Gina's DIY All Sky Camera - Mark 4 - with ZWO ASI185MC

[Gina]

A new version of my ASC using a new imaging camera for lower noise and higher resolution but with the same Fujinon zoom lens.  Previously Mark3.

I bought a ZWO ASI185MC for planetary imaging but the weather and short dark periods at this time of year are discouraging me from this.  This camera should make a great all sky camera in conjunction with the Fujinon zoom lens.  The 2.5mm lens supplied with it doesn't quite give the 180° coverage I would like and doesn't have an aperture control.

The ASC needed a rebuild of the drive system and electronics as I'm changing to two 28BYJ-48 12v stepper motors for focus and aperture control instead of a micro servo motor for the aperture, which proved unreliable.

[Gina]

Here's a photo of lens and camera.  The USB3 plug is quite long and the case will have to accommodate this as I want to seal it against damp.  Of course it also has to cover the two stepper motors that operate the lens.

[Gina]

For all sky use the camera doesn't need USB3 so a USB2 plug which is shorter could be used.  The stepper motors could be put either side of the plug I think.  The electronics board can go underneath and any LED shielded from the imaging area.

[Gina]

I'm wondering if the ASI185MC camera sensor is thermally connected to the case to dissipate heat.  If it is I could further conduct the heat away to the environment by mounting it on an aluminium plate and fixing this to the aluminium mast.  It may be that the noise is too low to matter but another idea is to cool the camera further with a Peltier TEC.

[ChrisLX200]

17 minutes ago, Gina said:

I'm wondering if the ASI185MC camera sensor is thermally connected to the case to dissipate heat.  If it is I could further conduct the heat away to the environment by mounting it on an aluminium plate and fixing this to the aluminium mast.  It may be that the noise is too low to matter but another idea is to cool the camera further with a Peltier TEC.

Yes it will be, the case is used as a heatsink. On the other hand, you want some heat in there to keep exterior condensation and dew at bay (unless you also use an auxilliary heater). The thermal noise can be controlled to a large extent by using darks.

ChrisH

[Gina]

Thank you Chris

I plan to have it in a sealed dry chamber if I can, probably with bags of desiccant (colour change silica gel) but I need to heat the dome to keep condensation off the outside of it so may divide it into two temperature zones though since heat rises, this may not be necessary.  Anyway, I'll see how it goes without extra cooling.  Changing the design as I go along is not a problem - have 3D printer (or 3) and can print new parts   Changing the electronics is also no problem.

Talking of electronics I have in mind using a Raspberry Pi maybe with WiFi or Ethernet link to my main PC indoors instead of laptop in my warm room.  This will be for the future though I think and I'll probably use my tried and tested Arduino systems for control and connected to local laptop running either Win 7 or Linux Mint to get things going.

[Gina]

Testing the camera and lens together shows that a thin spacer is needed to get focus at infinity - same as with the QHY5L-II-C.  In daylight with the exposure at the minimum of 32μs it needs the aperture closed down quite a bit.  Hence the need for aperture control.  Also, it needs to be able to be closed to take darks.  Manual focussing with the focus lever is not accurate enough and hence the need for remote fine focussing.  The little geared (approx 64:1) 28BYJ-48 stepper motors are ideal when coupled to the levers by 3D printed parts (is in Mark 3).

Next step is to measure up and design the plate to carry the motors and the cages to operate the lens levers.  These can be based on the Mark 3 designs.

[Gina]

Here is a snap of the view through the living room window focused as well as I could by hand.  Cropped and saved as PNG for upload here.

[ChrisLX200]

A wireless option would be very nice indeed, I wish I had one - I would then put it on the roof where it would have a much better horizon view. I could then see the clouds approaching from further away! My usual exposures are 3 seconds for this job (mono QHY5L-II camera) which gets pretty deep with an f/1.4 lens, and one frame every 3 seconds should not be an onerous task for WiFi?

ChrisH

[Gina]

Another snap saved as JPG this time.

[steppenwolf]

Quote

Testing the camera and lens together shows that a thin spacer is needed to get focus at infinity

That's strange that you need a spacer, Gina - my ZWO ASI 120mm based ASC (from the same stable) achieves infinity focus easily with adjustment either side when using the same Fujinon lens as yours. I would have thought that ZWO's range would all have the same backfocus so it is a bit of a puzzle.

My heated transparent dome (using Nichrome resistance wire and a PWM  generator) works very well indeed - I have yet to have a dew issue! Darks are an issue for automated use but I just throw a black cloth over the dome to capture them - shortly to be replaced with a black box that will lower over the ASC manually (for now!).

[Gina]

It may be that the ASI185MC having a bigger sensor is using a different zoom setting which affects focus.

The Fujinon lens blocks off all light when the combined aperture/shutter is closed which is very handy for darks.  I too use Nichrome wire for my dew heater and find it very effective (that was on the Mark 3 - I shall be using the same heater on the Mark 4).

[Gina]

Taking the focus and aperture cages and motor pinion from the Mark 3, this photo shows the stepper motor arranged to clear the ZWO camera.  The next version will be slightly different.  The motor mounting plate will sit on the step in diameter of the camera so the motor will be slightly lower.  To have the two motors almost side by side rather than opposite the gear quadrants will overlap but be at different levels and be driven by motor pinions at different heights.  Also I can design the lower gear (aperture) to sit directly on the camera rather than having a spacer.

[steppenwolf]

Quote

It may be that the ASI185MC having a bigger sensor is using a different zoom setting which affects focus.

Yes, this is a possibility, the focus certainly does change with focal length change. As a rough approximation, I estimate my focal length at about 1.5mm.

[steppenwolf]

I like this solution to focus and aperture control - I really do need a 3D printer!

[Gina]

I could print the parts for you Steve   Having a 3D printer already the cost is insignificant.

[steppenwolf]

That's very generous of you, Gina, thanks. I haven't decided for certain that this is the lens that I am going to stick with as I am also experimenting with a fixed focal length Arecont lens at the moment but I would like to keep your offer in mind if I may?

[Gina]

By all means Steve   Please let us all know how you get on with the new lens.  I can't say I'm entirely happy with the Fujinon lens - it loses resolution around the edges but this is likely to happen with a fisheye lens.  One thing I noticed with the 2.5mm FL lens that came with the camera was that it was already well focused for infinity.  The ASI185MC image sensor almost covers the image - I think they say 170°.  The biggest problem is no aperture control - the image whites out in daylight with minimum exposure.

I'm really interested in ultra widefield astro imaging and prepared to spend some money on a good ASC.  I'm wondering if I've damaged my Fujinon lens as the image is not centred on the image sensor.  I think we all know that a fixed focus lens will outperform a zoom lens and I would prefer a prime lens over a zoom if I can find one with the right FL and aperture/shutter control.  In previous ASC incarnations I looked into several ways of adding a shutter before someone suggested the Fujinon lens.

One way out of the problem of an ASC that covers both night and day would be to have two and if I used a different lens for nighttime I could use the QHY5L-II-C for daytime and the ZWO for nighttime.  That would mean I wouldn't need aperture control for nighttime imaging.

[ChrisLX200]

I suppose one question would be - do you really need it to work during the daytime? What is the value? Having the camera functional at twilight as darkness falls would be useful I guess - it would be a predictor of whether it will be clear very early on, but apart from that a live view of either clouds or blue sky is pretty much useless? I do have a standard webcam which I operate during the daytime (MS LifeCAM) which is adequate, automatic exposure/focus, and who cares if it doesn't work during the night because the AllSky cam takes over. I was thinking of swapping the LifeCAM out for a decent IP cam with PTZ capability.

ChrisH

[Gina]

That pretty much corresponds to what I was saying about having two ASCs one for day and one for night.  I've been comparing the ZWO lens with the Fujinon and I think the ZWO is superior in quality.  It is f1.2 compared with f1.4 of the Fujinon and it's prime compared with zoom.  The focus has already been set for infinity and won't need remote control though I'm not sure about the dome - I found this changed the focus slightly.  I might be able to arrange an external shutter for darks.  The view to the north doesn't quite go down to the horizontal so I might get away with slight lack of coverage.

[Gina]

I've put the ZWO lens on the camera and connected it to my laptop.  The image is far from well covered by the sensor   It would need to be almost as high as it is wide to give 180° of view.

[Gina]

The camera I intend to buy for DSO imaging, the ZWO ASI1600MM-Cool USB 3.0 Mono Camera, later on would do the job admirably other than not showing star colours.  But I can't afford that yet.  The colour version is cheaper but would be for ASC only and I think that would make the ASC more expensive than I want.  Another possibility might be to use one of my Atik 460EXs but the pixel size is bigger.

Once I get to try the new ASI1600MM for DSO imaging I might find these better than the Atik CCD cameras - they are certainly extremely interesting and right at the forefront of imaging technology.  At the forefront of technology is where I like to be   This new camera plus the SW Esprit scope looks like an outstanding combination and I'm getting exited at the thought of trying it   Again, I digress...

[Gina]

This lens from Orion looks interesting   Half the focal length of the ZWO so the whole 180° image would fit on the ASI185MC camera sensor.

[Gina]

Here is a diagram showing the coverage of various lens focal lengths for 180° FOV.  Outermost with 7mm diameter is the ZWO 2.5mm lens.  The Orion lens at 1.25mm is half that and the inner circle.  The ideal FL would be about 1.7mm as in the intermediate circle.  I won't be going with the Orion 1.25mm FL lens as that wastes too may pixels.

[ChrisLX200]

Depends whether you go for a C or CS mount lens, the latter mostly cover 1/3" sensors whereas the C-Mount lenses tend to cover 1/2" (12.7mm) sensors. Either will fit your camera, the C-Mount would need an extra 5mm spacer. C-Mount also _tend_ to be of better optical quality as this format is often used for machine vision devices but, as in all things, there is considerable variation. For 1/3" sensors a 1.8mm CS lens gives you 180deg coverage with a circular fish-eye FOV - and a total waste of time I would have thought for your ASI185 because you can get all there is to see onto the 1/3" sensor! A 2.1mm f/l lens gives you a 150deg FOV and is a good compromise as it cuts off much of the useless horizon area. The rectangular shape of the ASI185 sensor doesn't lend itself well to AllSky camera use because inevitably there would be [a lot of] waste pixels if you wanted to cover the whole sky, a much better prospect though if you were willing to accept a smaller FOV as you'll take advantage of the extra resolution. Stock Optics is a useful source of lenses, and Bern @ Modern Astronomy stocks some really nice lenses (though none are automatic iris models).

ChrisH