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Planewave CDK 12.5" First Light

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#1 Jared

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Posted 15 September 2012 - 12:32 PM

I managed to get first light with the new Planewave 12.5" CDK from my backyard in Oakland last night and am pleased with the preliminary results.

Equipment:
  • Planewave CDK 12.5" @ f/7
  • Astro-Physics 900 GTO mount
  • SBIG STL-11000M C1 camera
  • Astrodon Gen II Luminance filter
  • Astrodon MOAG off axis guider
  • SBIG AO-L adaptive optics unit running at both 5 Hz and 0.5 Hz
  • Manual focus based on visual inspection of star images only

Conditions:
  • Seeing 6/10 Pickering
  • Transparency 3.2 NELM (based on SQM readings)
  • Casual polar alignment based on Losmandy PAS only--likely within 15 arcmin of pole, but no better
  • Collimation based on Cheshire and laser only--no stars used for collimation

First, let's get some caveats out of the way. This is a first light report, and I have not in any way "optimized" the system yet. I performed a basic collimation using a cheshire eyepiece and a laser collimator, but have not given the scope a final "tweak" under the stars. I believe there is still some work to do on this--I think the secondary is very slightly off center, and the focuser probably needs a bit of shimming as well since rotating the focuser shifts the laser onto and off of the secondary spot. Also, please keep in mind when comparing CCDInspector results on different scopes that the percent field curvature is only valid for comparison across similarly sized CCD's. A value of 20% curvature with a 35mm chip might be more like a 5% value with a smaller chip. Finally, I haven't yet gotten the autofocus working, so focus is very rough--based simply on a visual inspection of diffraction spikes on Vega--trying to make them as narrow as possible.

O.K., enough caveats and explaining away of imperfect results... Let's get to what I found.

First, a CCD inspector measurement of a 5 minute subexposure of M15.

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#2 Jared

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Posted 15 September 2012 - 12:39 PM

As mentioned, there is still some work to do, but the FWHM numbers are pretty good for a rough pass (measured value across the frame was 2.2"). I'll get on the collimation and tilt issues. Curvature is quite reasonable--the Planewave does well across a 35mm imaging circle, but it is not a perfectly flat instrument. I believe this is consistent with Planewave's own charts from their website for field curvature. I was getting 2.1" FWHM on stars in the center and 2.3" FWHM on stars in the extreme corners with no sign of astigmatism in the corners--stars there looked just as round as in the middle, just bloated. Better collimation and focus should slightly improve all these numbers.

By the way, I measured 18% vignetting in the extreme corners which is also consistent with Planewave's published statistics and is easily addressed with flats.

Here is a stack of 6 total five minute exposures of M15, resized for display on Cloudynights. Only processing was calibration (darks, flats, bias), curves adjustment in Photoshop, and a black point setting via the levels tool.

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#3 Jared

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Posted 15 September 2012 - 12:41 PM

Here is a crop from the middle of the frame at 100%

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#4 Jared

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Posted 15 September 2012 - 12:42 PM

Here is a crop from the top-center of the image showing what a stretched, bright star looks like.

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#5 Jared

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Posted 15 September 2012 - 12:44 PM

Not quite as round as I'd like... Definitely some work to be done on the collimation.

Here is a 100% crop from the top left corner. The frames were dithered, so you might notice a slightly darker band where there was no data from some frames.

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#6 Jared

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Posted 15 September 2012 - 12:44 PM

And top right...

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#7 Jared

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Posted 15 September 2012 - 12:45 PM

And bottom left...

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#8 Jared

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Posted 15 September 2012 - 12:46 PM

And bottom right.

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#9 Jared

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Posted 15 September 2012 - 12:52 PM

Overall, a very promising first result.

Interestingly, I shifted M15 slightly off-center so that I could get a guide star bright enough to allow me to try the AO-L at a range of frequencies. Within the limitations of just six subexposures, it didn't seem to make much difference whether the AO-L was running at 4.5Hz (0.2s exposures w/ some latency) or at 0.5Hz (2s exposures). The image with the best FWHM was with the fast refreshes. Second best was with 2s exposures. Third and fourth were short exposures, and 5th and 6th with longer. FWHM measurements in CCDInspector ranged from 2.18" in the best exposure to 2.4" in the worst. Perhaps a slight advantage under these seeing conditions for faster refreshes? Hard to know for certain. Lots more experimentation required.

Next steps are to try and fix the minor collimation issue, perform a Rodier test (just to satisfy myself that the optics are reasonably good), and look at whether there is any residual tilt that needs to be addressed by shimming the focuser. I'm very happy with these preliminary results, though.

#10 David Pavlich

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Posted 15 September 2012 - 01:08 PM

Pretty impressive, Jared! Collimation is pretty close now. How's the CDK to collimate?

David

#11 Jared

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Posted 15 September 2012 - 01:20 PM

It's less finicky than my 10" RC for collimation--likely due to the spherical secondary mirror.

#12 GlennLeDrew

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Posted 15 September 2012 - 03:18 PM

Easier collimation is definitely due, in part, to the spherical secondary. Having no discrete optical axis means there is requirement for centration, outside of fully intercepting the image-forming light cones. If the secondary is oversized, it can be appreciably off center and no injury to the image results. Simply tilt as required.

#13 Nezar H

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Posted 15 September 2012 - 05:44 PM

hi jared ,

really great result .. have you tried to use the telescope for visual use, if yes ; please let me know how do you find it? i may go with the CDK24"! as all in one telescope .. Almost all in 1 !

thank you.

#14 Jared

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Posted 15 September 2012 - 06:03 PM

I used it visually for a few minutes, but it was really just to get the finder scope and main scope collimation. I also took a peak at the ring nebula, and it looked fantastic. I'll give you more of an answer on your main thread.

#15 djnct

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Posted 15 September 2012 - 10:14 PM

Looks terrific- congrats and best wishes for your new scope!
Dan

#16 Jared

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Posted 16 September 2012 - 11:15 AM

Managed to get the collimation tweaked last night and shimmed the focuser very slightly. Seeing conditions were a bit better, and I managed to get FWHM stars under 2" on a couple of my luminance frames.

Wahoo!

Here is the CCDInspector report...

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#17 Jared

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Posted 16 September 2012 - 11:16 AM

Here is the image...

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#18 astroRoy

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Posted 16 September 2012 - 11:21 AM

Beautiful. I just shot this the other night and I can see from yours that my focus is still a little soft. I'll use yours for a guide - and a goal! Nice work, wonderful processing, awesome setup.

Roy

#19 Jared

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Posted 16 September 2012 - 03:17 PM

Thanks for the compliments, Roy!

#20 pop

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Posted 16 September 2012 - 04:24 PM

Very nice picture.

Why you change AT10RC to this CDK 12.5? It is a lot more expensive scope but less spot size and have dedicated flattener?

Thank you very much for your nice and useful post.

POP

#21 KDizzle

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Posted 16 September 2012 - 04:38 PM

Jared,

Your ccdinspector screens inspired me to check my own equipment, especially since we both have similar stuff (great minds think alike, right ? ;)). I'm not really sure I understand the screens though or what they are telling me that I should be doing something about. Is the idea to get curvature/tilt/collimation as close to 0 as possible? Could you explain what the different values mean to you and what you do about them?

#22 EricCCD

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Posted 16 September 2012 - 07:56 PM

Jared,

Congrats! Looking very good and promising! Will you have full-res images of the stars at the corners of the M27 image available like you had your M15?

Eric

#23 Jared

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Posted 16 September 2012 - 09:15 PM

Very nice picture.

Why you change AT10RC to this CDK 12.5? It is a lot more expensive scope but less spot size and have dedicated flattener?

Thank you very much for your nice and useful post.

POP


I wanted the 12.5" for the extra light grasp and focal length. Plus, the AT10RC doesn't give round stars in the corners of a really large imaging chip, which the Planewave does. I suspect the AT Ritcheys could do better with bigger chips if they had dedicated flatteners. They aren't really in the same league in terms of optical quality or fit and finish--the Planewave is a significant step up. I haven't done a Rodier yet on the Planewave, but just based on quick star tests the spherical correction is better.

I think the AT Ritcheys are a wonderful value--I prefer them in general to the Celestron Edge scopes. The Planewave is better still.

#24 Jared

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Posted 16 September 2012 - 09:27 PM

Jared,

Your ccdinspector screens inspired me to check my own equipment, especially since we both have similar stuff (great minds think alike, right ? ;)). I'm not really sure I understand the screens though or what they are telling me that I should be doing something about. Is the idea to get curvature/tilt/collimation as close to 0 as possible? Could you explain what the different values mean to you and what you do about them?


The FWHM values tell you how small your stars are--both the best values and the worst across the frame. Lots of things can affect these values including collimation, seeing, focus, tilt (affects the range of values), and curvature.

The curvature value is usually not something you can adjust except, perhaps, by changing the spacing between your field flattener and your imaging chip. It's basically a measure of how much bigger the stars in the corners are, so the size of the chip affects the value.

The tilt value lets you know whether your camera is square to the optical axis. It measures differences in star sizes in the different corners.

The collimation values measure how far out of round the stars are at the center of the field.

I think I've got that all correct. I'm responding from memory, so anyone feel free to correct me if I've got any of that wrong.

#25 Jared

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Posted 16 September 2012 - 09:30 PM

Jared,

Congrats! Looking very good and promising! Will you have full-res images of the stars at the corners of the M27 image available like you had your M15?

Eric


Happy to provide corner crops tonight when I get home.






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