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Equipment Discussions >> Cats & Casses

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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Planewave CDK 12.5" First Light
      #5422364 - 09/15/12 01:32 PM Attachment (225 downloads)

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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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5422372 - 09/15/12 01:39 PM Attachment (199 downloads)

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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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5422374 - 09/15/12 01:41 PM Attachment (131 downloads)

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

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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5422376 - 09/15/12 01:42 PM Attachment (106 downloads)

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

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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5422380 - 09/15/12 01:44 PM Attachment (106 downloads)

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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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5422384 - 09/15/12 01:44 PM Attachment (64 downloads)

And top right...

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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5422385 - 09/15/12 01:45 PM Attachment (44 downloads)

And bottom left...

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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5422389 - 09/15/12 01:46 PM Attachment (48 downloads)

And bottom right.

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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5422401 - 09/15/12 01:52 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.


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David PavlichAdministrator
Transmographied
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Reged: 05/18/05

Loc: Mandeville, LA USA
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5422418 - 09/15/12 02:08 PM

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

David


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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: David Pavlich]
      #5422432 - 09/15/12 02:20 PM

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

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GlennLeDrew
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5422582 - 09/15/12 04:18 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.

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Nezar H
member


Reged: 08/11/11

Re: Planewave CDK 12.5" First Light new [Re: GlennLeDrew]
      #5422752 - 09/15/12 06:44 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.


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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: Nezar H]
      #5422775 - 09/15/12 07:03 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.

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djnct
sage


Reged: 11/26/05

Loc: CT
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5423090 - 09/15/12 11:14 PM

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


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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: djnct]
      #5423654 - 09/16/12 12:15 PM Attachment (97 downloads)

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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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5423655 - 09/16/12 12:16 PM Attachment (134 downloads)

Here is the image...

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astroRoy
professor emeritus


Reged: 07/09/11

Loc: Southern Oregon
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5423664 - 09/16/12 12:21 PM

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


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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: astroRoy]
      #5424028 - 09/16/12 04:17 PM

Thanks for the compliments, Roy!

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pop
sage


Reged: 02/11/05

Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5424116 - 09/16/12 05:24 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


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KDizzle
scholastic sledgehammer


Reged: 09/12/08

Loc: Woodinville, WA
Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5424131 - 09/16/12 05:38 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?


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EricCCD
Carpal Tunnel
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Reged: 04/14/04

Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5424383 - 09/16/12 08:56 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


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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: pop]
      #5424507 - 09/16/12 10:15 PM

Quote:

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.


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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: KDizzle]
      #5424525 - 09/16/12 10:27 PM

Quote:

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.


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Jared
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Re: Planewave CDK 12.5" First Light new [Re: EricCCD]
      #5424533 - 09/16/12 10:30 PM

Quote:

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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pop
sage


Reged: 02/11/05

Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5425084 - 09/17/12 09:43 AM

Quote:



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.




Thank you very much for your nice answer. I will wait your nice photos in using this scope.

Best Regards,
POP


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Jared
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Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5425577 - 09/17/12 02:35 PM Attachment (65 downloads)

Quote:

Quote:

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.




O.K., here are the four corner. These are the absolute extreme corners--the full image had very slight cropping to remove some areas where the different frames didn't overlap--so it won't perfectly match the earlier view.

This is a stack of ten separate five minute calibrated luminance frames. I manually applied a single curves adjustment in Photoshop, but no other processing of any kind.

It appears I still have a small amount of focuser tilt--you can see more curvature and astigmatism in the bottom right corner than in the rest. Still, it's pretty good for a 42mm imaging circle viewed at 100%.

First, the top left...


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Jared
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Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5425579 - 09/17/12 02:36 PM Attachment (49 downloads)

Top right...

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Jared
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Reged: 10/11/05

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Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5425581 - 09/17/12 02:36 PM Attachment (34 downloads)

Bottom left...

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Jared
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Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5425582 - 09/17/12 02:37 PM Attachment (36 downloads)

Bottom right...

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Jared
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Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5425583 - 09/17/12 02:38 PM Attachment (50 downloads)

And the center of the frame...

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jmiele
Patron Saint?
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Reged: 12/04/10

Re: Planewave CDK 12.5" First Light new [Re: Jared]
      #5426427 - 09/17/12 10:46 PM

Looks like a winner to me Jared. Well matched for you expertise. I expect we will be seeing some spectacular imaging from you this winter.

Also, Glad the visual seems to be usable as well. That's a bonus. I know you tossed that image together quickly to give us all a peek, but the processing really is quite good. Also seems the STL is a nice match for it. Thing I like about the SBIG ST(L) stuff is the ability to use the AO Devices. They are a big help at taking a shot back at seeing.

Anyway, as expected - excellent stuff sir.

Best,
Joe


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wiruna
member


Reged: 08/11/06

Loc: Sydney, Australia
Re: Planewave CDK 12.5" First Light new [Re: jmiele]
      #5517226 - 11/13/12 03:00 AM

Hi Jared
I'll soon have a similar setup--same mount, same scope, different camera. I have a question about your use of the AO unit. Is it necessary with the AP900 and the 2500mm focal length or could you get good enough tracking without using it?
Geoff


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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Planewave CDK 12.5" First Light new [Re: wiruna]
      #5520414 - 11/15/12 12:38 AM

I've been doing a fair amount of comparing my results using AO vs. ordinary auto guiding, and so far the AO system has shown a small but consistent improvement in FWHM values versus the guider. The differences seem to be around 0.2" FWHM improvement on nights of average or better seeing when I have a guide star bright enough to allow 2 Hz corrections, and more like 0.1" FWHM when guiding at 1 Hz or slower.

I don't have anything really conclusive yet. Traditional guiding on the AP-900 is certainly enough to achieve stars that appear round, so you don't need an AO unit to get good results, but I do believe that the AO system gives a small but measurable advantage in resolution.

Before I can post more than preliminary results, though, I've still got some optimization to do. I'm in the process now of adjusting mirror spacing to introduce a small amount of over correction into the system since my Planewave has a small amount of under correction in the optics when spaced according to the Ronchi grating. Also, I haven't done a drift alignment on the mount yet, so even my AO images include the occasional "bump" from the autoguider when the AO gets near the limit of its travel--I suspect this may be "robbing" me of a little bit of resolution that would be there if I had better alignment.

The goal I have set for myself is to consistently produce (on nights of reasonably good seeing) five to ten minute subexposures with less than 2" FWHM across the field of view. I'm pretty sure e equipment is up to the task once I get everything dialed in, but that's getting pretty close to my typical seeing limited conditions (based on short subexposures of a couple or three seconds), so everything has to be working juuusssstt riiiiiggghhtt.


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