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Thermal Sensitivity of Casses, SCTs, and the C14 Edge HD

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#1 jhayes_tucson

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Posted 10 May 2018 - 11:52 AM

Introduction
A while ago in this thread (https://www.cloudyni...in-mirror-slop/) I posted some charts showing the thermal sensitivity of some telescopes along with the equation for computing the thermal sensitivity of a Cassegrain telescope with zero expansion glass and an aluminum structure.  I’ll repost them below so that you can review them here.  The expression for a Cassegrain was derived using first order optics and based on an educated guess, I stated that the result should be reasonably close to what you might expect from a C14 SCT…and the Edge systems as well.

 

In that discussion, a question was raised about how much of a difference having non-zero expansion mirrors might actually make and while I hadn’t actually done the calculation, I promised to try to answer that question.  It was certainly a valid question that was worthy of a bit more investigation.  Unfortunately, it took me longer to get around to it than I expected, but I finally got it done.  So, the purpose of this post is to share the results.

 

One of the things that I did with this computation was to use the first order parameters for the C14 Edge telescope.  The first order properties can be derived with good accuracy from published data as well as from direct optical measurements that I’ve made on my own of C14 Edge components (to tighten up the accuracy a bit.)  However, because of my work with Celestron, I also happen to have access to proprietary optical design information for the C14 Edge system, which means that I cannot share any of the optical parameters that went into these results.  The reason that I took this approach was to ultimately be able to make a direct comparison of the simple models to the C14 Edge as a real product.  The good news is that I can describe what I did and how I checked it to reduce (or eliminate) chances of error in the results, which are presented in the table below.   (I've had to put the chart in the next post because of the attachment size limits.)

 

Thermal Sensitivity of a Cassegrain Telescope
The first row of numbers shows the amount of image shift per degree C that you can expect with a Cassegrain telescope using the same 1st order properties as the C14 Edge.  This model assumes that the support structure is aluminum and that the mirrors have zero CTE (such as ULE substrate material.)  The DOF is the depth of focus in microns (also called the critical focus zone.)  The DOF is computed using the Airy criteria, which is given by 2.5 (F/#)^2 at a wavelength of 512 nm.  This criterion is a bit less restrictive than the Rayleigh limit, which restricts the P-V wavefront error to be less than ¼ wave over the pupil to maintain a diffraction-limited image.  In the chart, the next column labeled C/DOF (C ,) is the temperature change (in C) required to shift the focus by the full DOF.  The +/- Tol ( C) is the temperature sensitivity of the system, which is the temperature tolerance required to hold focus within the DOF.   This is simply half the value of the previous column.  The larger this number, the less sensitive the system is to thermal variations.  These results are all shown to two decimal places (which is unnecessarily precise) simply to better compare results.

 

The second row serves as a cross check.  I entered the optical prescription for the Cassegrain system into OSLO (edu) and performed a simple ray trace to confirm the numbers given by the first-order model.  You might notice that there is a small difference between the ray trace and the model, which is due to the fact that very small variations in the prescription numbers (like in the 6th place—and beyond) will cause small rounding errors in the result.  Ray tracing relies on very small angles and double precision numbers to achieve accuracy so these small variations are hard to avoid without using double precision numbers everywhere—and I didn’t do that.  Since this is a straight Cassegrain system, I used paraxial focus to look at the focus shift, which should agree with the model and as you can see, the agreement is nearly perfect.

 

The third column shows the results for an all-aluminum telescope and it serves as another sanity check.  In the previous discussion, it was pointed out that an all-aluminum telescope should be perfectly athermal and that’s correct.  I ray-traced this model with the correct CTE numbers to demonstrate that I was able to get the correct result of zero (to within a small rounding error.) 

 

The fourth column gets more interesting because it shows the same Cassegrain telescope with borosilicate mirrors—the same material used in most commercial SCTs.  As you can see, using non-zero expansion glass in the telescope does indeed help it to be a little bit less sensitive; but, not by enough to make a meaningful difference in practical terms.  Still, increasing the CTE in the mirrors helps a little bit with thermal sensitivity.  (NOTE:  There are other, more important, reasons to make mirrors out of very low expansion glass so don’t take this as a reason to ignore the benefits of using mirrors with fused silica or ULE substrates!)

 

Thermal Sensitivity of the C14 Edge Telescope
The data in the last column shows the sensitivity of the C14 Edge telescope and it was the most challenging to compute.  OSLO (edu) does not have the capability of introducing thermal variations into a design.  So I had to carefully cut and paste full precision numbers between the design program and Excel where I could accurately compute both the change in dimensions using the CTE for all of the aluminum parts and glass types, as well as the change in index due to the dn/dT characteristics of the glass elements.  Then I had to carefully cut and paste the results back into the OSLO to do the ray trace.  There’s plenty of opportunity to bungle the numbers in this process, so I did the calculation twice, independently of each other, to insure that I got the same results.

 

In this case, using paraxial focus makes no sense, so I used the monochromatic minimum RMS spot size to determine the best focus position at 587.56 nm.  The reason that I couldn’t use the polychromatic solution is that OSLO (edu) won’t easily handle the change in dispersion with temperature for the refractive components.  This restriction will not affect the result in any meaningful way.

 

As you can see the temperature sensitivity of the C14 Edge is very close to the sensitivity of the equivalent straight Cassegrain with borosilicate mirrors (within about 1%.)  This result shows that holding the temperature to within about +/- 0.5 degrees will insure that any Cassegrain or SCT with the same first order properties will remain in focus as estimated by the original model.

 

Discussion
These numbers show how much the focus will change relative to the sensor over a temperature change of one degree C.  They also show the temperature sensitivity of the telescope, which is a measure of how much the temperature must change before the telescope goes out of focus under ideal conditions.  Remember that this computation relates to the difference between two identical telescopes at different temperatures while both are in thermal equilibrium.  That means that when the ambient temperature changes, it may take a while before all of the components are again at equilibrium.  In general, the external aluminum tube and mirror cells will change temperature more rapidly than the internal mirrors simply because the external parts are exposed directly to the outside sky and ambient air.  That means that the focus shift may not be a simple linear model during rapid temperature shifts.  It is certainly possible to model focus shift characteristics due to dynamic thermal changes, but that’s beyond the scope of this study.

 

Of course, the amount of focus drift will be altered if part of the telescope structure is electrically heated for any reason.  If you heat the tube, that will alter the temperature change of the structure; however, it’s important to recognize that nothing should be heated whenever the temperature-dew point spread is larger than about 5-6 degrees C.  Heat generates thermals and it is better to turn off all heat when there is no danger of dew.  Remember that these results show the effect of temperature change on the telescope without any heaters running.

 

Atmospheric seeing conditions will also affect the sensitivity of a telescope to changes in temperature.  This is because poor seeing effectively increases the blur diameter of the integrated star image on the sensor.  When that happens, the size of a star image is driven more by changes in seeing than by changes in focus so it takes more of a focus shift to be noticeable.  The last three rows of the chart show the effect of seeing on the C14.  When the seeing blur diameter is on the order of an arc-second, the C14 will have a temperature sensitivity tolerance of  +/- 0.62C; however, that tolerance rapidly grows to +/- 2.5C when the seeing deteriorates to a 2 arc-second blur size.  Many folks report that they see very little focus change over the course of an imaging session with their SCT and this is most likely the reason.   Combine mediocre seeing with a stable environment and even a C14 system will appear to produce well focused images without a lot of refocusing.   Smaller SCTs will be even less sensitive under similar conditions.  Of course this is true for most telescopes under similar conditions.

 

Understand that the sensitivity of most computer controlled automated focusing systems will also vary with the quality of the seeing conditions.  This is true for V-curve focusing as well for astigmatic focusing methods.  Since the data used for focusing is typically taken with relatively short exposures, most of these methods can achieve and hold focus to minimize star size well within the limits of the long-term local seeing conditions.

 

The important take away (and the point that I was trying to make previously) is that SCTs are quite sensitive to thermal variation relative to other telescope types—as shown in the chart.  When the seeing is good, a C14 is very sensitive to temperature variations—and it is particularly noticeable through large changes in the ambient temperature during an evening.  I used to record the temperature every night when I operated my telescope locally and it was not uncommon to see change rates of nearly 4C/hr as shown in the attached chart.  Under these conditions, I learned that my C14s had to be refocused at least every 10-15 minutes to achieve good results, which roughly matches the predicted thermal sensitivity of the system.  Since my typical exposure time is 20 minutes, I use a focusing system that holds focus in real-time while the shutter is open.  Remember that smaller telescopes will be less sensitive than the C14 that was used as an example here.  As we've seen, the results for the simple model are reasonably close to the real telescope so the model can be used to estimate the sensitivity of smaller scopes with reasonable accuracy.

 

There is no doubt that some users may be able to run for hours with very little focus change; but, that’s only going to occur under relatively stable thermal conditions and/or under relatively poor seeing conditions.

 

 

John

Attached Thumbnails

  • Figure 1 - Cassegrain Sensitivity.jpg
  • Figure 2- Telescope Sensitivity List.jpg

Edited by jhayes_tucson, 10 May 2018 - 11:55 AM.

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#2 jhayes_tucson

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Posted 10 May 2018 - 11:53 AM

Here is the chart showing the results along with some temperature data from a night of imaging here in the high desert of central Oregon.

 

John

Attached Thumbnails

  • DoF for Cass and C14 Edge 2 -sm.jpg
  • Figure 4 - Temp Change Chart - sm.jpg

Edited by jhayes_tucson, 10 May 2018 - 12:08 PM.

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#3 Edd Weninger

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Posted 10 May 2018 - 05:22 PM

Thanks for this.  Looks like an enjoyable evening read.  



#4 Codbear

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Posted 10 May 2018 - 06:52 PM

John,

 

As a new owner of a new Celestron 14 Edge, I found your post a fascinating and very relevant read. I experienced some of those Oregon high desert temperature deltas when I was in Madras last August for the eclipse, but was prepared for the drop when I slept outside on my zero gravity chair the night before!

 

In your "Temp Sensitivity Comparison" chart, you have the 14" Edge listed in the section that is most resilient to temperature changes with the description of "scopes using CF." I could not find anywhere in your analysis what CF stood for, but I would assume that since the 14" (as well as other scopes) are also listed in sections without CF that it stands for "cooling fans?"

 

If this is indeed the case, is there a specific type of cooling fan or fans that make the 14" Edge so much more capable of temperature changes, such as Tempest and/or Starizona cooling fans?



#5 jhayes_tucson

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Posted 10 May 2018 - 07:22 PM

Sorry...CF stands for carbon fiber.  In that calculation, I used the numbers for a properly made CF tube structure.  Unfortunately, virtually no one supplying replacement CF tubes for the C14 knows how to properly make the tube so the CTE is not very low--and is not linear.  CF can be really good but you have to use the right material and the weave has to be properly oriented to reduce the CTE significantly.  Unfortunately, the folks at Public Missiles (where I got my tube) only know how to get the dimensions right and how to make it very pretty; but, they don't supply a tube that has low CTE.  So all "real-world" C14s are going to be pretty sensitive to temperature changes.

 

John



#6 Codbear

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Posted 10 May 2018 - 09:41 PM

Ok thanks for clearing that up John...I was hoping it was something as simple as Cooling Fans but alas not to be. 

 

I am visual only and am in a temperate climate about half an hour north of San Francisco. As an example, tonight between 8:00 and 11:30 the temperature only drops a net 2.75 C.  I have an outside telescope room that has french doors that open the room up to the outside, as well as air conditioning in the room.  

 

If I may ask John, which - or what combination - of the following procedures would you recommend to bring the 14" Edge to initial ambient temperature?:

 

1. Open french doors to the outside

2. Use Tempest and/or Starizona fans

3. Use the air conditioning in the room.

 

I will have the 14" on JMI Wheeleys and will roll it out 10 ft to my observing area.

 

Thank you,

 

Sam



#7 jhayes_tucson

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Posted 10 May 2018 - 10:18 PM

Sam,

This study is really aimed at folks doing imaging where focus requirements are critical. For visual use, none of this stuff will matter very much because your eyes are so good at accommodation.  I used to run my scope as a roll out observatory and here is what I found to be the most effective.  Roll the scope out as soon as the sun went down.  Run tempest fans continuously.  By the time it got really dark (after maybe an hour) the scope would usually be stable enough to start an imaging run.  Sometimes I'd have to wait for another half hour but I'd just toss subs if they weren't any good at the beginning of a run.

 

John



#8 freestar8n

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Posted 11 May 2018 - 12:28 AM

First - I don't understand why people keep referring to EdgeHD scopes as "C14".  They are distinct product families.  The naming can be seen on the celestron page here:  https://www.celestro...s/optical-tubes  When EdgeHD came out - it was given a completely different name to distinguish it from the C family.

 

"C14 Edge HD" is a total of 11 characters - while the actual name is simply "EdgeHD 14" at 9 letters - and can be shortened to EdgeHD14.

 

The RASA also isn't in the C family - and you wouldn't say "C14 RASA".  

 

OK - regarding the simulation results.  As I have said many times - a simulation is merely a presentation of how you think a system behaves - but you have no idea if it is correct until you verify it empirically.  In this case I don't think there is any need to do so - because its predictions are clearly exaggerated when it comes to sct's - in a general sense anyway.

 

So - when a model does not match experiment - that can lead to new insights into what is missing from the model.  And I have already described a number of fairly obvious factors overlooked.

 

My counter-example of a pure aluminum telescope was simply meant to point out that other factors, such as optical changes with temperature - are being ignored.  But that was just the start.  The main things missing are:

 

1)  Dew strap and associated temperature gradients

2)  Thermal history of the system and how it was exposed to the sky

3)  Radiative coupling to the fairly constant temperature sky and ground

4)  Thermal mass of the tripod and eq. head

5)  Thermal environment of the area surrounding the scope, such as a line of trees, a wall, observatory structure.

 

The model seems to assume the entire structure is slaved to the ambient temperature - and that is obviously not the case.  And much of the equilibrium will be radiative - with the sky and ground fairly constant through the night and independent of what ambient temperature is doing.

 

The main thing is - this simulation is done without empirical verification that it actually has predictive value - and without that verification there is no reason to assume it is realistic.  And given all the counter examples of small fwhm images with occasional re-focusing - I think it is safe to say it is not realistic.

 

I don't know exactly what it is missing - but as a step I think it needs to be done with a very detailed finite element model to capture temperature variations throughout the system - along with physically accurate radiative couplings - and *a dew strap*.  I mean - this simulation assumes no dew strap at all - yet almost all of us use them - and just below the corrector.  That is quite an omission in a thermal study.

 

Frank



#9 Codbear

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Posted 11 May 2018 - 01:00 AM

Sam,

This study is really aimed at folks doing imaging where focus requirements are critical. For visual use, none of this stuff will matter very much because your eyes are so good at accommodation.  I used to run my scope as a roll out observatory and here is what I found to be the most effective.  Roll the scope out as soon as the sun went down.  Run tempest fans continuously.  By the time it got really dark (after maybe an hour) the scope would usually be stable enough to start an imaging run.  Sometimes I'd have to wait for another half hour but I'd just toss subs if they weren't any good at the beginning of a run.

 

John

John,

 

Thanks so much for your guidance...that really helps me focus in on what I need to do to be able to bring my 14" to ambient. I am awaiting delivery of my Tempest fans I have already ordered and am contemplating permanently pier mounting the 14" Edge and using a Telegizmo 365 to keep it covered. 

 

Out of curiosity, did you use a homemade set of wheels or something akin to JMIs to roll your scope out and was it an SCT?

 

I posted a question about the stability of an Edge 14" mounted on an AP1100 with a Losmandy Folding HD Tripod on a set of Large Universal JMIs in the "Mounts" section and got zero responses, so either my question was too focused or CN is full of lawyers who didn't want to incriminate themselves!lol.gif



#10 jhayes_tucson

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Posted 11 May 2018 - 01:07 AM

Frank,

 

1)  You are nit-picking.  If you read through this, you'll see that it doesn't matter whether it's a C14 XLT (which is basically just a Cassegrain configuration) or a C14 Edge HD, which I showed has a sensitivity within 1% of the straight Cassegrain.  Are you fussing over that 1% or are you just fussing?

 

2)  If you read through this, you'll see that I discussed the issue of dew heaters.  They are irrelevant when they are unpowered, which they should be most of the time.   The question is how much does the focus change with temperature?  Yes, dew heaters might change things a bit but that's not relevant.  If we wanted to hold the entire scope at constant focus, we could wrap the whole thing in a continuous shroud of heaters but then this calculation tells you how carefully you would have to hold the temperature to keep the system within focus.

 

3)  I agree that radiative cooling is a big deal and that's why I did this study.  When things cool off for any reason (conductive, radiative, etc.,) the focus will change.  What does the thermal mass of the mount have anything to do with it?  If you read what I wrote, the results are for systems in thermal equilibrium.  I don't know about your system, but my mount is made of aluminum so it will have zero effect on the system in equilibrium.

 

4)  You do not have to do a finite element analysis of the system to predict the change in focus.  The thermal environment only determines how much of a temperature change you might expect.  If you read what I wrote, you'll see that I indicated that if the temperature of the OTA isn't changing by very much, then the scope won't change focus by very much.

 

5)  I don't get your resistance to these conclusions.  Do you believe in engineering?  You seem to be throwing out irrelevant objections.  You haven't done any calculations to show that I'm wrong, and you haven't presented any data whatsoever.   Go take your telescope and put it in a temperature controlled room and record the change in focus as a function of temperature.  If you design the experiment correctly, maybe you can get some data that contradicts the theory and then we'll have to go figure out why.  Otherwise, don't give me a bunch of anecdotal stories about how often you have to refocus when you have your telescope outside and you haven't recorded either the temperature or the seeing values.  If you can get away with infrequent focusing with your system, that's great but it says nothing--absolutely zero about the thermal sensitivity of your system.

 

 

John


Edited by jhayes_tucson, 11 May 2018 - 05:09 PM.


#11 jhayes_tucson

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Posted 11 May 2018 - 01:19 AM

John,

 

Thanks so much for your guidance...that really helps me focus in on what I need to do to be able to bring my 14" to ambient. I am awaiting delivery of my Tempest fans I have already ordered and am contemplating permanently pier mounting the 14" Edge and using a Telegizmo 365 to keep it covered. 

 

Out of curiosity, did you use a homemade set of wheels or something akin to JMIs to roll your scope out and was it an SCT?

 

I posted a question about the stability of an Edge 14" mounted on an AP1100 with a Losmandy Folding HD Tripod on a set of Large Universal JMIs in the "Mounts" section and got zero responses, so either my question was too focused or CN is full of lawyers who didn't want to incriminate themselves!lol.gif

 

I have a customer pier.  I've posted pictures of it all over the place but I can't find a single one so I'll attach one here.  The AP1100 is a nice mount for the 1100 but I personally wouldn't want it on a tripod unless the tripod were really stout.  I run my C14 on an AP16000.  This is an old photo but it shows the pier pretty well.

 

John

 

 

Certain folks around here get grumpy if the topic wanders too much so we'll have to start another thread if you want to discuss this any further.

Attached Thumbnails

  • C14 Edge on Mount V2 - sm.jpg

Edited by jhayes_tucson, 11 May 2018 - 01:21 AM.

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#12 freestar8n

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Posted 11 May 2018 - 02:06 AM

John - I leave my dew heater on all the time at constant medium power.  It's a Kendrick dew heater - and many people use them.  So again you are making assumptions that do not match actual common usage.  You claim that heating is bad - but is it?  Even if you aren't forming dew - it will balance the radiative cooling in a way that could well be thermally beneficial.  You are simply assuming it is bad - yet it is how I operate and I get very good results with little focus drift.  The thing you say you shouldn't do - may in fact be the thing that is causing your focus to drift.

 

I do long exposure deep sky imaging all night with the dew heater on - and I periodically collimate and capture a nice clean Airy pattern - indicating good collimation and diffraction limited performance - with no indication of thermal trouble.  I don't use fans or anything else at all.

 

Your subject refers to a telescope called "the C14 Edge HD" - and there is no such thing.  It is not a critical point - but it is odd that you and others want to refer to it in a way that is incorrect and requires extra characters.  This causes genuine confusion because I see people refer to their "C14" and in fact it is EdgeHD.  C14 is one thing - EdgeHD 14 is another.  Simple, clear, fewer characters - and follows the branding.  No need for a space between Edge and HD - because there isn't one.

 

The main thing you don't seem to appreciate is that your conclusions do not match my experience at all - after imaging with a variety of sct's around the world.  The assumptions of your model do not match typical usage - and the predictions do not match actual focusing behavior.  I am not nit picking at all about this.  I am saying you claim the sky is falling - and it isn't.  Your order of telescope focusing needs may in fact be upside down - and I wouldn't be surprised if long refractors in fact require much more re-focusing than an sct.  It's a very simple model with very simple assumptions - and when something so complicated is modeled in such a simple way - it is crazy to me to assume it has predictive value without experimentally verifying it.

 

Your picture shows a telescope completely exposed to a cold sky - and I guess you are minimizing any dew heating.  You may also be rolling it out not long before you image.  Those are all issues of setup and technique that would result in much more focus drift than necessary.  If the scope is somewhat sheltered - as it is for many backyard setups - and if it is left outside and covered - there will be much less drift - and none of these details is factored into your model.   Is it really on concrete and asphalt?  If you are concerned about tube currents affecting seeing - I would seek a different location first - e.g. a grassy backyard with trees around.

 

Frank



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Posted 11 May 2018 - 08:49 AM

Interesting data. But surely, bad conditions and thermal issues are more than just a problem of defocus, right? Is there any data correlating temperature differences to wavefront error, or a simple measurement of how fast a certain OTA can cool?

 

I also found it surprising that bad seeing results in a wider depth of focus. Does this just mean you're not going to achieve a perfect focus anyway, so that lower level of focus is the best you can do? Like, it's easier to shoot for a C+ than an A+?



#14 Edd Weninger

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Posted 11 May 2018 - 12:48 PM

John,

 

A temperature drop of 9C from about 11:00pm to about 5:00am !!  You need to move to northern Arizona or do something to adjust focus.  I'll have to look for some data, but on clear nights here, I'm used to seeing our temperature drop steeply from sundown to about 10:30, then drop very little until sunrise.  For instance, last night, IIRC, temperature dropped from about 77F at sunset to about 58F at 11:00pm.  Then from 58F to about 56F just before sunrise.  We're quite high up here.  I wonder if that is a factor.

 

Nice mobile tripod.  Custom design?  Taxiway in the background?  

 

 

Here is the chart showing the results along with some temperature data from a night of imaging here in the high desert of central Oregon.

 

John


Edited by Edd Weninger, 11 May 2018 - 12:50 PM.


#15 gomezdb

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Posted 11 May 2018 - 01:05 PM

popcorn.gif popcorn.gif


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#16 jhayes_tucson

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Posted 11 May 2018 - 02:51 PM

John - I leave my dew heater on all the time at constant medium power.  It's a Kendrick dew heater - and many people use them.  So again you are making assumptions that do not match actual common usage.  You claim that heating is bad - but is it?  Even if you aren't forming dew - it will balance the radiative cooling in a way that could well be thermally beneficial.  You are simply assuming it is bad - yet it is how I operate and I get very good results with little focus drift.  The thing you say you shouldn't do - may in fact be the thing that is causing your focus to drift.

 

I do long exposure deep sky imaging all night with the dew heater on - and I periodically collimate and capture a nice clean Airy pattern - indicating good collimation and diffraction limited performance - with no indication of thermal trouble.  I don't use fans or anything else at all.

 

Your subject refers to a telescope called "the C14 Edge HD" - and there is no such thing.  It is not a critical point - but it is odd that you and others want to refer to it in a way that is incorrect and requires extra characters.  This causes genuine confusion because I see people refer to their "C14" and in fact it is EdgeHD.  C14 is one thing - EdgeHD 14 is another.  Simple, clear, fewer characters - and follows the branding.  No need for a space between Edge and HD - because there isn't one.

 

The main thing you don't seem to appreciate is that your conclusions do not match my experience at all - after imaging with a variety of sct's around the world.  The assumptions of your model do not match typical usage - and the predictions do not match actual focusing behavior.  I am not nit picking at all about this.  I am saying you claim the sky is falling - and it isn't.  Your order of telescope focusing needs may in fact be upside down - and I wouldn't be surprised if long refractors in fact require much more re-focusing than an sct.  It's a very simple model with very simple assumptions - and when something so complicated is modeled in such a simple way - it is crazy to me to assume it has predictive value without experimentally verifying it.

 

Your picture shows a telescope completely exposed to a cold sky - and I guess you are minimizing any dew heating.  You may also be rolling it out not long before you image.  Those are all issues of setup and technique that would result in much more focus drift than necessary.  If the scope is somewhat sheltered - as it is for many backyard setups - and if it is left outside and covered - there will be much less drift - and none of these details is factored into your model.   Is it really on concrete and asphalt?  If you are concerned about tube currents affecting seeing - I would seek a different location first - e.g. a grassy backyard with trees around.

 

Frank

 

Frank,

Huh?  Where did I claim that the sky is falling?  All I've done is to compute the temperature sensitivity of various telescopes relative to focus.  The specific data for the TEC oil-spaced refractors actually came from a calculation done by Yuri Petrunin at TEC.  Just because you run a heater all the time doesn't tell you anything.  How much of the structure is being heated?  Is it being heated to a constant temperature or a constant temperature difference relative to ambient?   Do you have any data showing how much focus changes relative to ambient conditions with your heater running vs without the heater?  Whether or not the telescope is sheltered or on concrete or asphalt is completely irrelevant.  All that matters is the temperature change at the telescope.  That's what causes the focus to change.  Your statement that, "...your conclusions do not match my experience at all..." isn't a data point.  If you think that my numbers are all wrong then gather some good data to make your point.  Put your telescope in a temperature controlled room, set it up in double pass, and record the focus change as a function of temperature.  I'll bet you $100 that the results will come out to agree with my predictions to within the measurement accuracy of the experiment.

 

As for dew heaters, I've long thought that none of the anti-dew controllers work the right way.  I use a Dew Buster and it might work like your Kendrick system to hold a constant temperature differential relative to the ambient temperature.  That's a valid approach to combating dew but it makes no sense to run it when there is no threat of dew.  I built an Arduino controller on my system that powers the Dew Buster only when the temperature-dew point spread is below six degrees.  So my dew control system is only powered for maybe 6-10 nights out of the year when dew or frost might be a concern.  It may certainly be possible to heat the system without creating significant air currents but I know or sure that there aren't any air currents being produced when the system is turned off.

 

Finally, perhaps you didn't read what I said about that picture.  It is an old photo.  My scope is now located in an observatory in New Mexico--that also has a concrete floor.  Regardless of location, I've found that when the temperature changed significantly throughout the night, I had to refocus quite frequently--like on the order of every ~15 minutes.  That's not a data point but it jives with the sensitivity that I've computed for the telescope.

 

John


Edited by jhayes_tucson, 11 May 2018 - 05:18 PM.


#17 jhayes_tucson

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Posted 11 May 2018 - 03:15 PM

Interesting data. But surely, bad conditions and thermal issues are more than just a problem of defocus, right? Is there any data correlating temperature differences to wavefront error, or a simple measurement of how fast a certain OTA can cool?

 

I also found it surprising that bad seeing results in a wider depth of focus. Does this just mean you're not going to achieve a perfect focus anyway, so that lower level of focus is the best you can do? Like, it's easier to shoot for a C+ than an A+?

If the system is in thermal equilibrium, there shouldn't be any issues with wavefront errors.  There are a lot of things that you might try to measure but I was only looking at focus sensitivity.

 

Remember that the depth of focus (also called the critical focus zone) is the amount of defocus that you can have without causing any significant change in the size of the focused spot.  Poor seeing increases the effective size of the focused spot (called the blur size) which means that the tolerance in focusing accuracy goes down with poor seeing.  That's because you can't make the spot any smaller by focusing more precisely.

 

John


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#18 jhayes_tucson

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Posted 11 May 2018 - 03:47 PM

John,

 

A temperature drop of 9C from about 11:00pm to about 5:00am !!  You need to move to northern Arizona or do something to adjust focus.  I'll have to look for some data, but on clear nights here, I'm used to seeing our temperature drop steeply from sundown to about 10:30, then drop very little until sunrise.  For instance, last night, IIRC, temperature dropped from about 77F at sunset to about 58F at 11:00pm.  Then from 58F to about 56F just before sunrise.  We're quite high up here.  I wonder if that is a factor.

 

Nice mobile tripod.  Custom design?  Taxiway in the background?  

 

Edd,

For a long time, I measured the temperature every night when I imaged so I have a lot of data.  Some night are better than others but it's possible to experience some pretty significant temperature swings.  [Oops!  I just looked back at the chart that I posted above and realized that I grabbed the wrong one from an older version that had a mistake.  I'll post the correct version here.  Sorry for the goof.]

 

Yes, that is a custom pier design and yes, that is a taxiway in the background.  For many years, I operated at the Bend airport.  The skies were pretty dark and it worked pretty well.  However, as more hangars were built, the traffic got heavier and lights became more of a problem so that was one of my reasons to move my scope to New Mexico.  In NM, I get more clear nights in the winter (less in the summer) and I can sleep at night.  Unfortunately, the seeing in NM is not as good as it can be in central Oregon so nothing is perfect.

 

John

Attached Thumbnails

  • Figure 4 - Temp Change Chart corrected - sm.jpg


#19 Codbear

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Posted 11 May 2018 - 04:06 PM

John,

 

Thanks for photo and the additional information. If I have any more questions I'd like to PM you if you don't mind so I won't divert the thread!



#20 freestar8n

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Posted 11 May 2018 - 07:35 PM

Frank,

Huh?  Where did I claim that the sky is falling?  All I've done is to compute the temperature sensitivity of various telescopes relative to focus.  The specific data for the TEC oil-spaced refractors actually came from a calculation done by Yuri Petrunin at TEC.  Just because you run a heater all the time doesn't tell you anything.  How much of the structure is being heated?  Is it being heated to a constant temperature or a constant temperature difference relative to ambient?   Do you have any data showing how much focus changes relative to ambient conditions with your heater running vs without the heater?  Whether or not the telescope is sheltered or on concrete or asphalt is completely irrelevant.  All that matters is the temperature change at the telescope.  That's what causes the focus to change.  Your statement that, "...your conclusions do not match my experience at all..." isn't a data point.  If you think that my numbers are all wrong then gather some good data to make your point.  Put your telescope in a temperature controlled room, set it up in double pass, and record the focus change as a function of temperature.  I'll bet you $100 that the results will come out to agree with my predictions to within the measurement accuracy of the experiment.

 

As for dew heaters, I've long thought that none of the anti-dew controllers work the right way.  I use a Dew Buster and it might work like your Kendrick system to hold a constant temperature differential relative to the ambient temperature.  That's a valid approach to combating dew but it makes no sense to run it when there is no threat of dew.  I built an Arduino controller on my system that powers the Dew Buster only when the temperature-dew point spread is below six degrees.  So my dew control system is only powered for maybe 6-10 nights out of the year when dew or frost might be a concern.  It may certainly be possible to heat the system without creating significant air currents but I know or sure that there aren't any air currents being produced when the system is turned off.

 

Finally, perhaps you didn't read what I said about that picture.  It is an old photo.  My scope is now located in an observatory in New Mexico--that also has a concrete floor.  Regardless of location, I've found that when the temperature changed significantly throughout the night, I had to refocus quite frequently--like on the order of every ~15 minutes.  That's not a data point but it jives with the sensitivity that I've computed for the telescope.

 

John

I have provided focus data previously and pointed to it multiple times.  And every time you talk about how sct's hold focus poorly, you ask me to provide data.  You have a simulation - and I have data that contradicts your results.  So - it should be no surprise - your simulation does not match reality - and no conclusions can be drawn from it.  People have been imaging well with sct's for years with occasional or even no re-focusing.  

 

"All that matters is the temperature change at the telescope."  Again - that is what your mind is telling you - but it is just an assumption - and one I completely disagree with.  I would say "All that matters is the temperature *of* the telescope - and all its gradients - and how all the components of this complex system interact."  And the key here is that ambient temperature is not a good proxy for the temperature *of* the telescope - when radiative cooling and the dew strap are playing a dominant role.

 

If you wheel a telescope out from indoors it will have some thermal history that will put it completely out of equilibrium with the environment.  At that point the entire system will be equilibrating and cooling and there will be rapid changes over a long period of time that are not at all slaved to whatever the ambient temperature is doing.  So if you wheel a telescope out under a wide and cold sky - there may indeed be rapid focus change - but it is due to factors completely ignored in your model.  Focus would be changing even if the ambient temperature were constant.

 

I have not imaged with a long refractor - but I think they may be more prone to focus drift than sct's.  If you have a long refractor tube with a dew strap at the top - the overall system may be more prone to thermal contraction than an sct with a dew strap in the front.  So it may well be that the predictions of your model are exactly backwards regarding sct vs. refractor.

 

My Kendrick dew heater is about 20 years old I think.  It has no feedback at all - and just heats at a constant level based on fixed duty cycle set by a potentiometer.  There are much more sophisticated ways to do this - but I am doing fine both with deep sky imaging and diffraction limited results - and periodic focus with the primary mirror every 30 minutes - which takes 2-3 minutes due to winding out backlash twice.

 

Frank



#21 jhayes_tucson

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Posted 11 May 2018 - 09:36 PM

So, where is your data?   All I need to see is how much the focus position moves as a function of temperature--when the scope is in equilibrium.

 

John


Edited by jhayes_tucson, 11 May 2018 - 09:42 PM.


#22 aacc66

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Posted 11 May 2018 - 11:02 PM

John,

 

I noticed that you have something that looks like reflectix on your Edge 14 picture.

You are probably aware of a topic here in the cloudynights forums about reducing the temperature change in the OTA instead of cooling it with a fan.  And some of the proponents of this idea suggest using reflectix.

 

BTW: I am not suggesting that you are proponent of this idea.

 

But it is a tempting one; instead of trying to chase the temperature change with a fan, you try to keep OTA stable.  I can imagine that heat will be bleeding to the outside through the aperture of the OTA. Does your calculations indicate how much losses will you get if you from the aperture if you can keep the rest thermally isolated?

 

As a side note: I have a piece of code that reads the temperature from the TC of my focuser, and tries to correct focus with every temp change (rather rudimentary).  If you have large temperature drops during a session. will those changes affect the nature of your flats that you usually take at the end of the session?

 

Alvaro

 

 

 

 

I have a customer pier.  I've posted pictures of it all over the place but I can't find a single one so I'll attach one here.  The AP1100 is a nice mount for the 1100 but I personally wouldn't want it on a tripod unless the tripod were really stout.  I run my C14 on an AP16000.  This is an old photo but it shows the pier pretty well.

 

John

 

 

Certain folks around here get grumpy if the topic wanders too much so we'll have to start another thread if you want to discuss this any further.



#23 jhayes_tucson

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Posted 12 May 2018 - 12:40 AM

John,

 

I noticed that you have something that looks like reflectix on your Edge 14 picture.

You are probably aware of a topic here in the cloudynights forums about reducing the temperature change in the OTA instead of cooling it with a fan.  And some of the proponents of this idea suggest using reflectix.

 

BTW: I am not suggesting that you are proponent of this idea.

 

But it is a tempting one; instead of trying to chase the temperature change with a fan, you try to keep OTA stable.  I can imagine that heat will be bleeding to the outside through the aperture of the OTA. Does your calculations indicate how much losses will you get if you from the aperture if you can keep the rest thermally isolated?

 

As a side note: I have a piece of code that reads the temperature from the TC of my focuser, and tries to correct focus with every temp change (rather rudimentary).  If you have large temperature drops during a session. will those changes affect the nature of your flats that you usually take at the end of the session?

 

Alvaro

 

Alvaro,

Yes, my system is wrapped in Reflectix.  I do that to reduce radiative cooling of the tube relative to the ambient air temperature.  For what I want to do, aluminum foil would work better but it's not very robust stuff so I went with Reflectix.  Since my scope sits in an observatory, it probably doesn't matter very much.  I run Tempest fans continuously on my scope to dissipate tube currents and to keep everything at the same temperature as the surrounding air.

 

Even a large temperature drop during a session won't have a very big effect on flat calibration.  So I don't think that's something that you need to worry about.

 

John



#24 freestar8n

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Posted 12 May 2018 - 01:48 AM

I have pointed the data to my focus curve and drift many times.  If you look at your own threads on related topics where you asked me for data, you will see me reminding you I have already provided it.  I did my own thread on focus drift in sct imaging - and I have pointed you to it many times.  It should be easy to find.  I have no idea why it has never registered with you that my data does not agree with your dire predictions of focus drift.

 

The main thing is - I should not have to explain this at all:  You have done a simulation.  Fine.  Now you need to do the experiment to see if it is valid. 

 

A simulation is nothing more than a model of what you think is happening.  I have already pointed out numerous extremely important factors that your model ignores - so even without data I see no reason to think it would have any predictive value.

 

Frank



#25 gnowellsct

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Posted 12 May 2018 - 11:07 AM

In my part of the world dew is ALWAYS a threat. I can't use a controller that tries to turn the dew heater on and off because the only effective tactic is to leave it on the whole night.

The only thing I *have* measured is temperature deltas. The sky side vs. ground side temperature delta is orders of magnitude greater (considering total area) than the tiny influence of the dew heater.

I can't speak to temperature/focus relationship. I will note that John is about as astute as they come when it comes to the science of SCTs. I hope he gave some friendly advice to Public Missiles on what type of carbon fiber to use. I've heard other complaints that their carbon fiber doesn't do much to stabilize focus.

It seems to me that sky side v. ground side temperature deltas ought to cause differential rates of contraction in the tube. I've measured the difference at about 6 degrees F.


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