Considering Drilling Holes In My New CDK?

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Rouz Astro avatar
I have been working on my current CDK14 scope with very good results. Here is a review of the scope:
https://rouzastro.com/reviews/
  The CDKs do come with fans on the rear backplate that blow air into the OTA. The larger scopes and newer versions have 4 x side fans as well to blow air across the primary - mine is brand new too but the CDK14 doesn't have side fans. I'm considering adding 4x fans to the side and wondering if there will be any extra benefit? The new DR350 which is a short FL CDK14 does have these fans as well. Beside more scrubbing of the boundary layer, another advantage could be the primary will be closer to ambient and may need less heat to prevent dew. These scopes to have heaters. Any ideals are welcomed.     The new short version of the CDK14 attached (note 3x rear fans + 4x side fans).

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Rouz Astro avatar
CDK17 (new version)  and RCOS OTA:



Rouz Astro avatar
Looks like even the new CDK20 has the side fans.
John Hayes avatar
Rouz,
It takes the very tiniest airflow to break up baffle plumes and to remove tube gradients.  My guess is that they might not do much but if I were going to add them, I would add just two–one for intake and one for exhaust and I wouldn't add anything bigger than about 1" in diameter.  My sense is that I wouldn't want them blowing right across the front surface of the mirror but that's a total guess.  I'd be inclined to line them up with the edge of the mirror.  In the world of interferometry where we can easily see air flow, I have a good sense of what happens when a fan blows air directly into the test wavefront and it's not good–like at all.  I'd only run these things to bring the primary mirror into equilibrium with the air temperature; not for imaging.

John
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andrea tasselli avatar
Bad idea. I have 2 side fans on my MN and they didn't do any good. In fact they probably made things slightly worse. Now they collect spiders.
Rouz Astro avatar
John Hayes:
Rouz,
It takes the very tiniest airflow to break up baffle plumes and to remove tube gradients.  My guess is that they might not do much but if I were going to add them, I would add just two--one for intake and one for exhaust and I wouldn't add anything bigger than about 1" in diameter.  My sense is that I wouldn't want them blowing right across the front surface of the mirror but that's a total guess.  I'd be inclined to line them up with the edge of the mirror.  In the world of interferometry where we can easily see air flow, I have a good sense of what happens when a fan blows air directly into the test wavefront and it's not good--like at all.  I'd only run these things to bring the primary mirror into equilibrium with the air temperature; not for imaging.

John


Useful insights, thank you John.

I could also see effects of heat/air on the C14 with high frame rates and very high resolution during planetary imaging. Tried all manners of modifications, in the end I ended up insulting its extensively but that's a close system vs the open CDK.


I'm very hesitant myself, fluid mechanics can be counterintuitive at times. My guess is that the airflow from behind a flat primary causes a turbulent wake on the front side. The edges should be cleaned up but a boundary layer may persist near the center?
Small fans can possible push that wake inwards, decreasing the separation from the surface, thus scrubbing more boundary layer. 

Its hard to say, but I'm also thinking there must have been a reason why Planewave added them to almost every CDK, as you have noticed, they don't always address every little thing so this might be more significant?

Rouz,
Rouz Astro avatar
andrea tasselli:
Bad idea. I have 2 side fans on my MN and they didn't do any good. In fact they probably made things slightly worse. Now they collect spiders.

*Interesting, thank you for sharing.
I tried 2x small internal only fans inside an SCT to "stir" and blow air off the primary, the results weren't better than no fan.

The CDK is a bit different as its an open system and I run the rear fans during imaging so I'm not sure they behave the same. But again, not sure, I've got a setup that is working well.
The old adage may hold true here - If it ain't broke don't fix it!

CS
Rouz
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Rouz Astro avatar
@John Hayes 
I forgot to mention, I keep the fans on during imaging, not sure if that's what you or others do.

The fans did not induce any vibration effects as some suspect, I didn't detect any increase in FWHM. If anything, the camera fan mms away from the sensor is more probable to do that.

Not having water cooling now, that would be hard to test. But I did test that on a 0.08 arcsecond/pixel planetary setup and couldn't see any issues with fan vibration there (also QHY camera).

 What I did notice was that the focus point did shift significantly with the fans on vs off, can't explain why. Tested multiple times, it shifts some 2000 steps on my 0.11u/step focuser.
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John Hayes avatar
Rouz Astro:
@John Hayes 
I forgot to mention, I keep the fans on during imaging, not sure if that's what you or others do.

The fans did not induce any vibration effects as some suspect, I didn't detect any increase in FWHM. If anything, the camera fan mms away from the sensor is more probable to do that.

Not having water cooling now, that would be hard to test. But I did test that on a 0.08 arcsecond/pixel planetary setup and couldn't see any issues with fan vibration there (also QHY camera).

 What I did notice was that the focus point did shift significantly with the fans on vs off, can't explain why. Tested multiple times, it shifts some 2000 steps on my 0.11u/step focuser.

Rouz,
1)  If I had to guess, I'd say that Planewave added the fans because customers are asking for them; not because of extensive testing and analysis.
2)  Last summer (in Chile) when the air was VERY still, I found that my data looked a little better with the fans off.  It's hard to say with any certainty that that's due to the fans or the atmosphere but as long as I don't have plumes and everything is stable, the fans aren't doing anything.  Understand that when conditions are right, the temperature can be stable to +/-0.7C over the ENTIRE night at my location.  That doesn't happen all the time but in general, I try to leave the fans off as much as possible.  This is the opposite of what I experienced with my C14 system.   Of course, YMMV.
3)  Hmm...that is a pretty substantial focus shift since it is more than the depth of focus.  My first guess is that it must be due to a temperature related issue.

John
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andrea tasselli avatar
Rouz Astro:
The CDK is a bit different as its an open system and I run the rear fans during imaging so I'm not sure they behave the same. But again, not sure, I've got a setup that is working well.
The old adage may hold true here - If it ain't broke don't fix it!


Rear fans are OK-eish, especially in bringing down the mirror temperature in locations subject to wide day-night temperature obscillations. What you don't want to do is to break the boundary layer without good reasons. In fact one could devise a Peltier-cooling/heating system to bring down (or up as needs require) the mirror temperature to within 1 degree from ambient temperature and keep it there.
Yuxuan avatar
Rouz, do you feel that there is space for improvement in terms of resolution for long-exposure astrophotography on your CDK 14? Even on my iDK 10", the resolution (<2.0" FWHM when good seeing) is decidedly seeing limited across a full frame sensor.

My only motivation to upgrade would be light gathering abilities.
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Rouz Astro avatar
Rouz,
1)  If I had to guess, I'd say that Planewave added the fans because customers are asking for them; not because of extensive testing and analysis.
2)  Last summer (in Chile) when the air was VERY still, I found that my data looked a little better with the fans off.  It's hard to say with any certainty that that's due to the fans or the atmosphere but as long as I don't have plumes and everything is stable, the fans aren't doing anything.  Understand that when conditions are right, the temperature can be stable to +/-0.7C over the ENTIRE night at my location.  That doesn't happen all the time but in general, I try to leave the fans off as much as possible.  This is the opposite of what I experienced with my C14 system.   Of course, YMMV.
3)  Hmm...that is a pretty substantial focus shift since it is more than the depth of focus.  My first guess is that it must be due to a temperature related issue.

John

*John,
Similarly, I see very little difference in FWHM with the fans off vs on when the temperatures are stabilizing.
The technical person at PW did say the mirror outer mask divers air to scrub the boundary layer, fans not needed. But they also said you can use an OAG before the reducer and that is working well.
Also, 50mm round is not vignetting anything with the reducer (tested by removing the internal wheel), they advised 50x50mm which I found was not necessary. 

It could be that the fans are more based on customer demand or perhaps to help cool the optics initially.

As for the focus shift, its instant on my CDK14. The moment the fans are off, the focal point moves. I suspect a physical shift from the pressure somehow. It is very strange, I bet PW are not aware o this. Perhaps it only applies to the 14" not sure.
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Rouz Astro avatar
andrea tasselli:


Rear fans are OK-eish, especially in bringing down the mirror temperature in locations subject to wide day-night temperature obscillations. What you don't want to do is to break the boundary layer without good reasons. In fact one could devise a Peltier-cooling/heating system to bring down (or up as needs require) the mirror temperature to within 1 degree from ambient temperature and keep it there.

*Given these have heaters behind the mirrors, and the primary is tinned (conical), they I suspect a Peltier wont be necessary.
The fans and open truss do help cool the mirror considerably easily compared to closed OTAs and full thickness mirrors.
Rouz Astro avatar
Yuxuan:
Rouz, do you feel that there is space for improvement in terms of resolution for long-exposure astrophotography on your CDK 14? Even on my iDK 10", the resolution (<2.0" FWHM when good seeing) is decidedly seeing limited across a full frame sensor.

My only motivation to upgrade would be light gathering abilities.

*Not really related to this topic (I don't mind, hopefully other wont either)

Given todays small pixels, I doubt you will run into a situation when the aperture is limiting your resolution. The seeing blur will overwhelm most of the difference in theoretical resolution of a 10inch vs 12 vs 14.

Its very different with planetary imaging, you can at times approach the potential theoretical resolution with lucky imaging. There a 14inch will provide significant resolution over a 10 inch.

Seeing is variable so you can't really work out a rigid scale in my opinion. My approach is to always image at bin1 (max resolution) that will oversampled. Then you can "tune" the image to a scale that works for that session/season. I get low 2s at times and over 3.0" in some months. The lowest ones are in 1.7" for brief periods. 

So why did I go from a 12 to 14? 
The 14 does have a better performance and one of the largest image circle both in terms of illumination and spots sizes. I wanted to use to reducer with a FF camera that is tough for most OTAs given the 0.66 reduction factor. Camera sensors are getting larger but to get what I'm getting with my reducer/camera combo in native format would have cost over 50k in camera, filters, FW, etc... The image  from a native CDK14 + IMX411 at 0.3"/pixel will most probably have little to no real resolution benefit over the reducer + IMX455 at 0.46 "/pixel.  Both will have the same FOV. The IMX411 setup will a lot less practical I'd say.

As image scale will run into diminishing returns after a certain limit (2~3.5x your seeing - 2 might be a bit too low), my ideology is to put that aperture to better use, use it to increase signal.
A good astro image is not just resolution but high SNR as well. Being in a location that is mostly cloudy, clear skies are rare so I traded excessive resolution for "speed". My approach is using both optical reduction a digital (down sampling) to get the most signal. Obviously get the highest QE and lowest noise camera you can as well.

There are some exotic scopes with fast ratios and large image circles too, but they tend to be pricey as well. For example:
https://www.italiantelescopes.net/ti-55-astrograph/

The spots and illumination and f ratio are ideal (for me), but the prices are not very easy to swallow! 

CS
Rouz
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Yuxuan avatar
Rouz Astro:
Yuxuan:
Rouz, do you feel that there is space for improvement in terms of resolution for long-exposure astrophotography on your CDK 14? Even on my iDK 10", the resolution (<2.0" FWHM when good seeing) is decidedly seeing limited across a full frame sensor.

My only motivation to upgrade would be light gathering abilities.

*Not really related to this topic (I don't mind, hopefully other wont either)

Given todays small pixels, I doubt you will run into a situation when the aperture is limiting your resolution. The seeing blur will overwhelm most of the difference in theoretical resolution of a 10inch vs 12 vs 14.

Its very different with planetary imaging, you can at times approach the potential theoretical resolution with lucky imaging. There a 14inch will provide significant resolution over a 10 inch.

Seeing is variable so you can't really work out a rigid scale in my opinion. My approach is to always image at bin1 (max resolution) that will oversampled. Then you can "tune" the image to a scale that works for that session/season. I get low 2s at times and over 3.0" in some months. The lowest ones are in 1.7" for brief periods. 

So why did I go from a 12 to 14? 
The 14 does have a better performance and one of the largest image circle both in terms of illumination and spots sizes. I wanted to use to reducer with a FF camera that is tough for most OTAs given the 0.66 reduction factor. Camera sensors are getting larger but to get what I'm getting with my reducer/camera combo in native format would have cost over 50k in camera, filters, FW, etc... The image  from a native CDK14 + IMX411 at 0.3"/pixel will most probably have little to no real resolution benefit over the reducer + IMX455 at 0.46 "/pixel.  Both will have the same FOV. The IMX411 setup will a lot less practical I'd say.

As image scale will run into diminishing returns after a certain limit (2~3x you seeing), my ideology is to put that aperture to better use, use it to increase signal.
A good astro image is not just resolution but high SNR as well. Being in a location that is mostly cloudy, clear skies are rare so I traded excessive resolution for "speed". My approach is using both optical reduction a digital (down sampling) to get the most signal. Obviously get the highest QE and lowest noise camera you can as well.

There are some exotic scopes with fast ratios and large image circles too, but they tend to be pricey as well. For example:
https://www.italiantelescopes.net/ti-55-astrograph/

The spots and illumination and f ratio are ideal (for me), but the prices are not very easy to swallow! 

CS
Rouz

Rouz, I asked because I wondered what improvements for deep sky imaging you were aiming for by adding the side fans. Or were you planning to do planets with it?
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Rouz Astro avatar
Yuxuan:

Rouz, I asked because I wondered what improvements for deep sky imaging you were aiming for by adding the side fans. Or were you planning to do planets with it?

*I was thinking with the colder weather and possibly larger temp. drops, the side fans could be helpful.
Was hesitant to begin with and it seems its not worth the effort and risk.
 
I didn't try planetary imaging with the CDK14, the larger secondary (than the C14) leads me to believe it might not do as well. The sample planetary images I saw in the Planewave gallery weren't very inspiring either.

Also, the planets are mostly too low from 50 degrees north.

CS
Rouz