DIY Solar Scintillation Seeing Monitor with FC support

RTJoe · Post by **RTJoe** » Fri Jul 03, 2015 4:47 pm

Hello

Here is my first prototype of a DIY Solar Scintillation Seeing Monitor (based on https://www.ecu.edu/cs-cas/physics/uplo ... final2.pdf). It's currently build on an Arduino prototype board to check basic functionality.

: IMG_1205.jpg (167.41 KiB) Viewed 34422 times

: IMG_1206.jpg (186.33 KiB) Viewed 34422 times

The software is also just a first "quick and dirty" solution to display measured values. But - and I think that is an interesting feature - it has already the capability to start FireCapture recordings automatically when a specified seeing trigger level is reached.

: ScreenShot01.jpg (58.51 KiB) Viewed 34422 times

The blue line on the left chart is the seeing value, the red line a moving average. The right chart displays the current input value (intensity) of the sensor.

I have just started to test the output. At least the results seem to be not completely unreasonable and the seeing measured this morning at cooler temperatures was clearly better than yesterday evening above the heated roofs.

There is a long ToDo list:

- rebuild on perfboard, housing, …

- validate results (compare measures with image quality)

- user friendly software (improved display, configurable trigger for FireCapture, log file support, …)

- maybe external display for standalone operation (site evaluation)

Joachim

Post by **marktownley** » Sat Jul 04, 2015 7:49 am

Interesting! Let us know how it develops. I've moved this to the mods section as it will get more attention there.

Valery · Post by **Valery** » Sat Jul 04, 2015 1:46 pm

Very interesting development! Thanks for letting us know about this.

One important note:

As the FC has "start" and "pause" triggers, it will be better to make a following option:

User put the movie full duration (in seconds) and the seeing monitor starts and pause
and then restarts again, pause again, restarts again the record within this time movie duration.
This will cut the unwanted recording during poor seeing.

Say, user put the seeing maximal value, say, 1,2". Movie time limit - 40sec. So, within 40sec
the FC will record all frames when the seeing was better than 1,2".

Valery

PEterW · Post by **PEterW** » Sun Jul 05, 2015 3:55 pm

Just found this one by random surfing.... I have an arduino so will look to make my own one. Maybe add a display rather than just uploading to the serial output.

Cheers

PEter

RTJoe · Post by **RTJoe** » Mon Jul 06, 2015 8:04 am

Thank you for your comments.

Valery:This will be a very useful feature. I will implement it soon.

Joachim

robertp · Post by **robertp** » Sun Jul 26, 2015 9:15 am

Hello Joachim,

great to see that someone else is also working on the diy version. I also started to build it beginning of June. I've chosen the same approach (small breadboard and prototyping shield) and finished the electronics, but I'm still missing the IR-sensor. I ordered it more than 7 weeks ago from http://www.tme.eu but it still is backordered.
May I ask where you got your sensor from?

Best regards and steady skies,

Robert

RTJoe · Post by **RTJoe** » Tue Jul 28, 2015 1:05 pm

Hi Robert

It’s really difficult to get the sensor used in the original project. I have currently replaced it by a BPW34 and also replaced the 220 ohm feedback resistor by a 2K variable resistor to get about 1V output.

Meanwhile I have switched to a soldered board and an Arduino Nano:

: SSSM04.JPG (147 KiB) Viewed 34200 times

Currently I’m working on a sensor with longer cable because I want to attach it to the solar scope.

Good to know some other people working on such projects. Please let us know about your progress and results.

Joachim

robertp · Post by **robertp** » Tue Jul 28, 2015 7:05 pm

Hello Joachim,

thanks for the hint with the BPW34, I will also try this one until the VTP4085H arrives. I ordered 3 of the VTP4085H, so if they ever arrive and you are interested to get one, I can send you one (I also live in Germany).

Robert

RTJoe · Post by **RTJoe** » Wed Jul 29, 2015 7:56 am

Hello Robert,

thank you for this kind offer, it would be great to compare the results of both photodiodes!

Joachim

RTJoe · Post by **RTJoe** » Thu Aug 27, 2015 9:13 am

Hello!

The DIY SSSM is making progress and before readers with less interest in the evaluation of the measurements stop reading, I would like to start with a question regarding Firecapture:

I have some problems when trying to resume a paused recording with keyboard commands. My software controls Firecapture by sending „key pressed“ commands to the FC window. In the FC settings I can assign the commands to start, stop and pause the recording. Everything works fine except that I’m not able to resume a paused recording; neither with a second „pause“ command nor with a new „start“ command. Does anybody have an idea how to resume a paused recording using the keyboard (not the mouse)?

Back to the DIY SSSM: for evaluation I have written some software to record the output of my ASI camera and log precisely the frame number and the corresponding SSSM seeing value. Then I processed the recorded AVI with AVIStack2, which is able to export the image quality data to a CSV file (usually I use AutoStakkert, but I have not found a possibility to export the quality graph).

The correlation coefficient of seeing and image quality was a negative value in all tests. That’s great, because larger seeing values mean lower quality. Here is the graph of one test run (685 frames):

Please note that the left y-axis (seeing values) and the right y-axis (image quality calculated by AVIStack2) have opposite directions.

I must admit that the graph is the one with the best correlation, about -0.5. The average correlation of my test runs was about -0.35. I guess that for a given seeing there is still a wide range of image quality possible. Therefore it might be not useful to start and stop recordings immediately based on the seeing values, but to wait for a longer period within some range.

Has anyone already tried to compare the results of the commercial SSM with the image quality? I would appreciate your reports and how you use the measured values for imaging.

BTW: During the SSSM test with an ASI120MM, I accidentally captured this:

Meanwhile a display is connected for usage without a laptop. Now only some housing is missing.

Joachim

Valery · Post by **Valery** » Sat Aug 29, 2015 9:04 am

Hi Joe,

Very interesting progress. Keep coming.

Hope you will find the best solutions and your SSM also will be available for us, solar nuts.

Thanks for sharing about your works progress.

Valery

PDB · Post by **PDB** » Fri Nov 04, 2016 9:42 am

Hello Joachim,

also started to build one, but have a question on whicht type of cable you used to connect the sensor. In the original doc a coax cable is recommended, but looks like you have used someting different. Suppose it is also a shielded cable?

Rgrds,

Paul

RTJoe · Post by **RTJoe** » Fri Nov 04, 2016 10:31 am

Hello Paul,

yes, I have used a shielded cable. The part you have seen in the pictures above was just a small piece from the board to an RCA socket at the housing.

I have just build another device and you can see the cable on the image:

: SSSMon device; IMG_2127.JPG (592.68 KiB) Viewed 33157 times

From the RCA socket I use a standard video cable to attach the sensor.

BTW:
I'm currently working on a major update of the FireCapture plugin software. Solarchat user rxdeath has tested the AiryLab SSM support and it works fine. The next version will also include a live plot of the seeing measurements inside FireCapture, support for firmware with more samples per measurement and a log file with seeing values for each captured frame. Stay tuned...

Wish you success with your seeing monitor project!

Joachim

PDB · Post by **PDB** » Fri Nov 11, 2016 10:19 am

Hello Joachim,

making some progress, but still waiting for the LMC6484 to arrive. (Hopefully on monday). Meanwhile did some testing with the firmware loaded in the Arduino just putting some voltage (3,3V and 5V) to the analog pins. There is a small glitch in your modified sketch versus the original schema.

The schema connects to analog pins A0 and A1 (also reading them in the sketch). Your firmware is reading pin A1 and A2, so I wondered long time why the output in the SSSM program was not what I expected. Not a big deal, just changing the analogRead did the trick. (Already soldered my board to connect to pin A0 and A1, so that was difficult to change).

Kind regards,

Paul

RTJoe · Post by **RTJoe** » Sun Nov 13, 2016 4:06 pm

Hello Paul,

sorry for the confusion caused by using A1 and A2 instead of A0 and A1. I should add a comment into the sketch. My intention was to keep A0 free, because the LCD keypad shield 1602 uses A0.

Wish you a lot of sunshine for testing!

Best regards,
Joachim

PDB · Post by **PDB** » Sun Nov 13, 2016 4:26 pm

Thanks Joachim,

looking forward for your updated FC plugins as well. This should be a great help.
Hopefully I can start checking if I soldered all wires and components correctly on the print. Not having electronics background, that sometimes goes wrong. (Having written software since the the 70's that is more my business).

Kind regards,

Paul

RTJoe · Post by **RTJoe** » Sun Nov 13, 2016 5:02 pm

Hi Paul,

same with me, I'm also in the software business. Fortunately embedded software...

Some hints:
- It's really necessary to have full(!) sunlight for testing. I was very surprised of the very small change of voltage at pin 1 of LMC6484 when holding the "flashlight" led of a smartphone 1-2cm in front of the sensor.
- You can already check the voltage at pin 5 of LMC6484, should be 2.5V
- Without sunlight, you should measure about 2.5V at pin 1 of LMC6484 and the ADC of Arduino
- Use a variable resistor instead of the fixed 220 Ohm resistor to adjust input level
- If you have the possibility to use an oscilloscope, this would be vary useful in case of problems

Wish you a lot of success,
Joachim

PDB · Post by **PDB** » Wed Nov 16, 2016 9:44 am

Waiting for the sun, but did some measurements and testing. Discovered that I soldered 1 wire to the wrong place; after correcting that something started to work. Holding the smartphone led almost in contact with the BPW34 photodiode I was able to generate input values even higher than 1. Seeing values also showed some variation then. So looks like now it is just waiting for real life test.

Regards,

Paul

PDB · Post by **PDB** » Thu Nov 17, 2016 9:57 am

Hi,

with a bit of sun and passing clouds did some quick tests this morning. The board seems to be working but I get different results if I connect it to the Nano or the Arduino Mega. Results from the Mega seems more what I expect. The only difference I see between the 2 setups is the supply voltage. The Nano has a 0.3 voltage drop when power is given over USB. I measure 4.7 instead of 5. On the ATMega I measure 4.9V. Can that explain the difference?

SSSM with Nano (all seeing values with sun around 4)

: SSSM.JPG (70.83 KiB) Viewed 32981 times

SSSM results with Mega (much more variation in seeing values)

: SSSMMega.JPG (83.64 KiB) Viewed 32981 times

The test board (before the LMC6484 arrived ...) Still planning to make a smaller 1 but need to solve this issue first before I solder a Nano to the print.

: 20161110_143949_resized.jpg (183.31 KiB) Viewed 32981 times

Regards,

Paul

RTJoe · Post by **RTJoe** » Fri Nov 18, 2016 9:47 am

Hi Paul,

you are right, the results of the Nano version are strange while the results of the Mega version seem to be reasonable.

I also have discovered the lowered voltage for USB powered Nano, but this should not make a (big) difference because the measures are normalized to the measured intensity. You can use the more stable voltage by using external power of 7-12V on the Nano. But my devices are working with 4.7V USB power or external power without problems.

According to the input plot you had a lot of trouble with clouds. Maybe you should try again with clear sky (I know it's hard to wait...) and recheck the connections to the ADC inputs of the Nano.

One more remark: Is the big capacitor on your test board a standard electrolytic capacitor? Note that the schematic requires a non-polarized or bipolar one:

: NPCapacitor.JPG (9.82 KiB) Viewed 32961 times

Best regards,
Joachim

PDB · Post by **PDB** » Fri Nov 18, 2016 10:00 am

Hi Joachim,

The capacitor is of NP type so that should be ok. When directly displaying the values from the A0 pin, I see they are slightly lower with the Nano. There might just be some interference on the circuit, but to find that out i will need to find a scope. Could be from my USB ports.
(Had already been thinking of connecting a 9V battery to the VIN pin and GND to power the Nano and circuit board.)

Kind Regards,

Paul

GreatAttractor · Post by **GreatAttractor** » Fri Nov 18, 2016 12:41 pm

Great reading, guys! I'm thinking of building one myself.

RTJoe wrote:Hello!

Then I processed the recorded AVI with AVIStack2, which is able to export the image quality data to a CSV file (usually I use AutoStakkert, but I have not found a possibility to export the quality graph).Joachim

Good point; I'll add it to Stackistry in the next release (coming soon).

PDB · Post by **PDB** » Fri Nov 18, 2016 2:48 pm

some response to our thread on "Cloudynights" http://www.cloudynights.com/topic/55698 ... eshooting/

Paul

Post by **solarchat** » Fri Nov 18, 2016 5:38 pm

please be VERY careful with any posts from “Apollo".

He is one of two members that have ever been removed from this forum because of the content of their posts…believe me it takes an EXTREME occurrence for me to do that.

RTJoe · Post by **RTJoe** » Mon Nov 21, 2016 9:07 am

GreatAttractor wrote:
RTJoe wrote:Hello!

Then I processed the recorded AVI with AVIStack2, which is able to export the image quality data to a CSV file (usually I use AutoStakkert, but I have not found a possibility to export the quality graph).Joachim
Good point; I'll add it to Stackistry in the next release (coming soon).

Great idea! The next version of the plugin can write log files with seeing values for each captured frame. By comparing the seeing measures with the image quality we should be able to analyze possible improvements.

Looking froward to the next Stackistry release,
Joachim

RTJoe · Post by **RTJoe** » Wed Nov 23, 2016 10:30 pm

Hello,

plugin version v0.92 is released now.

This version now "officially" supports the SSM from AiryLab in FireCapture. Thanks to user rxdeath for testing:

: SSSMonScr_AiryLab.jpg (152.42 KiB) Viewed 21625 times

One main addition is the new chart window with graphical plot of the seeing values. This windows is a FireCapture window, it's no longer necessary to use additional software to display a chart.

: SSSMonScr_ChartEN.jpg (89.34 KiB) Viewed 21625 times

The plugin writes the average seeing value during a recording run into the FireCapture log file and optionally can write an additional log file with the seeing value for each captured frame. The format of this log file is CSV, it can be easily processed or imported e.g. into Excel.

A new firmware is also available, providing calculation of an alternative seeing value based on more (and longer) scintillation measures.

Plugin and Firmware are available for download here: http://www.joachim-stehle.de/sssm_eng.html. Please use the latest release version 2.5 of FireCaputue.

Joachim

PDB · Post by **PDB** » Thu Nov 24, 2016 8:08 am

Perfect job Joachim!

I will test it out a soon as I get some decent sunshine. The SSSM monitor with the Nano now seems to function as expected while testing during a short period of sunshine. (But then trees got in the way ...)

Kind regards,
Paul

PDB · Post by **PDB** » Fri Nov 25, 2016 6:32 pm

Hi,

so first real outside testing today.
This is what I got in the SSSM

: Measure1.jpg (174.31 KiB) Viewed 21559 times

I had the impression seeing was worse than what the device was reporting. Only could get this poor result. (but of course Quark runs at f/24.5 and that seemed to be to much today)

: PP2016-11-25-1019_5-PDB-Ha__SSSM_1.8.jpg (24.26 KiB) Viewed 21559 times

little bit later seeing got worse (also through the camera)
This data was collected using the larger number of samples

: Measure4.jpg (152.96 KiB) Viewed 21559 times

The plugin works as expected, starting at the right moment, pausing when needed. Now need to figure out what settings work best.

Kind regards,

Paul

MalVeauX · Post by **MalVeauX** » Fri Jun 02, 2017 2:48 pm

Any particular updates from anyone who has completed a monitor?

I'm interested in building one just for fun to monitor seeing while I'm at my scope.

I have no experience with boards, circuits, etc, though. May be beyond what I can do without someone helping or a video or something.

Very best,

PDB · Post by **PDB** » Fri Jun 02, 2017 5:32 pm

Hello Marty,

not that difficult to make (i have no experience in electronics either), but you need to be able to do a little bit of soldering on a strip- or veroboard.
What I found the most difficult thing was to mounting the sensor.
Difficult parts to find were the original sensor, so replaced in by the BPW34. That one is tiny and difficult to solder, but beeing cheep, I ordered 4 so i had some spare parts. Also the NP capacitor can be hard to find.

Seems I have thrown away my veroboard design (was just a sketch on paper). Will "clean" my desk a bit, maybe I can find it back.

As for the device, it works very well and is a great help reducing the captures to usefull and much smaller files than before.

Regards,

Paul

MalVeauX · Post by **MalVeauX** » Fri Jun 02, 2017 7:26 pm

Hi Paul,

Thanks. I suppose I'd have to start by finding out how to even interpret the schematic on the original PDF to know what any of it means. I actually found all the bits, the boards, the sensor and the amp, so I can get the parts quite inexpensively (though I have no idea what any of the wires, resistors, capacitors, or anything of that stuff is). More just a matter of knowing what I'm doing putting them together and how to load the software and how to have it output to a laptop and produce data to interpret.

I bought two of the sensors (VTP4085H), two of the LMC6484 amps, and an Arduino uno R3 board.

From here, I just need to know if I need to get a solder setup, or if I can get some kind of wires that allow me to use them without soldering so I can see if it works without it being permanent and ruining stuff. Jumper wires? I'm looking at small bread boards and wire kits that are commonly listed with the arduino boards. But I'm not sure of any of it.

I figure one to learn on, one to do right (maybe), once I get everything I need and figure this stuff out. And if it works out, maybe I'll build a few since I can get the parts so easily and inexpensively.

Too bad there's not a simple video tutorial out there to show one of these being built with instruction!

Very best,
Marty

PDB · Post by **PDB** » Fri Jun 02, 2017 7:32 pm

Marty,

give me a few days and I will reconstruct the board layout I made for a strip-board. (not that difficult)

Paul

MalVeauX · Post by **MalVeauX** » Fri Jun 02, 2017 7:39 pm

Thanks Paul, appreciate it.

BTW, I edited in some more information prior to this post:

"

I bought two of the sensors (VTP4085H), two of the LMC6484 amps, and an Arduino uno R3 board.

From here, I just need to know if I need to get a solder setup, or if I can get some kind of wires that allow me to use them without soldering so I can see if it works without it being permanent and ruining stuff. Jumper wires? I'm looking at small bread boards and wire kits that are commonly listed with the arduino boards. But I'm not sure of any of it.

"

Example: not sure if I can use this (to avoid soldering at first to at least learn with):

https://www.amazon.com/dp/B004RXKWDQ/re ... UTF8&psc=1

Very best,

PDB · Post by **PDB** » Fri Jun 02, 2017 7:48 pm

Need to check. In theory you could place the components on a breadbord and then connect with wires to the Arduino, but of course theory an practice do not always go hand in hand.

Very best

Paul

MalVeauX · Post by **MalVeauX** » Fri Jun 02, 2017 10:49 pm

Hrm,

So just going back and reviewing some material:

Looks like from the program, basically the IR sensor is measuring light intensity and its becoming converted to a voltage value that is monitored. Basically the intensity of the light change over time is being correlated to seeing quality. Does that sound correct?

I've watched some YouTube tutorials on the Arduino boards, basic circuitry, etc. Looked at the software to load the sketches onto the Arduino too. Seems easy enough for the most part. Looks like I can use solderless jumper wires to practice on a breadboard to make sure the circuit works before moving to a permanent build, so that's good.

So now I really just need to learn to read the schematic on the original PDF so that I understand what the symbols mean and how to follow the circuit design because when I compare the schematic in the PDF to the image of the breadboard, I'm not able to follow it very far before losing where the circuit is going. For example in the PDF I read about 5 instances of a resistor (values 220, 47k, 10k, 10k, and 20meg) and I see them on the board, but I guess I'm getting lost on where it translates to the mirror side of the board and back.

So really I don't think I'm able to read the schematic yet.

Looks like the microprocessor is referenced 4 times (U1 through U4) and that there are two capacitors (non polarized) in the schematic? One that's 3.3 micro-ferads and one that is 20 pico-ferads, right?

Also, I probably need to figure out how to connect the photodiode to a cable, like the shielded RCA, so that I can mount that on the scope and run the cable to the board near the computer. That will take soldering I'm sure, so I will have to get into that a bit too. But I don't mind testing it without a cable at first just to ensure its working and to get the voltage correct at that stage (to 1V). I think I'll get a variable resistor to attenuate it to 1V, even though it says 220 ohm resistance, but says to change it to get to 1V).

Very best,

PDB · Post by **PDB** » Sat Jun 03, 2017 9:15 am

Marty,

will send you a private message.

Rgrds,

Paul

MalVeauX · Post by **MalVeauX** » Sat Jun 03, 2017 9:23 pm

Hey all,

So I'm trying to make sure I get the right components per the original PDF, just want to make sure I'm reading it correctly and found the correct items if anyone familiar wouldn't mind confirming:

10k Resistor x 2: http://www.mouser.com/ProductDetail/KOA ... YulfaVM%3d

20 Meg Resistor x 1: http://www.mouser.com/ProductDetail/Ohm ... lJaLh0I%3d

47k REsistor x 1: http://www.mouser.com/ProductDetail/KOA ... wvink34%3d

Capacitor 3.3 micro Ferad non-polar x 1: http://www.mouser.com/ProductDetail/Vis ... %252b7o%3d

Capacitor 20 pico Ferard x 1: http://www.mouser.com/ProductDetail/Vis ... W9kYh4o%3d

Very best,

PDB · Post by **PDB** » Sun Jun 04, 2017 1:27 pm

The 3.3 mF capacitor should be of type NP. Could not see that from the spec. Looks like a standard one. (there seems to be a way to use normal capacitators, if you use 2, but then the value will be different, and that is far beyond my knowledge)

Paul

MalVeauX · Post by **MalVeauX** » Sun Jun 04, 2017 3:42 pm

Turns out all most all of that above is the wrong stuff. I didn't know the difference between carbon film, metal, etc, none of this is in the schematic so I have no idea what the differences really are, there's so much as it is.

If anyone is familiar with the stuff and can link the items per the original PDF, that would be really helpful.

+++++++++++

Looks like I need 1/4watt, 5% carbon film. I'd have never known! Hah.
I guess the size of the capacitors and all that eluded me as well.
Also, no idea how to know if the potentiometer for the variable resistance would have the ranges needed to achieve something around 220ohm, or whatever it turns out to be to achieve the 1V at the photodiode.

++++++++++++++

Let's see if I can get any of this right:

10k Resistor x 2:
http://www.mouser.com/ProductDetail/KOA ... %2f2Y24%3d

20Meg Resistor x 1:
http://www.mouser.com/ProductDetail/KOA ... XXOMQTs%3d

47k Resistor x 1:
http://www.mouser.com/ProductDetail/Yag ... CvOsHqQ%3d

Is this an appropriate 3.3 uF non-polar capacitor?
https://www.parts-express.com/33uf-100v ... mpaign=pla

Is this an appropriate 20 pF capacitor?
http://www.mouser.com/ProductDetail/Vis ... hgodkEcL8A

No idea how to even select an appropriate variable resistance, or potentiometer. Or if I follow the original schematic, a 220ohm is used, so should be possible to just do that again. But if I extend the sensor off the board with a cable, will that change anything?

Very best,

PDB · Post by **PDB** » Sun Jun 04, 2017 4:41 pm

As variable resistor use one for 2K (that will give all the range from 0 to 2K). When testing you adjust it until you get an input value between 0.6 to 0.9 when the cell is in full sunlight. (even 1K value could be ok)

The capacitor looks ok.

Rgrds,

Paul

MalVeauX · Post by **MalVeauX** » Sun Jun 04, 2017 4:58 pm

Thanks,

Way more complicated even just finding parts that are appropriate than anything it seems!

Very best,

MalVeauX · Post by **MalVeauX** » Thu Jun 08, 2017 1:21 pm

So, I have most of the parts now.

I have:

VTP4085H sensors
LMC6484 I/O OP Amp
Arduino Uno R3 Controller
Breadboard & Jumper Wires

I haven't picked up any resistors, capacitors, etc, yet because I can't seem to find exactly what is needed due to my inexperience and lack of education in this kind of stuff. I at least know the type of basic resistors I need, but I'm not confident with the capacitors, and I'm really not sure of the variable resistor pot to make sure its the right kind and range of values. And then, I still am not confident on how to set it up on the board, but I can at least play with it a little with the jumper wires. I have multiples of everything above in case I torch it.

I've linked some resistors, capacitors and a variable resistor pot above; if someone can help me confirm what I can/cannot use. I'm trying to follow the original PDF design as closely as possible. If I can make it work, I will then look into extending the sensor off the board and onto an audio cable or something to get it onto the telescope. But that's later, if I can get this to work.

?

Very best,

RTJoe · Post by **RTJoe** » Wed Jun 14, 2017 11:16 am

Hello,

I guess the parts you have selected should work, but here is some information about the parts I have used:

This is the 3.3 uF non-polar capacitor I have used:
http://www2.mouser.com/ProductDetail/WI ... DLXjc8zLm2

Please check the value of your 20 pF capacitor; when I click on your link there is a 68 pF capacitor displayed.

The resistors don't seem to be critical, but I have spend some cents more and used metal film types with 1% tolerance only.

I would recommend (and have used) a trimming potentiometer of 250 or 500 ohm instead of the fixed value 220 Ohm resistor. This allows easy adjusting of the input value.

Of course it's a good idea to start building on a breadboard with jumper wires. If everything works, you might start soldering.

Best regards,
Joachim

MalVeauX · Post by **MalVeauX** » Thu Jun 15, 2017 6:25 pm

Hi Joachim,

Thanks for the info.

That capacitor you linked, how is it designated to be non-polar?

Thanks!

Very best,

RTJoe · Post by **RTJoe** » Fri Jun 16, 2017 7:48 am

Hi Marty,

the capacitor I have linked is a film capacitor, these capacitors are nonpolarized by nature. The capacitor you have linked is an electrolytic nonpolarized (bi-polar) capacitor, that means actually two polarized capacitors in series. I guess both capacitors should work, but I'm no expert about the details.

Best regards,
Joachim

MalVeauX · Post by **MalVeauX** » Fri Jun 16, 2017 11:41 am

Thanks, that helps!

Very best,

MalVeauX · Post by **MalVeauX** » Sat Jun 17, 2017 1:04 am

Any chance there's a 20pF capacitor on there? I just looked under film and could only find a 22pF. Just trying to follow the PDF at the beginning of this thread, if that helps.

Very best,

GreatAttractor · Post by **GreatAttractor** » Sat Jun 17, 2017 3:35 am

All right, I'm joining the SSM club! Parts have been ordered - I'll use BPW34 diode too, and an LM224AN op-amp; parameters similar to LMC6484, we'll see how it goes...

I wanted to understand what's what, so I've tried to recall the electronics course (wasn't my favourite one) from studies, and also read the original article on solar scintillation by Dr. Seykora. Eq. (5) therein (page 391) gives the angular diameter of the average “seeing cell”:

ω = θ (ΔI / )

where θ is Sun’s angular diameter, ΔI the RMS of irradiation intensity changes (our second ADC input), is average intensity (our first ADC input).

The 4.46 coefficient in output formula is due to the fact that we want to express ω in arc seconds, and the circuit magnifies ΔI by a factor of 20M/47k ≅ 425.5 (which we have to cancel out in our final calculation), while we can take the Sun to be ~1900 arc sec in diameter, so:

ω [arc sec] = 1900/425.5 * (ΔI / ) ≅ 4.46 * (ΔI / )

As for the 10 000 samples taken for each returned seeing value, it seems it's due to Arduino's ADC read-out rate:

It takes about 100 microseconds (0.0001 s) to read an analog input, so the maximum reading rate is about 10,000 times a second.

i.e. we sample the RMS and the average over a period of 1 second.

The photodiode also checks out; according to specs, it produces 50 µA photocurrent at 1 mW/cm² irradiance. In full Sun (assuming 1000 W/m² = 100 mW/cm² irradiance), and converted to voltage and amplified 220x (due to the 220-ohm resistor) by the first op-amp, it should be about 1.1 V. Of course it's good to have a variable resistor to fine-tune it.

I'm wondering about filtering; Seykora's article mentions using filters which gave 220 nm FWHM at 510 nm. If the SSM is used during an Hα imaging session, shouldn't we e.g. slap a red filter on the diode to give more meaningful seeing values? Would there be any noticeable difference anyway?

P.S. Marty, I'm taking my chances with a 22 pF too.

PDB · Post by **PDB** » Sat Jun 17, 2017 3:41 pm

Hmm ... my answer from this morning seems to have vanished.
Interesting remark GreatAttractor. Now I remember the filters in the original document. I did a small test with a red Wratten 25 filter (that was all I had on hand). What I expected the input value dropped by approximately 0.2 but calculated seeing values did not change very much. (Difficult to compare in a short test, not reallt scientific, under varying sky conditions, more work needed.

Some remarks on the builds
A 22 pF capacitor will work (using it as well)
I am using the cheap Arduino Nano's but find these fairly unreliable. Also the mini USB connector is not very sturdy. With the SSSM i get sometimes hangups (could als be due to the one wire LCD display I am using, that sometimes hangs on startup as well). But I also made a focus controller with the same nano model and it works very well excexpt that it sometimes reset and all stored values are set to default. (Very annoying if you count on these positions to make sure the focuser does not go outside it boundary. The very strong stepper motor could the ruin the gearbox or the rack-and-pinion assist rail) So I decided to do some rebuilding and test it with an UNO which looks much better and has better usb connectors.

Regards,

Paul

GreatAttractor · Post by **GreatAttractor** » Sun Jun 18, 2017 9:01 am

I've just realized it's perhaps better not to use filters; if all/most of us use just a naked diode, it'll make sense to compare seeing values among all SSM users (not depending on people using exactly the same filters etc.).

Also, if I'm not mistaken, the U2 op-amp together with the 47k resistor and the 3.3 µF capacitor form an active high-pass filter with cut-off frequency of 1/(2π · 47k · 3.3µ) ≅ 1 Hz, so I suppose that's why the average () is obtained by summing over 1 s. The faster-changing portion of the intensity signal goes to the RMS calculation.