Gina's DIY All Sky Cam - Complete Redesign

[Gina]

My other thread contains so much detail of the design I have now rejected that I thought a new thread was indicated.  I am pretty much starting from scratch as apart from the lens, camera and dome everything will be new and totally different.

Camera - QHY5

Lens - Fujinon YV2.2x1.4A-2 - f1.4 1.4-3.1mm FL fisheye zoom lens

Dome - Acrylic 80mm OD - sold as housing for CCTV outdoor cameras

Since the QHY5 may be damaged by focussed direct sunshine falling on the sensor I shall be providing a shutter mechanism to shield the sensor in daylight.  Ideally the shutter should close if the power supply fails or the sensor would be vulnerable to sun damage.  This precludes any mechanism that relies on power to close the shutter.

Having tried various "blind alleys" for the shutter mechanism I am going to try a very lightweight shutter blade driven by an analogue meter movement as has been used by another member (or members).   There are so many problems with my other idea of using Baader solar film to use the camera in daytime that I have decided to just go for a night-time only "All Sky Cam".

The Fujinon lens has a manual shutter (of a sort) operated by a lever on the side, but the power available from a meter movement would not be enough to operate it.  Hence, I am taking the QHY5 sensor PCB out of the case and mounting it separately and putting the shutter between lens and image sensor.

The meter movement I am using is a very cheap plastic cased ammeter (without shunt) which I have modified by cutting off the dial and gluing a small piece of kitchen foil to the pointer.

[leelee970]

Looks interesting, I think I'll follow this one seeing as I've a spare QHY5.

Lee

[Gina]

Testing the meter operated shutter.

Unpowered with shutter covering sensor.

And with 3mA flowing through the meter coil and shutter open.  Bench PSU supplying 3v through 1K resistor.

[Kropster]

That shutter mechanism is a stroke of genius!

[Gina]

Now sorting out a circuit to drive the meter.  The LDRs I've got drop to about 25 ohms in sunlight and 70 ohms with bright cloud going up to 2K when I put my hand over the LDR.

Here is a circuit diagram.  The 6.2v zener diode means the any LDR resistance over about 1.8K will start moving the shutter open.  If I find this a bit high I could add another resistor in series with the diode and reduce the 1.8K.  The extra series resistor would then limit the meter current in the dark.  The circuit could be refined further to make the meter more responsive as the daylight fails but I'm loathe to make the circuit any more complicated than necessary.

[Gina]

That shutter mechanism is a stroke of genius!

I'm afraid it's not my idea

[Horwig]

Beware the weight of the foil shutter Gina, it's weight on mine was overcoming the return spring, so if the assembly was out of level the shutter would move off its zero position.

H

[Gina]

Ah - I had a feeling it was you who used the meter idea Huw but I couldn't find the posts.

Yes, the weight of the foil far outweighs the return spring on my meter movement and I know it will need to be level or if anything slightly biassed towards the foil covering the sensor.  I could arrange stops to limit the movement.  The shutter only needs to move about a third of FSD to clear the sensor so I estimate the FSD current to be 10mA.  By having end stops for both limits of shutter movement I could make the dark current to the meter 10mA and obtain three times the force exerted by the meter coil spring at present.  I think the meter coil would take several times the FSD current quite safely but I'll try 10mA to start with I think.

[Cath]

You must have a box full of canon shutters Gina, can't you use one of those ?

[Gina]

I did try one of those Cath, but the motor overheated if you held the shutter open and it's kept open for long exposures in "T mode" with an electromagnetic latch which needs power to close the shutter.  This makes it non-failsafe

[Cath]

I did try one of those Cath, but the motor overheated if you held the shutter open and it's kept open for long exposures in "T mode" with an electromagnetic latch which needs power to close the shutter.  This makes it non-failsafe

oh I see.

Well, that's easily overcomable.

If you use a very simple/basic drive circuit to open/close the shutter, and if you give it enough energy reserves (a super capacitor is many times more than enough) to operate without external power for a short period, it could simply close the shutter when external power is lost and/or when the external 'open' signal is lost, or when it looses whatever signal you feed to it to tell it to open.

[Gina]

Here's a modified circuit diagram running the meter at just over 10mA in dark conditions.  The shutter would tend to open when the LDR resistance dropped below around 560 ohms or maybe somewhat less if the balance is biassed towards shutter closed.  I think this might work quite well

[JamesF]

I honestly believed there is some risk of sensor damage if the lens is uncovered in full sun Gina.  However, a comment from Sam @ ZWO some time back suggested that it may well not be an issue.  His English is fairly good, but not so good that I'm willing to risk an error in translation, so I am wondering if it might be a useful exercise to set up the lens focused on the Sun and measure how much heat it generates at focus.  We all know about setting fire to stuff using a magnifying glass I'm sure, but we're talking about lenses that are considerably smaller here.

I'm just wondering if this is a problem that doesn't actually need to be solved?

James

[Horwig]

Good point James, does the sx all sky have a shutter?

H

[Gina]

oh I see.

Well, that's easily overcomable.

If you use a very simple/basic drive circuit to open/close the shutter, and if you give it enough energy reserves (a super capacitor is many times more than enough) to operate without external power for a short period, it could simply close the shutter when external power is lost and/or when the external 'open' signal is lost, or when it looses whatever signal you feed to it to tell it to open.

Yes, that's true but beginning to get complicated.

[Cath]

If it does require a shutter, then a small solar cell could also be used to activate a thin narrow band filter or shutter or strong ND filter - no external power needed then.

[JamesF]

Good point James, does the sx all sky have a shutter?

H

If it does, it's not at all obvious.

James

[Horwig]

Truth is, for me, with the amount of cloud of late, it's not going to be an issue   

Huw

[Gina]

I honestly believed there is some risk of sensor damage if the lens is uncovered in full sun Gina.  However, a comment from Sam @ ZWO some time back suggested that it may well not be an issue.  His English is fairly good, but not so good that I'm willing to risk an error in translation, so I am wondering if it might be a useful exercise to set up the lens focused on the Sun and measure how much heat it generates at focus.  We all know about setting fire to stuff using a magnifying glass I'm sure, but we're talking about lenses that are considerably smaller here.

I'm just wondering if this is a problem that doesn't actually need to be solved?

James

Yes, I've seen several examples of ZWO cameras being used with the same Fujinon lens for all sky cams where there seemed to be no protection from the sun whatsoever.  I don't know how you could measure the temperature of such a tiny area as would be produced by the sun when focussed with these fisheye lenses.  I guess you could put some paper at the focus and take the whole lot outdoors in the sunshine and see if the paper got burnt or discoloured.  I have Microsoft webcams with a 60 degree FOV that I have used in full sunshine without noticing any damage, so maybe the QHY5 might be alright.  Both are CMOS technology, I gather.

[Gina]

If it does require a shutter, then a small solar cell could also be used to activate a thin narrow band filter or shutter or strong ND filter - no external power needed then.

Yes I thought of that.  Thanks for the suggestion though

[Gina]

Interesting point about cameras that have shutters!  I had forgotten that.  There are indeed some astro cameras that have internal shutters and with the shutter not exactly in the focal plane, the heat produced on the shutter may well be tolerable.  So we need to find out which cameras have shutters.  Do we have any definite info about the ZWO ASI series for instance?  I know the QHY5 doesn't have a shutter.

[Gina]

Been doing some Googling...  This is an interesting quote regarding ZWO cameras :-

Features of the New ZWO ASI174MC Camera

Recommended for the Sun, Moon and some deep sky objects
(To image the Sun you'll have to use a proper front solar filter or Herschel wedge depending on your telescope type. Please contact us if not sure!)

Note the part I have highlighted in red!

[JamesF]

The SX Oculus uses a ICX205AL sensor.  I don't think that has a shutter.  It's clearly intended to be able to leave it out in the Sun, too.

James

[JamesF]

Been doing some Googling...  This is an interesting quote regarding ZWO cameras :-

Note the part I have highlighted in red!

I think that's probably relating to use with a telescope rather than a small CCTV-type lens.

James

[Horwig]

here's the Oculus pdf, no sign of a shutter. They even go as far as quoting the chip spec as 'no need for a mechanical shutter'. but this might just be copyrighter's exuberance!

http://www.sxccd.com/Oculus/Oculus_Product_Brochure.pdf

This camera even uses auto iris to shut down to f11 in daylight, spot size must be tiny then, still no sign of shutter

http://www.alcor-system.com/us/AllSkyCamera/OMEA_cameras.html

Whilst this one DOES have a shutter

http://www.alcor-system.com/us/AllSkyCamera/EUDA_cameras.html

The sbig allsky 340 has a shutter, but that is for taking darks only

https://www.sbig.com/site/assets/files/18247/allsky340_manual.pdf

I know that this is not a scientific asserssment, but it looks as if the shutter might be a luxury we don't need.

Huw