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Dpac with pt grey firefly mv

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#1 tim53

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Posted 10 February 2019 - 06:29 PM

Today I worked on tweaking the dpac rig I’ve set up for testing mirrors. 

 

I posted my latest tests of the 8” f/8 mirror I bought for the Schaefer scope.   I had stripped the coatings last time, which made it harder to find the reflected ronchigram, but I managed. 

 

So now I’m retesting the 8” f/6 mirror I bought off CL many years ago. I thought it was a so so mirror, but it looks decent under this test. 

 

Inside focus

 

96712E42-D9F7-4E15-A34A-1197CB145287.jpeg

 

Outside focus 

 

35019046-B167-4826-A6DC-437B6ACE47CC.jpeg


Edited by tim53, 11 February 2019 - 10:13 AM.

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#2 tim53

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Posted 10 February 2019 - 06:32 PM

Those are screenshots with my phone. I can also stack the video but it takes more work to avoid thermals in the frame. 


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#3 tim53

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Posted 10 February 2019 - 06:33 PM

Based on the general appearance of the mirror, where I bought it, and the similarity to other oc mirrors, I think it was probably purchased from them. 


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#4 Chuck Hards

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Posted 10 February 2019 - 07:06 PM

What's the line frequency of the Ronchi screen, Tim?


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#5 tim53

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Posted 10 February 2019 - 07:36 PM

133


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#6 starman876

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Posted 10 February 2019 - 07:36 PM

Looks like a good mirror Tim.  Congrats.



#7 Chuck Hards

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Posted 10 February 2019 - 07:39 PM

133

Thanks, that's what I've been using lately for most stuff.  I tried a much finer one thinking it would add to test senstitivity but it also introduced artifacts that kept throwing me.  I have an Edmund glass Ronchi at 100LPI that I used for most of my life  until I picked up an assortment a couple of years ago.


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#8 starman876

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Posted 10 February 2019 - 09:27 PM

133 is what I use also.

I have found that at half the focus length of the lens I get what looks like some strange lines. Do not know if that is because I use camera and can watch how the lens responds every inch of the focal length as i move the camera away from the lens.
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#9 tim53

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Posted 11 February 2019 - 10:19 AM

When I went back to the 8" f/8 mirror, the Firefly camera had been on for a couple hours, and appears to have overheated, or the application is having troubles.  It is an old application, and the computer is 15 years old this year.  I much prefer my later, but not latest Mac, as it's as fast as a new one but still has Firewire 800.  But it's also one of the dreaded period with faulty logic boards (soldered on graphics chips), so it crashes a lot.

 

I may have to use a different camera for this, which means making another ronchi setup with the c-mount lens on one of the firewire cameras.

 

But I've also been cogitating about something like an endoscope camera, which would be very small.  And maybe a laser pointer, also small, for collimation?  Hm...

 

-Tim.


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#10 DAVIDG

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Posted 11 February 2019 - 10:20 AM

 If your going to use a camera with a lens as your  primary sensor vs your eye then I  highly recommend you add an internal standard to be sure that nothing in your setup is causing a false result. So what can you use ? Since your using a Ronchi screen and looking for the straightness of the bands place  a straight piece of material directly in front of the mirror and about a third of the way from the center. of the mirror. This is the location were you would see the largest amount of bow in the Ronchi bands.  Now you have a standard that should show in your images as being perfectly straight and also something to judge the straightness of the band against. If the standard isn't straight you know something is wrong.

  Another thing to check is that the image of the mirror should be round. If not your diagonal is not aligned correctly. It looks to me that the diagonal is too close to the mirror so it is vignetting and your not getting an image of the very edge, so you could be missing a turned edge.  

 

                                - Dave 


Edited by DAVIDG, 11 February 2019 - 12:04 PM.

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#11 tim53

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Posted 11 February 2019 - 10:44 AM

Good suggestions!  I hadn't thought of that. I will do that next time I have the chance.  

 

I think i'd have a lot more success with visual if I could hold my head in a vise so the ronchigram stops wiggling.  And with my eye, I can't show my results here.  I may be making excuses for my approach, of course, but I think it's been harder to do dpac with a diagonal than it would be with a perforated flat.  The diagonal mirror might be a 1.33 incher, which is probably not big enough for an f/6 mirror.  Add to that the effect that the light source and camera axis are not right on the optic axis, but a small distance to either side, and vignetting would be worse (this is what I found with my complete OTAs when I tested them - Cave, in particular, seemed to like to use diagonals small enough to vignette past about a half-inch focal plane.  For planetary, of course, even imaging, that was plenty).

 

I do find it easier to get the collimation very close with the camera, because I can watch the image on the computer screen while I tweak the adjusting screws on the mirror cell.  But there is one place where I might still be off-axis and not have it show in the camera:  If the initial collimation isn't actually at the optical center of the mirror, I could be testing slightly off axis when i have everything collimated on the computer screen.  For the f/8 mirror, I've been using the hole in the center of the cell plate that I drilled and used to guide where I drilled the adjusting screw holes.  And the mirror is resting on 2 decking screws.  So if the tapered mirror blank is not exactly 8" in diameter, I'd be aligning the laser through the uncoated mirror slightly off axis and getting the laser beam perpendicular to the mirror off axis.  For the f/6 mirror, which is coated, I don't have a center mark, so I've been eyeballing the location of center.  And since I'm viewing the reflection of the laser from one side of the test stand, I may be fooling myself by perspective into thinking its centered when it might not be.

 

One thing I noticed this time, while trying to use a laser collimator that's just got the tight dot beam and no grid pattern, is that the beam isn't centered in the body of the collimator.  I think it's adjustable, but I'd have to put it in the lathe to adjust it to be centered.  It's too bad, too, because the Lasermax is heavy, and when adding it to the Hotek blug, it cantelevers pretty far out from the focuser, and may be bending my support stand.  So that's another source of off-axis distortion that isn't preserved when I put the much lighter camera in the focuser.

 

-Tim.



#12 tim53

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Posted 18 February 2019 - 06:52 PM

So I had started a thread under ATM asking about using a borescope camera on axis for dpac testing.  Nobody seems to have tried it, and didn't seem to think it would work, since borescopes typically have wide angle lenses on them focused in the near field.  

 

So I've come back here, and have some questions.  I have tried several configurations with my firefly MV camera and get no better results than I showed above.  Usually, they're a lot worse, if I can find the reflection at all, that is.  Note that this test is of the 8" f//8 mirror that i've stripped the coatings from.  

 

I find that, once I've gotten the optics collimated, I still can't see the ronchi bands over the whole mirror.  As I said, the results I've been getting are worse than what I showed above, with the ronchigram being "football shaped" with the shadow of the secondary mirror on one side of the football.  The whole mirror is visible in the camera, too (see attached pic taken with one shop light on so I could see the secondary hub at the same time view the ronchigram).  Note that this was the best I could do with the adjustment screws on the primary cell.  I never did get the bands on the "left" side of the mirror, unless I rotated the camera/light, but then got the same size or narrower football.  So I don't know why I'm not seeing the ronchigram over the mirror.

 

I've tried just using the ronchi screen and LED and viewing by eye, but I can't find the reflection any easier by eye.  I can't adjust the mirror support while viewing either.  And I would rather not core the Perkin Elmer 12" flat if at all possible.  It's just too nice (and about 2 1/2" thick).  Being on the optic axis without having a **** secondary to adjust as well as the primary and flat, would certainly be my preference, so I've been thinking about building a support stand that would hold a 1 1/4" ID tube that I could put my laser in and then swap out for the camera/light/ronchi screen.

 

Questions: 

 

*How close does a camera lens need to be to the ronchi screen to image the ronchigram? 

 

The firefly MV uses a webcam lens, so its' pretty small and the front element is close to the front of the lens barrel.  

 

The Flea has a c-mount lens where the front element is a good 3/4" inside the light shield, so I can't get it any closer to the screen than that.

 

The iphone X's camera is only about 1/4" from the LED, and is even smaller than the webcam lens.  It and the LED can be placed very close to the ronchi screen, so if I could mount the iphone in the focuser I've been using and not have it wiggle every time I go to image with it, that would still be my preferred option.

 

*When viewing by eye, there's obviously a lens involved for focusing.  But this isn't afocal, since there's no eyepiece in the telescope.  So, does the camera need a lens in order to reach focus?  What are you focusing on when imaging the ronchigram?  Infinity?  Or the ronchi screen?

 

Here's what I saw with the firefly MV after I'd made a new "ronchi eyepiece" with it, the red LED, and a piece of ronchi screen taped to a 1 1/4" adapter for the 2" focuser I've been using.

Attached Thumbnails

  • ronchigram.jpeg

Edited by tim53, 18 February 2019 - 06:53 PM.

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#13 tim53

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Posted 18 February 2019 - 07:00 PM

I've got so many scopes in my shop right now that I need to sell more.  In spite of my current frustration, I would rather sell most of my classics and keep my favorite ATM projects.  So I'd really like to get this rig set up so I could show any prospective buyers/friends what they're getting when I do sell the classics, and so we can remain friends!wink.gif

 

Actually, testing complete tube assemblies would be a desirement, and I have had some success with that, though almost always with some vignetting, since Cave and others had a tendency to believe in fully illuminated fields only a half inch across for planetary, so their secondary mirrors are undersized.  In that case, separate tests of the primary and secondary would make sense.

 

-Tim.


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