Planewave CDK

[Peter Ward]

Given the decidedly varied quality in images put up on the Web from CDK telescope users, I'm interested to get some feedback from Planewave CDK17" users in particular.

I'm particularly interested in real-world cool-down times, thermal control and focus stability.


Thanks
Peter

[multiweb]

Might want to talk to Mike (placidus). He has a CDK20".

[rustigsmed]

I think greg has a 17" cdk.

[Gama]

I have a 20" myself, and have it apart at the moment.
Cooling is fine, however, mine was the first ones. The newer ones have side fans now as well. So this helps quite a bit more.
Thermal control (Again the original series) is with a differential pair of sensors. One is mounted on the primary near the base, and the second off the mirror to measure the ambient.
It turns on the fans at any point you want it to. Mine is set to 0.5 deg C, and works fine.
Cool down times are about an hour from opening of Dome slit.
I have had to "Fine Adjust" the focus on extreme temp swings however.
But this may also have been my camera, so a pinch of salt here..

The new models have even more control, so may need to get someone with the new bits on it.

Theo.

[Peter Ward]

Thanks very much Theo. Do you have an website/image gallery?

[Gama]

No images as such.
I mainly use it visually, and for nova searching.
The last two years its been in storage while I was building and moving into my new home (Farm). Its back in the observatory, now, but the mirror is going back for a clean, test and recoat.
I had some issues in the past, and Planewave are going to do this for me as part of their backup Warranty support. I must say, its been out of warranty for three years now, so to anyone else, its a good thing to remember when looking at suppliers. Support Support Support from anything you buy.
If there was a specific thing you wanted to know, let me know and I will answer it. No sugar coat, no bias.

Theo.

[Bassnut]

Itelescope love CDK, recon support is excellent. Why you would want to swap out the RCOS is a worry though (apart from more ap and now I suppose zero support from RCOS).

[Peter Ward]

Actually...I really covet one of these:

http://www.alluna-optics.com/rc-telescopes-16-inch.html

[clive milne]

Realistically Peter, the single biggest improvement you could make to your imaging equipment is to relocate what you already own.

[Peter Ward]

No argument from me ....:thumbsup:

[marc4darkskies]

You'd need a bigger dome.

[gregbradley]

Same as Theo. I find about an hour to cooldown. If temperature differential is more than about 1C there is a definite bad effect on focus. Lower than 1C it starts to become not so noticeable and after about .5C difference it seems pretty good. Focus stability seems good and similar to an RCOS in that regards. Being carbon fibre and temperature compensation on the carbon fibre rod joints.

As mentioned on some of the latest models (starts with the 17) there are sidefans so that could make a difference as well.

The main issue with the CDK17 is the same as what you would be experiencing with your 14.5 RCOS - seeing. Its quite sensitive to seeing at 3 metres.

I have also found flats touchy on this scope than any other. The optics tend to have a bit of a centre bright area like a Tak BRC250 does (the corrector?). But a nice dark sky and well done flats handle it but there's little tolerance there unlike some scopes that are fairly tolerant of flats.

Fairly easy to collimate, well made, they seem to continually upgrade their products. Also good support (they have sent me baffles for the corrector and baffle tube to cut down on stray light plus a new secondary shroud all for free).

I'd buy another Planewave and overall I like the scope a lot.

But as mentioned I don't see enough of a gain over your 14 to make it worthwhile. The bigger gain would come from a rural dark site with good seeing to be able to come close to Martin's images.

The reducer is good but has practically no backfocus so that gives problems with using it and a MMOAG or some guiding solution. Even an STXL and self guide filter wheel would be outside the optimum spacing for smallest spot sizes.

Martin got an upgraded mirror on his because the one in the scope they shipped to him was damaged in transit and they replaced it with a higher quality one.

CDK17 with STXL 11 + AOX at a decent dark site as Martin has shown is probably an ideal setup.

Greg.

[Peter Ward]

And boat too? :)

[marc4darkskies]

Huh? That's strange. So Planewave give you an option between a good mirror and a not so good one? I don't see that on their web site. :question:

[Peter Ward]

Thanks Greg, very useful info. :thumbsup:

[ericwbenson]

Hi Peter,

I have a 20" model as you know, it is about 4 years old. The support from PWI has been very good, they are very knowledgeable and treat advanced amateurs as such, I guess they know their clientele is generally more versed in OTAs than your average Tasco customer. I had several problems to overcome and they did their best to help me fix it as quickly as possible.

The first issue was a slight wedge in the corrector lenses causing dispersion so that every star was a little rainbow. The effect was small but I was able to measure it, easily with a one shot color camera. The dispersion looked like what you get when shooting low in the sky, but was present at zenith and rotated with the lens. They eventually sent me two replacement corrector lens sets with no complaints. The original lens set was from China and the wedge was detectable by observing the orbit of a laser spot as an individual lens was rotated in a v-block. They evaluated all their stock and determined that they ranged from bad to barely ok, they sent me the best one of the lot, it was better but not perfect. And so they went to a German supplier, I had to wait a bit for it, but then wedge gone, dispersion effect gone.

The next problem was the ghost reflections off anodized Al of the inside walls of the corrector cell. This took some detective work to track down, and I was actually lucky to have the spare lens cell assemblies here in Adelaide (the scope was installed in Ark by then) to play around with. Shining a collimated light beam (torch + 66mm refractor) thru the cell onto a screen in a darkened room revealed the problem. The solution was to take the cell apart and flock the interior walls of the cell. I explained the problem to PWI and they started making baffles, they offered me some but I had already flocked the tube so didn't need them.

While flocking the lens cell I flocked everything else that was anodized Al in there that I could (focuser, rotator, the OTA behind the lens cell I believe). This made a tremendous difference in reducing to the flat "hot spot".

The scope thermals are quite predictable. I open the slit just before or at sunset. The fans are set to operate if the primary versus ambient temp is greater than 1 deg. The fans will run for about 3-4 hours generally, unless the temp is dropping due to weather changing in addition to normal night time drop (then they will run all night). I only have the three fans on the back face. I have thought about installing fans on the sides to blow across the mirror but haven't inquired about it yet, too many other bigger problems lately...

The first focus run is always different from the last one from the previous night, and normally a second autofocus about 1-1.5h later will be the biggest shift of the night, autofocus every 2-3 hours after that will make only small adjustments. If I am late opening the dome the images for the first 0.5-1 hour will have extra aberrations off axis, and best focus will also be compromised.

I think most of these issues would be minimized if the mirrors were fabricated from zerodur...but then I wouldn't have been able to afford a 20"...so the trade off is acceptable with modern autofocus software.

Collimating is actually really easy, it's just three screws on the secondary. For the longest time I thought collimation was really hard and I played with secondary centration, camera tilt, etc. None of this really helped. It turned out that the mirrors, and corrector lenses had some residual astigmatism. So no matter what I did to collimate or tilt the camera, I got funny star shapes in the corners of the big chip. The center was always OK. So finally on the last night of my last trip I finally figured it out...rotating the lens cell in the baffle tube about 30 deg at a time I found a position where the lens + mirror astigmatism canceled, and stars were good all the way to the corners. Now remember in less than good seeing or with a smaller CCD no one would ever notice this effect, but now that I have spent untold hours fretting about this, I can spot it in some CDK images posted on the web.

The tube keeps it's collimation at different altitudes very well (I don't think I have detected an effect). This could be due to very good mechanics, the mirror being epoxied to the back plate, or a lack of sensitivity to collimation in the optical design, probably a combination.

The stock focuser is very sturdy however uses up too much backfocus when using an external rotator + MMOAG. I would recommend getting the integrated focuser + rotater (IRF90) since I think it is stiffer than the Bellerophon + Optec, and allows you to focus using multiple stars and a few exposures instead of a single star and many exposures. This would help reduce RBI from the bright focus star. I was going to upgrade to this unit in the near future, til my camera went kaput.

Get the optional shroud too, I take it for granted now, but it really helps keep the dust out and probably reduces the occurrence of dew on the secondary (I don't have that problem in Ark but near the coast you might). You can also get a dew prevention system (heater + sensor + software), I have the sensor + heater but haven't bothered to write software for it since dew has been a non problem.

As for site selection, well at 3m of focal length in poor seeing you will be binning and choosing larger targets such as nebula. The gain in aperture is really important in dark skies, but in suburban light pollution you will hit the sky glow wall of diminishing returns pretty fast.

Best
EB

[ericwbenson]

Martin had the very bad luck of a defective main mirror. The out of focus star image (donut) had a spiral pattern in it. He sent the whole scope back and PWI replaced the primary (I doubt they would have argued much!). The main annoyance for him was that it was a remote setup, I can sure relate to that!
I never heard what the cause of the defect was, but Greg is implying that it was shipping damage to the primary? I find that interesting, and perhaps plausible, since I had never seen such a severe and strange defect star pattern before, and would have thought that basic QC would have easily picked this up.

EB

[RobF]

Peter, surely that amazing 12" RHA and your RCOS does the job from suburbia? Sounds like aperture fever knows no bounds! :lol:

[gregbradley]

Martin told me that Rick from Planewave drove the new mirror up to Sierra Remote and installed it.

As far as an upgrade goes I believe that is what he said. Perhaps I got that wrong but judging by his images they seem sharper than what I get and I don't think all of that gain is from the seeing. I could be wrong.

The standard mirror is 1/4 wave and I believe they measure that at the eyepiece/camera not just the primary as it can shift after fitting if there is any warp or twist.

Greg.

[Gama]

Rick from Planewave is one of the most honest people I have dealt with.
If there's anything wrong with the telescope, he will tell you outright. Not blame something else. He will then fix it, as what is to happen to my mirror when it gets there.
So if you want the low down on what to expect from a Planewave telescope in terms of thermal issues then, I really suggest emailing Rick, and asking him.

Theo.

[Peter Ward]

I think I've met him (fleetingly) at a couple of trade shows...in any event, I'm going to the planewave factory in early December. Thanks one and all for the feedback to date.

:thumbsup:

[Gama]

You don't want to take a mirror up with you do you ?..
Im in the process of packing it..
Its only 20 odd inches, and weighs 20kG..
??????????????????????

Theo.

[Peter Ward]

Err..... No. :)

[multiweb]

Doesn't fit behind the seat :lol:

[Gama]

Use it as a good shaving mirror !.
Or better yet, a reversing mirror for the plane.


Theo

[Satchmo]

It does surprise me with these premium intruments that optical defects ( ie astigmatism , corrector lens wedge etc ) can slip through . It suggests that they are not given a thorough end to end artificial star test upon final assembly, to make sure it is meeting spec and that the customer is not going to see optical defects on the real sky. I would have thought that that kind of thing was what you were paying extra dollars for.

It is not rocket science to set up a collimating telescope of larger than the aperture to view an artificial star upon final assembly and tweaking. Celestron for example in their heydey used a 22" of long focus telescope to produce a collimated beam and final checking on hundreds of thousands of telescopes.

[ericwbenson]

The off-axis astig doesn't show up on a star test, only on really big CCDs. The wedge issue was only apparent with a one-shot color CCD in very good seeing on a star near zenith. I might have been able to see visually, but probably would have had to know it was there to distinguish it from atmospheric dispersion. In fact in the normal star testing I did the correction\roughness\on axis astig looked really good.



For a big telescope the artificial star need to be <very> far away (impractically) to be of any use, or you swamp the test image with S.A., so the collimating scope is not the problem, it's the <very> long hallway one needs, see here for example: http://www.telescope-optics.net/two-mirror2.htm#Close_objects_error
(http://www.telescope-optics.net/two-mirror2.htm#Close_objects_error)
Regards,
EB

[Satchmo]

For a final end star test for commissioning a telescope you just need a collimator telescope which produces a parallel light beam into the aperture of the telescope you are testing . Celestron used one of 22" aperture . In the 80's I made and tested hundreds of mirrors at Astro Optical Supplies by 'looking' into the collimated 14" F6 mirror of good quality which had a laser shining on a ball bearing at the focus of the collimator mirror . Also tested many Celestrons that came in, this way .

If you want to test a scopes off axis image quality , you just tilt the telescope to put the artificial star as far off axis as you want to look at ( and of course you could do all that at the focal plane of CCD if you wished ) . If you need to look at the polychormatic performance of the 'scope you just use a white light source .This is an optical testing technique that goes back to the 18th Century. If a manufacturer wants to rigorously test a compound telescope before delivery , this is the way to do it.

Hope this helps.