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SCT (EdgeHD) vs CDK Considerations

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#1 chadrian84

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Posted 13 August 2019 - 04:42 PM

I'm thinking about moving on from my EdgeHD scopes to CDK.  In consideration are Planewave 12.5 and 14.  I currently have two 11" EdgeHD and one 8".  I'm frustrated with Edge design for imaging due to:

 

- Recollimation seems to be required every session.  Especially at sub .5" pixel scales and because I set up / tear down each night.

- I think mirrors and/or the corrector plate shift slightly throughout the night causing changes in star shapes.

- Stars suffer from color fringing.  Looking at the Edge spot diagrams I don't think this is surprising (excluding the 9.25").

- Difficulty cooling the tube to ambient.  I live in a humid area so I haven't used fans yet.

 

 

Some questions about CDK are:

 

1) Is collimation easier with a CDK than an SCT?  Is the elliptical CDK primary more forgiving than the spherical SCT primary - how much so?

2) Any issues with star shapes changing during the night or from session to session?

3) Have people found the advertised 52mm corrected circle of the Planewave CDKs to be accurate - any color fringing?

4) How well does the focuser and ASCOM software work?

5) Is the open-tube CDK 14 a bad choice for humid environments?

6) Have people been happy with the Delta T cooling and dew prevention system?

7) Is a dew shield recommended for the 12.5"? - I assume not.

 

Comments on differences between these designs are welcome, especially from those who've used both.  My AP1100 mounts should have no trouble handling the 12.5 or 14.

 



#2 rgsalinger

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Posted 13 August 2019 - 05:20 PM

I have a 12.5 now for 30 months. It was last collimated about 15 months ago (it really needed to be cleaned) and then went to New Mexico and back here to California just recently. It survived the 2000 mile round trip just fine. I don't tear down nightly but I did buy a custom made padded case for the scope. I also use (but do not own) a CDK14, so I'm familiar with both telescopes.

 

I love the whole EFA/focuser/heater/fan combination. First, it's everything that you need for imaging. It's all in one integrated GUI you control. Makes life really easy. Second, because it's all integrated it just works. I love the fact that I can control the heating and mirrors based on the temperature sensors built into my scope.

 

The fans do a good job of cooling the scope. How long it takes depends on the weather, etc. I don't start imaging until 90 minutes after sunset in any case.

 

The focuser is wonderful but slow and noisy. The focusing software gives me better focus (measured in PI, not by the focusing software) than anything else. Still, mine is just F8 so the whole thing is pretty tolerant of being slightly off. 

 

I use a QHY16200 camera on the 12.5. I get round stars corner to corner. That's enough for me. On the 14 we have a PLI16803 and the stars are the same (pretty much) from corner to corner. 

 

There's no such thing a s dew shield for any of these scopes - there's no corrector plate to dew up. I've never used a 14 in a humid environment but between the fans and the heaters controlled by the sensors I don't know why that would be inadequate.

 

I have never seen "changing star shapes" but seeing changes make subs better and worse as the night goes on like any other scope. 

 

The 12.5 has the advantage of being much easier to handle. The 14 is huge and needs two people to do anything with it. I use an MX+ with my 12.5 and it's entirely adequate. So I can't imagine that you would be stressing an AP1100 at all. The 14 is only 8 pounds heavier so that's not a big deal. You will probably need some tube weights - I use 6 pounds and the 14 I also have access to has them as well. (It's riding on an AP1600.)

 

I have to tell you that despite being ludicrously expensive, I feel my 12.5 was the best thing that I ever bought for astro-photography and the support has been excellent from PW.

 

Rgrds-Ross


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#3 chadrian84

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Posted 13 August 2019 - 06:10 PM

Thanks Ross.  I guess the 14 is out of the question if it requires two people.  Hopefully I can hoist the 12.5 myself.  I plan to use the SGP focus routine.  I'm surprised to hear it's slow and noisy, maybe a Nitecrawler is a better choice.



#4 rgsalinger

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Posted 13 August 2019 - 11:17 PM

The focuser not THAT slow but it is noisy when moving long. Too bad you can't use the focus routine that PW gives you. I wish that they used some camera control other than MaximDL. One other option for auto focus is to use Voyager which also does full frame focusing. I'm experimenting with that tonight using a CDK14 that I manage for some friends. 

 

I think that the 14 has to be for someone who's got a permanent situation. Even the 12.5 is tricky if the pier is too high. I have two piers and I can get it on the lower one myself but I cannot get it on the higher of the two. Now if you had one of those saddles (Robin Cassady used to make them.) which allow you to just place the scope on the pier, you might well be OK. Both of my mounts have these beveled brass rods that attach to the OTA dovetail so I have to slide it in



#5 chadrian84

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Posted 14 August 2019 - 01:18 AM

The only control software I'm familiar with is SGP.  I've found the focus routine effective with room for improvement.  It sounds like Planewave has their own software package which I don't think I'm interested in.

 

Why do you need tube weights?  Are you not able to fit the tube on the saddle and get dec balanced without one, or do you use it for torque reasons?



#6 rgsalinger

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Posted 14 August 2019 - 09:44 AM

I used the weights for both reasons. I didn't like how much the tube extended from the saddle of the Versaplate. I tried to use more weight to get an overall shorter moment arm. The idea was to see if guiding improved. It didn't so I've reverted to less weight. I needed some weight to keep it in contact with all three of the brass plungers that both of my mounts use. Your saddle may be different.

 

I've never used SGP. So I can't be sure it's autofocus is inferior. However, the full field focusing gives me better results than I used to get with MaximDL and the SKYX autofocus. The results are on a par with Focusmax, but require no training runs or tweaking at all. The differences I measured in Pixinsight were around .3 arc seconds of FWHM pretty consistently. YMMV of course.

 

Rgrds-Ross



#7 Escher

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Posted 14 August 2019 - 02:24 PM

 

 

- Recollimation seems to be required every session.  Especially at sub .5" pixel scales and because I set up / tear down each night.

 

 

Just curious - Have you considered getting a Dome or Roll-Off instead of setting up each time? 

 

I know this only addresses some of the issues you mentioned, but the concept of being able to be up and running in 5 minutes is pretty sweet.  I love my POD... I just don't use it nearly enough.



#8 chadrian84

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Posted 14 August 2019 - 03:11 PM

I've considered it.  I'm currently in moderate LP, have a lot of clouds, and don't have a good place in my yard.  In five years I may be living in a dark location in AZ. 

 

I currently set up and tear down three AP rigs each night.  I usually don't mind.  It keeps me in harmony with the technical side of the hobby.  What drives me crazy is astro-dark rolling around and frantically trying to get my SCTs aligned - even though they were fine the previous outing.


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#9 Jeffmar

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Posted 14 August 2019 - 05:25 PM

It could be that your collimation issues are simply a result of the secondary screws not being tight enough. Long ago I changed to Bob's Knobs and have had good success with that. The only caveat to the knobs are tightening them up enough. Which ever way to do it you have to get the screws tight enough that your adjustments will last for a while. I recently drove around on bumpy dirt roads, for a few days, through the wilds of central Utah with my C11 packed in a Pelican case lined with foam. When I set up my scope again the collimation still looked good so I didn't have to touch it. I haven't needed to collimate either of my sct scopes for about 4 months.

 

I haven't heard of corrector plate shift but all you have to do is check the front of your scope to know if the plate is loose. There are also mirror locks on Edge scopes to prevent the primary mirror from moving. 

 

Just about any large scope is going to take some time to cool to ambient temperatures. You can either let the scope set in the cool temperatures for an hour before you use it or put a fan on one of the vents which is what I did. It speeds up cooling significantly.


Edited by Jeffmar, 15 August 2019 - 12:28 PM.


#10 chadrian84

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Posted 14 August 2019 - 07:56 PM

I've stripped the cheap factory collimation screws on two scopes, tried Bob's Knobs on all scopes, and currently use hardware store hex key screws.  I keep them tight and I haven't found much difference in any of the screw types.  All my scopes arrived from the factory clearly miscollimated and none of them with the corrector perfectly centered over the baffle.  The Hotech Advanced Collimator procedure likely made things worse as it instructs you to adjust the factory corrector plate positioning as well as alignment of the Moonlite focusers.  After screwing up my Moonlites via the Hotech instructions, I managed to recollimated/align them after purchasing a Howie Glatter laser.  I've used various methods with the Howie Glatter (and less so with the Hotech) to get the corrector plates perfectly centered over the baffle tubes.  I've had to remove some of the the factory foam corrector plate shims in order to properly center the corrector plates.

 

My current advice is to never adjust the four corrector plate retention screws or corrector plate positioning.  Keep it the way the factory set it, even if it's not centered.  Even though there are red factory corrector plate position markings, it's still a guessing game getting the corrector plate back exactly how the factory had it.  Presumably the factory decided their position was best even if uncentered.

 

With that said, I just went ahead and ordered my third Edge 11.  I won't touch the corrector plate on this one and will compare it to the others.  I've gotten great deals on all my Edge scopes, so I'm still under the cost of a single 12.5" CDK.  The CDK may be where I end up but I've decided to not give up on Celestron yet.  Even with an expensive CDK, there doesn't seem to be much guarantee of fewer problems than with an Edge.  I can still eek out photos I'm happy with, but the optical inconsistency of mishapen stars and miscollimation has been a pain.  I don't think there's much I can do about the star color fringing I've mentioned though, except for processing tricks.



#11 rgsalinger

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Posted 14 August 2019 - 08:54 PM

Well, I disagree that a CDK has not much of a guarantee of being problem free. I use three of them these days and the only one that needs a collimation touch up is over 4 years old. When you say expensive please bear in mind that you're getting a number of things with the telescope that you cannot get of will need to pay for if you want them in a C14. Price out thermostatically controlled fans and heaters, a focuser with zero backlash and one micron accuracy, a carbon fiber tube, a fixed mirror and F8 aperture without a reducer and you may as I did realize that the price difference between a C14 Edge and a CDK 12.5 is really not so much. On the other hand it's really hard to pass up a C11 at 3000 bucks. 

 

I really wonder about trying to collimate any C11 early in the evening. I think it's just asking for trouble. Where I image, the air settles down, the seeing improves and the mirror gets to ambient.Everything that I've read about collimating an SCT says that you need all those factors controlled or it's just not going to work. 

 

Rgrds-Ross


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#12 sink45ny

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Posted 14 August 2019 - 10:26 PM

I really really like my CDK17. Collimation, what's that?



#13 Astrojedi

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Posted 15 August 2019 - 09:53 AM

I've stripped the cheap factory collimation screws on two scopes, tried Bob's Knobs on all scopes, and currently use hardware store hex key screws.  I keep them tight and I haven't found much difference in any of the screw types.  All my scopes arrived from the factory clearly miscollimated and none of them with the corrector perfectly centered over the baffle.  The Hotech Advanced Collimator procedure likely made things worse as it instructs you to adjust the factory corrector plate positioning as well as alignment of the Moonlite focusers.  After screwing up my Moonlites via the Hotech instructions, I managed to recollimated/align them after purchasing a Howie Glatter laser.  I've used various methods with the Howie Glatter (and less so with the Hotech) to get the corrector plates perfectly centered over the baffle tubes.  I've had to remove some of the the factory foam corrector plate shims in order to properly center the corrector plates.

 

My current advice is to never adjust the four corrector plate retention screws or corrector plate positioning.  Keep it the way the factory set it, even if it's not centered.  Even though there are red factory corrector plate position markings, it's still a guessing game getting the corrector plate back exactly how the factory had it.  Presumably the factory decided their position was best even if uncentered.

 

With that said, I just went ahead and ordered my third Edge 11.  I won't touch the corrector plate on this one and will compare it to the others.  I've gotten great deals on all my Edge scopes, so I'm still under the cost of a single 12.5" CDK.  The CDK may be where I end up but I've decided to not give up on Celestron yet.  Even with an expensive CDK, there doesn't seem to be much guarantee of fewer problems than with an Edge.  I can still eek out photos I'm happy with, but the optical inconsistency of mishapen stars and miscollimation has been a pain.  I don't think there's much I can do about the star color fringing I've mentioned though, except for processing tricks.

The Hotech does more harm than good. The factory alignment is usually the best that you can achieve with that OTAs mechanical tolerances. Best not to mess with it.

 

I am also surprised your EdgeHD is not holding collimation. I have 3 and they all hold critical collimation for years. Have you tried sending it to Celestron for service? They did a great job on EdgeHD 8.

 

I have not personally owned a CDK but have had a chance to image with a 12.5. The Planewave ones are superb for imaging. If permanently mounted collimation seems to hold well from what I am told.  But imo need a permanent home otherwise they will not be as enjoyable.


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