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Comparison of the Boltwood II and Sky Alert Cloud Sensors

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#1 theastroimager


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Posted 09 June 2018 - 06:40 AM

This review is a side-by-side comparison of the Boltwood II Cloud Sensor from Diffraction Limited/Cynagon , and the SkyAlert Cloud Sensor from Interactive Astronomy. Both units were purchased new by the author. The sellers were not made aware that I would be doing a review of their products, so no temptation was held to send me anything other than a typical unit.

Click here to view the article

#2 cjdavis618


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Posted 09 June 2018 - 07:53 PM

Thanks for such a thorough review. Very helpful to decide what option would work best for me. 

I looked on the site for the Skyalert system and didn't see the  spinning anemometer option listed. Is that something you asked for specifically? I see other options like power monitoring, but not that spinning version of the meter. 

#3 RichA



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Posted 10 June 2018 - 05:42 PM

It's nice that someone would do a thorough review of two items that the regular magazines/sites probably would pass-over in favour of a telescope or mount review.  Good work.  The price difference of these units is considerable, but professional weather instruments from companies like Vaisala or Campbell Scientific is a lot higher still.

#4 pablotwa


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Posted 11 June 2018 - 07:17 PM

Disclaimer, I have no direct connection with any of the above companies other than having purchased the Sky Alert and Sky Roof for my ROR a few years back.


There are lousy vendors, medium vendors and great vendors but there's only ONE outfit above them all and that's Jim Collins from Sky Alert. (I know it sounds like BS..but believe me, it's absolutely and provably true)


When I have a problem (mostly self inflicted and not related to his products) and leave him an email he responds within minutes and in most cases phones me back in less than an hour...then he will ask for my Teamviewer code and fix the program himself...as many times as needed and never EVER with anything less than a positive attitude and enthusiasm for his product.


His Sky Alert saved my very expensive equipment so many times I lost count. I set it forget it and it works, every time, all the time.


Oh yes, one of the sensors went bad after 2 years and he fixed it..free of charge (I insisted I pay for the postage and I will but so far he hasn't even billed me for that).


You're not only buying a great product but your also buying the kind of support that really doesn't exist anymore.


Pablo Lewin

The Maury Lewin Astronomical Observatory

Glendora,CA. 91741

Edited by pablotwa, 11 June 2018 - 07:21 PM.

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#5 eastwd


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Posted 11 June 2018 - 10:09 PM

I looked on the site for the Skyalert system and didn't see the  spinning anemometer option listed. Is that something you asked for specifically? I see other options like power monitoring, but not that spinning version of the meter. 

I'm curious about this, too.

#6 Alain Maury

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Posted 15 June 2018 - 09:25 PM

Boltwood sensor is supported in Prism. Both anyway measure the temperature of the sky with an IR sensor, and therefore can't see cirrus clouds which are very high and therefore very cold. When doing photometry you want of course no clouds, including no cirrus.

Best way so far is to have an all sky camera with a software able to count the visible stars in the image, and see a drop when there are clouds. I don't know if many all sky cams do that, but the one I use does (see https://hyperion-astronomy.com/alphea ). First, I live in a place where clouds are rare (the second image from the left on the above page is from my all sky), I have a weather station, mostly for the wind indication. Don't use IR sensors anymore. And the all sky cam can tell me if there are clouds or not. Normally not.

#7 NMNights


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Posted 27 June 2018 - 06:08 PM

I just sent an email to Jim Collins and got a response in a few minutes.

The three vane anemometer IS available but has not been added to the web site yet.

It is an additional $195 and he can add it to the PayPal with a link.


I am just setting up my observatory in NM but one thing is for sure: if you don't have clouds, clouds aren't a problem. If you do there is a lot to consider before dumping a shooting session and/or slamming the roof closed. 

#8 Chris W

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Posted 28 June 2018 - 03:19 PM

Thanks for doing this review. A welcome change to the cursory 4/5 star reviews that tell you very little.

Very relevant to UK weather! The advance of technology leaves no prisoners.

#9 astrodoc71


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Posted 22 July 2018 - 08:16 PM

Excellent job and very thorough! I would like to clarify though that the Boltwood II, which I am using, is supported in CCD autopilot pretty seamlessly. The Boltwood outputs its data to a file and all you have to do in CCDAP is indicate the file path. It can then read the weather info from that file and you can set up a variety of weather based shut down routines etc. based on your preferences (from within CCD autopilot of course)

Thanks again for posting this!



#10 flyingcougar


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Posted 22 August 2018 - 11:10 AM

Thanks John for the informative comparison. I've been considering adding a weather station and both of these systems are on my research list.


I recently installed the SkyRoof on my observatory without any difficulties, and as others have mentioned, Jim Collins was very responsive throughout the entire process of questions to final installation. Looks like SkyAlert it is!



#11 chriscorkill


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Posted 24 August 2018 - 01:14 AM

Thanks for this wonderful review. I was just looking at getting a Boltwood 2 or the SkyAlert but I think after this review, I’ll get the latter. Now to research what Allsky camera is best!



#12 Stevous


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Posted 29 September 2018 - 02:48 PM

From this review I would be cautious in drawing a conclusion on comparative performance. Being a remote observatory controller, I was interested to read this, however, the questions I know that exist for an experienced user are not really based on the discussions above. 


My concern relates to whether or not the device worked correctly on detecting clouds? Did the device work correctly with large variations in sky temperature, for example, one day it was 5c ambient, and the next was 18c, but both nights it was clear? When you have swings in temperature, do you need to edit detection tolerances? Do you need to edit these tolerances throughout the year for these condition changes? What happened if the night was especially dewie, did the devices internal heater plus it intelligence correctly distinguish between rain and dew? How did they compare for measuring sky temperature?


These are but a few questions I thought would be discussed, and if you have answers to these that would be great. But dont answer them unless you have daily monitored these two side by side, as assumptions are no good to publish. We regularly check weather station detection results to allsky timelapses, to ensure cloud detection was working in alignment to the conditions, have you done this to confirm they each detected cloud correctly? 


Cloud sensors may not be accurate all the time, so you need to adjust thresholds, have you needed to do that for either unit? Do you set their detection to conservative thresholds so to get the best safe performance out of these?


What type of unsafe detection limits exist for each? Like, clear, cloudy, overcast, light, dark, bright, dry, drizzle...


Can either do scripts? For remote observatories, these are very important. These are automated actions that are the backbone of the system. Backyard users may think integrated mount control is handy, but this is not what I'd like to rely upon solely for a safe close.


The wind speed variation mentioned is not that important at low speed, is it (unless your equipment is exposed, but even then, these low speed errors will not impact)?  But, how accurate was the wind speed detection at 15km and above, say to at least 30km, when at that speed most observatories would start shuttling down? What third party device did you use to confirm the true wind speed?


People who haven't had extended experience with weather station devices may incorrectly assume they just work as they expect them to work, but they dont. Perhaps you can give us more information on the above as you accumulate more information, it would be interesting to hear. Thanks for the effort so far.




#13 Sixburg


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Posted 13 October 2018 - 02:02 PM


Great questions posed.  I only have experience with Sky Alert and AAG Cloudwatcher (the AAG about 6 years ago and the Sky Alert for the last 5).  


It strikes me that the relatively simple calculation for "cloudy", Sky Temp - Ambient = Sky Condition, is not robust.  As you indicated there are seasonal and daily situations that easily "trick" this calculation and any monitor that relies on this. The underlying determining algorithm for the AAG, as I understand it, is more robust and can be tuned to handle situations that fool other monitors.  


Jim and I spoke about ways to incorporate a better algorithm and anyone who has worked with Jim and Wes know that he sets the bar for responsiveness versus all other vendors (in my opinion).  They tried a few things, but nothing made it past experimental phase.  (Update, spoke with him today(13-Oct-2018) and they are going to take a look at it again maybe).  


What does the AAG that the others don't... I don't know the answer, but I believe it is not proprietary, but is based in part an a 1969 paper by Sherwood B. Idso  Ray D. Jackson, "Thermal radiation from the atmosphere".  I have a renewed interest in this because our needs have increase and I'd like to have something that does not have to be adjusted as much.  



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