Heads Up - Pegasus, Planewave, and the Delta-T

Hi, 

This is interesting.  Thanks for sharing.

I am curious what's the disadvantage of the current Delta-T?  It constantly applies heating even when there is no threat for dew on the mirrors.  Is this a bad thing?  Personally, I wouldn't want the heater to be turned on or off in the middle of the imaging.  This would lead to rather sudden changes of mirror temperature and therefore a fast drift of focus in a short time period.  I would rather prefer the mirror temperature changing smoothly (with the change of ambient temperature), even if this means heating when there is no threat of dew.

Wei-Hao Wang:
Personally, I wouldn't want the heater to be turned on or off in the middle of the imaging.

Btw, I've seen the Delta-T do exactly this. Depending on the difference between ambient and primary or secondary temps, the Delta-T may turn the heaters on or off quite often, or ramp up and ramp down power often. And if you look at some of the graphs posted in the thread I linked to above, you'll see some pretty wild gyrations of mirror temperature--particularly with the secondary. 

The best solution would be to have the Delta-T be sensitive to the dew point, not just ambient and mirror temps. Planewave used to supply a piece of software that would help with this, but they've since withdrawn it (see https://www.astrobin.com/forum/post/64320/). 

But use or non-use of this feature is, of course, completely up to you. I'm just reporting what Pegasus is telling me, and I do indeed intend to use it.

Wei-Hao Wang:
Hi, 

This is interesting.  Thanks for sharing.

I am curious what's the disadvantage of the current Delta-T?  It constantly applies heating even when there is no threat for dew on the mirrors.  Is this a bad thing?  Personally, I wouldn't want the heater to be turned on or off in the middle of the imaging.  This would lead to rather sudden changes of mirror temperature and therefore a fast drift of focus in a short time period.  I would rather prefer the mirror temperature changing smoothly (with the change of ambient temperature), even if this means heating when there is no threat of dew.

Any temperature difference between the mirrors and the air will cause convection.  It does not take very much convection to introduce ~0.5 - 1.0 waves of wavefront error into the light path.  This is something that is VERY easy to see in interferometry.  Just putting your hand into a light path can introduce many waves of optical path difference due to the heat flow from your hand.

The primary driver of temperature difference is radiative cooling to the sky.  Since the secondary mirror points down, it generally remains at or near the air temperature so this is a problem that affects mostly the primary mirror.  Under the right conditions, the temperature of primary mirror can be as much as 5-6C below the air temp and if that’s less than the temp-dew point spread, you’ll get dew (or frost if it’s cold enough).  In general, the best way to run the Delta-T is to set it up to keep the primary mirror within ~0.5C of the ambient air temperature.  The big problem is that it’s hard to heat the mirror perfectly uniformly and to measure everything to within +/- 0.25C so this setting might not be sufficient to prevent dew should it occur.

My own approach is to minimize heat to the mirrors until it is actually needed to prevent dew.  I’ve found that the best way to insure that the optics are at the same temperature as the surrounding air is with properly arranged fans.  Convection prevents micro-convection at the surface of the mirror and helps to insure that the mirror surface stays at the same temperature as the air.  The trick is in “properly” arranging the fans.  Poor air flow can make the image worse; whereas proper air flow can make improve imaging.   I’ve seen both in my telescopes but I can’t give you hard rules for getting it right beyond experimenting.  I can say that the side fans on my ASA600 blow filtered air over the surface of the primary and that approach works extremely well.  The fans work so well that the telescope does not even have mirror heaters.  The rear fans on my CDK20 did not work well at all.  So YMMV.

John

It would be interesting to know what settings you wind up using on the UPB dew heaters once you get this configured!

Timothy Martin:
I may dump the CDK and go with AGO or ASA.

Maybe just ASA since AGO no longer makes scopes. I had a 10 inch AGO and collimation was probably the hardest of any scope I have had and I have had an OGS, two RCOS and the AGO as well as the present Planewave. I have to say that overall the Planewave has been the best although still could be better.

The sad  thing about PW is that they have a scope that is 90% there and would be 100% if they would just listen to what the customers are saying.

PlaneWave scopes do work well once you work out the gremlins. We host 4 different PlaneWave scopes and the experience with all four has been completely different. Our CDK14 has been flawless. The only work we have done was to collimate the scope with SkyWave. Our CDK20 was purchased from another user who did all of the work to get that system performing well, which John has plenty of documentation about his journey on. The fans still cannot be on during imaging at all, or we end up with triangle shaped stars. This is something we are exploring a custom solution for as we would at least like a little airflow to break up the boundary layer on the mirror. The scope does take excellent images though so this hasn’t been a high priority for us.

The DR500 takes a lot of work to collimate, adjust tilt, and we still see some optical flaring on very bright stars. We’ve explored a number of fixes for this and haven’t found the silver bullet quite yet. We aren’t alone with this issue though as other owners of DR’s in general report similar results in very good setting conditions on brighter stars.

The CDK700 is just flawless. Setup and collimated it, did some work to solve a minor modeling problem, but that was it. We’ve seen some interesting issues with derotation here and there that we’ve started looking into but we expect this system will just produce data like a champ.

If they could get the experience across their products to be consistently more like what we saw with the 14 and 700, that would be more ideal. I’ve also heard very good things about customers experiences with the CDK17.

Habib Sekha:
My Delta-T stopped working after two nights.

Stopped working or stopped communicating with the software and (therefore) cannot be set to an operating mode? I have found the USB connection is not reliable.

Timothy Martin:
. That's just poppycock. 12V is 12V. It's clear to me that this is a poorly designed product that is manufactured with little quality control using cheap components--but at a premium price.

Agreed. That would also explain why some die right away and others soldier on for a very long time. Poor quality parts often have highly variable quality as well with some being crap and others not too bad. It is a total crapshoot as to which one you will get.  

In any case, that is why us owners need aftermarket alternatives for as many of the systems as possible. Some are easy (like the focuser/rotator), others not so much. No telling how long PW will survive (just ask RCOS owners about that situation )

Just another example (albeit not a critical one) is the RA "toe" counterweight shaft that came with my L-350. Supposed to be stainless but actually rusted in less than 6 months. PW said to "send it back" and they would look at it (I had sent them the photo below) A good company would just send a new one and let you send the old one back once you got it.  Very clearly just very low grade cheap stainless. Photo is after I cleaned it with steel wool, it was much worse before and  clearly not what it should have been. Worse yet, it showed early signs of this when I got the mount - meaning it was shipped that way!  I may still send it back since I am not using it at this time.

It may seem like this is a "Bash Planewave" thread but I have also pointed out that overall the scope has been good and has given me some of the best images I have ever taken. Not all of that is the scope, of course, a lot is the site. 

The problem is that the system would be basically perfect with better attention to the issues that a large percent of customers have had and that PW is very well aware of. It is not like the Delta T issue is uncommon.  Add that to the nonsensical way that  PW has designed the thermal control and anyone can see the problem.

Willem Jan Drijfhout:

Timothy Martin:
Just FYI, Pegasus released the feature that allows separate aggressiveness settings for each dew port on the UPBv3. It requires you do apply both a software and a firmware update.

Thanks for the heads up Timothy. Do you know if this also works for the UPBv2?

Would be great to get rid of the Delta-T. A few weeks ago I lost communication with the Delta-T, for no apparent reason. After my initial Delta-T came dead in the box and had to be repaired/replaced as well as the many stories here on AB, I was afraid this would be the end of my second copy. Interestingly enough, life was brought back into it when it was plugged into a USB2 port of the UPBv2, rather than one of the USB3 ports it had been in so far. This should not matter, all USB3 ports on the UPBv2 are fully backward compatible. But hey, it worked, so no further questions asked.... The USB connection of the Delta-T seems very unreliable indeed. So for anyone having issues, you might want to try USB2 and see if it works.

Not sure how long this solution will survive, so would be happy to try any other solution.

Unfortunately, this only seems to work on the UPBv3.

Willem Jan Drijfhout:

Timothy Martin:
Unfortunately, this only seems to work on the UPBv3.

Ah, that's a bummer. Well, if it works, it is well worth the upgrade I presume.

We'll see. So far, it has worked pretty well at preventing dew, but I'm probably overheating right now and degrading the sub quality. We've only had a couple of nights where humidity has climbed since I sent the Delta-T off for repair. When it comes back, I'll be able to tune the aggressiveness setting for each mirror more precisely. Once that's done, I can report back my findings--with the caveat that it only applies to the CDK12 (and given Planewave's quality and component variability, it may only apply to my CDK12)

Bill McLaughlin:

Habib Sekha:
My Delta-T stopped working after two nights.

Stopped working or stopped communicating with the software and (therefore) cannot be set to an operating mode? I have found the USB connection is not reliable.

The power LED goes on, the computer can't detect it so no way to communicate with it. If it is an USB connectivity issue then it is from inside the Delta T box. I tried exchanging the USB cable, tried various USB connections on the UPBv3 and and the mini PC, and after reading Willem Jans's post I also made sure I tried some USB2 connections. Also tried another 12V adapter just in case the original one stopped supplying enough current.


I agreed with the distributor over here in Europe that they would send a new one (free of charges) out today.  They never heard of a Delta T malfunction before... yeah sure...!

Bill McLaughlin:
It may seem like this is a "Bash Planewave" thread but I have also pointed out that overall the scope has been good and has given me some of the best images I have ever taken.

So far I like the optics of the CDK-14 I received.


That rod in the picture does not look like stainless steel.

Willem Jan Drijfhout:
The USB connection of the Delta-T seems very unreliable indeed. So for anyone having issues, you might want to try USB2 and see if it works.=14px

Thank you for this suggestion Willem Jan! Always good to try this option in case of malfunction. I'm glad that with this 'workaround' you are able to use it.


I tried this, this morning...no luck. I should get a replacement within a few days.