligated to do due diligence and actually try both configurations myself before opening that discussion. (Not that its intimidating questioning the titans or anything....) ; )



#13 Starman1

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Posted 08 August 2014 - 12:00 PM

Vignetting and baffle diameter is merely a matter of selecting the baffle size that vignettes your widest field by zero percent.

The secondary itself will vignette some (unless it's chosen for 100% illumination over the entire chip), so why add to that with a tube baffle?

If you look at the baffle diameter, a simple tangent function will suffice:

The clearance between the mirror and the tube should be equal to 1/2 the field size in degrees of your widest field.

So, the adjacent side is equal to the edge of the mirror up to the tube opening.

The opposite side is the distance from the mirror edge to the tube (or baffle)

And the angle is the angle between a vertical line from the mirror edge and a straight line from the mirror edge to the inside lip of the front of the tube (or baffle).

You can easily see that the farther an aperture is from the mirror, the larger it needs to be in inside diameter to avoid vignetting.

 

I'll use my 12.5" as an example:

Maximum field = 1.3 degrees

so the angle between mirror edge and inside diameter at the top of the UTA will be 0.65 degrees.

Distance from mirror edge to lip of tube (rounded off for simplicity) = 60 inches.

So, tan .65= x/60

where X = mirror to tube I.D. (or baffle I.D.)  the actual I.D. of the baffle or UTA = Mirror diameter + 2X.

On my scope, X = 0.68", so an I.D. of 12.5 + .68 + .68 = 13.86" would work with that field size.

The mirror-to-UTA lip is actually slightly less than 60" on my scope, and the New Method of collimation slightly tips the optical axis toward the focuser anyway, and i might use an eyepiece with a larger field stop (like a 31 Nagler) so a slightly larger I.D. of the UTA is desirable.  In fact, it is 14.25".

 

So how large would a baffle be, placed at distance X from the mirror.

Well, at the mirror, a baffle can = the mirror size without vignetting.

At 60", it needs to be 13.9"

Which means that 30" off the mirror, it needs to be (12.5" + 13.9")/2 in size, or 13.2"

Similarly, you could easily figure out the baffle diameter at any distance from the mirror.

 

If your scope is a full tube type, you may discover that the tube I.D. itself vignettes the field of view.  That is fairly typical.

In that case, you don't need baffles but external rings to surround the tube (and be bolted to it) to provide greater stiffness to the tube.

Alternately, an additional piece of tube could be attached to the tube either internally or externally to increase the wall thickness of the tube.








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