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

[Gina]

Tonight looks better - forecast is partly cloudy.  I need to get the matt black paint out for the knurled focus ring on the lens - or cover it with flocking paper (or maybe the black ABS I used for the spider because that shows no reflection).

[Gina]

Ten to nine and improving

[Gina]

2140.

[ChrisLX200]

I can't decide what the weather is going to do here, Sat24 is not promising at all by the looks of it, but right now it's clear so I have the 178MM outside.

ChrisH

[Gina]

Hope you get clear sky after dark Chris   Clouds are coming and going here.

[ChrisLX200]

This is with the 178 @ 4secs, gain 200. 2.5mm lens.

 

Oh, I have the camera attached to the side of the shed rather than out in the middle of the lawn - so it's quite near to the hedge and shrubs!

ChrisH

[Gina]

Half past ten and a clearish patch of sky in the middle.  Exposure up to 20s now.

[Gina]

hmm...  Hole in the clouds is disappearing

[ChrisLX200]

It's coming and going here too... I got up to 15sec exposure. You can see the poor edge performance of this 2.5mm lens if you click the image and view full size. The centre (and around Lyra) the stars are quite nice, but near the horizon they turn into comets!

 

ChrisH

Edit: I just noticed, SharpCap's timestamp is wrong - for some reason it's an hour behind! Computer clock is correct though...

[Gina]

Yes, I can see that.  My Fujinon zoom lens does that too but I don't think it's quite that bad.  Could it be the dome causing a problem?  I found that if I didn't have the front of the lens at the centre of curvature.

Total cloud cover here now   I'm running a video capture in the hope of clearer sky later but I'm not hopeful.  My exposure is now 60s.

[ChrisLX200]

There is no dome   The camera and lens are mounted on a rail attached to the side of the shed, no casing. I just found a 2.1mm lens I can try but it's only a small 1/3" CS lens.

ChrisH

[Gina]

I see.  Clearly your 2.5mm lens is not the same as mine.  I think a 2.1mm 1/3" lens should be alright.  Might give a smaller image circle than the height of your sensor though.  Just have to try it and see.

[ChrisLX200]

Out of interest, I found this table which shows the sensor coverage for different format lenses:

ChrisH

[Gina]

That's useful

[ChrisLX200]

Well tonight was never going to be any use for DSO imaging - too many clouds around   Still, it was a good opportunity to test the ASI178MM and so I created a short video. I've uploaded it to YouTube and you can view it in 4K resolution if you go to their website. SharpCap saved the individual frames as PNGs (each 20sec), then I used VirtualDub to stitch them together as an AVI encoded using H.264.

ChrisH

[ChrisLX200]

A couple of points of interest:

The focus locking mechanism for these small lenses is crude in the extreme - one screw on the side which just pushes against the threaded barrel to hold it place and stop it rotating. However, it seems that tightening this screw cocks the barrel over to one side thus throwing the lens off-axis. This is the cause of the edge distortion of my 2.5mm lens  - it's only really noticeable on the RH side of the frame which is what made me suspicious. Leaving the locking screw loose reduces the problem markedly!

I tried the 2.1mm CS lens (1/3" format) on the ASI178MM and it produced seemingly identical FOV to the 1/2.5" format 2.5mm CS lens. I expected the 2.1mm lens to offer a larger FOV as it has a shorter f/l compressed into a smaller image circle. Puzzled by that one but the image quality was inferior to the 2.5mm lens so I won't be using it anyway.

ChrisH

[Gina]

That's strange Chris.  I too would have thought the 2.1mm lens would have given move FOV.   I know this applies to normal lenses but maybe fisheye lenses are different.  I know the physics of normal lenses but not really fisheye.

[ChrisLX200]

It seems every time I think I know how something works I am proven wrong!  

ChrisH

[Gina]

Oh, I know what you mean!!  With knobs on

[Gina]

Been looking into thermally coupling the camera to the aluminium mast to cool it.  ATM being enclosed in a plastic box it warms up to something like 30-35°C.  This causes hot pixels with a 60s exposure.  With the camera in the open air with no case it was alright - no hot pixels or other noise. 

This would be the ideal solution - turned on the lathe from cylindrical aluminium stock.  Only problem is that there's a lot of metal to be removed and my lathe doesn't like doing that - it would take a long time.  I do have a suitable lump of ali - 70mm diameter by 50mm long.  The camera could be attached by central quarter Whitworth screw and/or by the M4 threaded holes by screws through the flange.

[Gina]

This would involve less turning - going inside the aluminium tube I'm using as a mast.

[Gina]

This is a scale model in SketchUp.   To make this would need a large diameter parting off so I think I'll modify the design.  The flange could be thicker - the M4 screws can be longer to match.  I plan to attach the 3D printed case with the M4 screws with the bottom of the case below the flange.  With a thick flange it would be possible to route cables down the inside of the mast by having a radial hole through the flange joining up with an axial hole through the middle.  New model coming up...

[Gina]

Yer 'tis   This makes use of the entire piece.  The flange is 20mm thick and the part that goes into the pipe is 30mm long.  I have faced off the top in the lathe and can just turn the piece round in the chuck to turn the rest.  The chuck is quite accurate enough for this purpose - the optical axis of the camera doesn't need to be accurately aligned with the axis of the mast.

[Gina]

No rain forecast until tomorrow afternoon so I've set up the ASC without case and dome - I hope to finish turning the aluminium thingy tomorrow.

Some clear sky forecast for tonight.  Here's a screenshot.

[Gina]

And the latest.