Gina's DIY All Sky Cam - Complete Redesign

[Gina]

I'm afraid it's not that simple.  There aren't any markings and infinity is not at the end of travel.  Focus is very critical.  The geared down manual control was usable but not easy to use.  Focussing outdoors in an exposed position at night in a "thin wind" isn't nice!

[Gina]

oacapture is working fine   See pic below.  But if I could make a suggestion, James, having the controls down the side of the preview window rather than on top would help with widescreen monitors/LCDs as the image is almost square.  EDIT OOps think I've found it

[JamesF]

oacapture is working fine   See pic below.  But if I could make a suggestion, James, having the controls down the side of the preview window rather than on top would help with widescreen monitors/LCDs as the image is almost square.

In the "Settings" menu, click on "General" and you'll see an option near the bottom "Display controls on right".  Check that, save and close the settings window and restart the application

James

[Gina]

Yes, found it - thank you Janes   Perfectly intuitive if you bother to look   Well done

[Kropster]

I'm afraid it's not that simple.  There aren't any markings and infinity is not at the end of travel.  Focus is very critical.  The geared down manual control was usable but not easy to use.  Focussing outdoors in an exposed position at night in a "thin wind" isn't nice!

I see.

But once you have finalised the setup, will you need to make any further focus adjustments?

[JamesF]

Focus position may change with temperature I guess.

James

[Cath]

I wonder exactly which part of Gina's setup is creating the temperature/focus gradient/dependence ?

[JamesF]

Presumably it's possible for it to be completely unrelated to Gina's kit?  Atmospheric pressure/density might also account for focus changes?  I'm guessing here...

James

[Gina]

I think it quite likely due to ambient temperature changes - we get that with telescopes which are less critical for focus.  My kit might be more susceptible due to the plastic shutter box.

I should perhaps add that I haven't noticed any focus change myself but I haven't been testing for long.

[Horwig]

Interestingly, this is what SBIG have to say about focusing their AllSky340

'Fortunately focus holds over temperature changes fairly well. Sometimes the day/night temperature cycling can initially cause some focus shift, but it should settle down after that.'

H

[Kropster]

With such a short focal length, wide angle lens, it surprises me that it needs much focusing at all.

Wide angle (short focal length) DSLR lenses have a huge depth of focus from close by 'to infinity and beyond'.

With a telescope it is a different matter. The longer the focal length, the smaller the depth of field and hence more precise the focusing needs to be.

But the world is full of mysteries!

[Gina]

Looks like tonight is not going to be much good for seeing the stars   Not sure if clouds or fog will win - the fog bank is getting nearer and there's high cloud.  AND the Dell laptop is refusing to connect to TeamViewer in the warm room though the wi-fi connection is fine - Firefox working well.

[Gina]

Brought camera back indoors to continue working on.

[Gina]

With such a short focal length, wide angle lens, it surprises me that it needs much focusing at all.

Wide angle (short focal length) DSLR lenses have a huge depth of focus from close by 'to infinity and beyond'.

With a telescope it is a different matter. The longer the focal length, the smaller the depth of field and hence more precise the focusing needs to be.

But the world is full of mysteries!

That is just NOT true.  Longer focal length have a corresponding longer depth of focus.  I think you are getting confused between depth of field in the view/object and the focal accuracy needed to get the image in focus.  I am talking about depth of focus in the image plane NOT the object plane. 

[Horwig]

Critical focus is related to focal ratio, not focal length. The faster the lens, the more difficult it is to focus. And the Fuji lens at f1.4 is going to be super critical

H

[Gina]

I think it's related to both but I'm prepared to be corrected.  In comparing my 1000mm f5 MN190 with my 400mm f5 Esprit 80 I find the Esprit is more critical to focus.

[JamesF]

My understanding is that the depth of focus varies as the square of the focal ratio.  The diameter of the Airy disk is 2.44 x wavelength x focal ratio, and the depth of focus is 2 x Airy disk diameter x focal ratio.  For green light of 560nm and an f/1.4 lens you'd therefore be looking at a focal depth of almost nothing at all (about 5.5um?).  I've also seen a formula quoted for focal depth as 4 x wavelength x focal ratio, which would give an even smaller result.

James

[Horwig]

This is I think has been the accepted method of calculating cfz: http://www.wilmslowastro.com/software/formulae.htm#CFZ

There has been discussion, don't remember where, but I think on Cloudynights, that when running an autofocus routine, I think with Focusmax, that the measured cfz was even narrower, possible by a factor of three.

Huw

[Gina]

OK - I stand corrected   Clearly the ratio of depth of field to depth of focus fo the image depends on FL.  I guess the difference I found in my telescopes was due to the focussing mechanism.  Thank you folks for clearing this up

[Gina]

My understanding is that the depth of focus varies as the square of the focal ratio.  The diameter of the Airy disk is 2.44 x wavelength x focal ratio, and the depth of focus is 2 x Airy disk diameter x focal ratio.  For green light of 560nm and an f/1.4 lens you'd therefore be looking at a focal depth of almost nothing at all (about 5.5um?).  I've also seen a formula quoted for focal depth as 4 x wavelength x focal ratio, which would give an even smaller result.

James

Seems to me that the Airy disk will be smaller than the pixel size for a fisheye lens so the criterion for fisheye lens focussing will be pixel size (if the lens is good enough to get that resolution - high resolution in a very wide angle lens gets expensive!).

[JamesF]

Yes, I'm not sure what happens when you get to a point where the Airy disk is smaller than the pixel size.  I guess ideally you still don't want to get into a situation where a star image that would appear near the edge of one photosite spills over into a neighbouring one but as you say, other factors may well have taken over by then.

James

[Kropster]

I stand corrected... all is clear..... in which case, it might make life easier to stop down a few stops (is that possible?) and expose for a bit longer.

Being ultra-widefield, exposure time should barely matter as far as star trails go.

Focusing would seem to be a major issue and almost impossible to maintain at f1.4!

[Gina]

I'm using a QHY5 camera which is less sensitive than some.  I'm already using exposures of 30-60s and could do with more when I can get dark frame subtraction.  Also, this lens doesn't have an iris for aperture control - only a very crude two bladed shutter controlled by a lever.

[Gina]

I've been looking into using a sliver of solar film for the shutter instead of kitchen foil - I think it's workable   It would mean I could stick with my tried and tested meter movement shutter control and yet provide daylight imaging.  It just depends how much light gets under the film.  With the film touching the sensor, resting against it by its own weight, I think this will be minimal.  I could improve things by flocking the enclosure but this may not be necessary.  With virgin film unframed there are no ridges and the film is nice and flat.

[Horwig]

I take it the qhy5 mk 1 is not supported by EZPlanetary. Shame, it's got enough control of gain and exposure duration to give you daylight images without nd film.

Just another thought Gina, in an attempt to reduce the effect of temperature induced focus shift, would it be worth considering painting, or covering the lens body with white? A black lens body under perspex must get REALLY hot.

Huw