The next eshine telescope

Papers from astrophotography articles that mention the Foven sensor in the SD10 camera:

This paper reminds us of a few things, namely that the camera (like all? CMOS and consumer-grade CCDs) has internal mechanisms for performing dark-subtraction – you do not get to do this by yourself. Exposure time is limited to 2min.

Note also these abstracts found in the ADS:
http://adsabs.harvard.edu/abs/2008AAS…212.2404B
http://adsabs.harvard.edu/abs/2013OptLT..48….1M

This paper may be of general interest:

http://arxiv.org/pdf/1306.3273v1.pdf

I have performed some tests of color-separation on the Sigma SD10. The internal IR-blocking filter was present.

I imaged a red, green and blue surface with the camera as is; with the camera and a Wratten25A red filter; and with the camera and a 720nm long-pass IR filter.

Plotting a line across the three coloured surfaces (red at left, green in middle and blue at right), and separating the R G and B fields in the image file, we get:

We see

Top panel. No external filters, only internal IR-blocking filter. As
expected, the Red target is bright in the R-band of the image, Green in
the G band, and Blue in the B band.

Middle panel. Now the red Wratten 25A filter is on – i.e. it passes mainly
red light, up to about 560nm, or so. The R band is brightest on the Red target (good); the Green target is supressed strongly everywhere; the Blue band is brightest on the red surface.

Bottom panel. The ratios of panel 1 and 2. Note that exposure times were
not necessarily the same. In the R-band Red and Blue targets seem to have
fared similarly, but the Green target was suppressed by the W25A filter.
In the B-band the Red target was least suppressed by the W25A filter.
In the G band we see strong suppression by the W25A filter for all targets.

Summary: W25A really kills the Greens, but Blues seem to make it through
a bit. Strange that.

The long-pass 720nm filter performs similarly.

The tendency for the G band to ‘drop out’ is seen especially in images of point sources – streaks occur as if the presence of a point source is especially bad for the system. bad news for stellar photometry? We shall see.

A Canon DSLR camera has an IR-blocking filter that has to be removed before the camera allows us to obtain data similar to the B,V, VE1, VE2 we wish to have. It is evidently complex to remove the IR filter form a Canon body – lots of small screws, unbending of glued bits and fragile parts everywhere.

An alternative is the Sigma SD10 camera with the Foveon sensor. The removal of the IR filter from a Sigma SD10 is trivial – one finger can pop the filter out in a second. The SD10 costs about 180 UKP (body only) while the SD15 costs 590 UKP.

We could do away with some of our woes if we used a DSLR CMOS camera instead of the current system. There are several benefits, and a few drawbacks:

PROs: With a DSLR camera we would get RGB colours at the same time, at the same sky conditions, and at the same focus setting. Alignment issues would go away. Shutter precision issues would go away.
With the modifications described in this paper

we would even get the NDVI index which is what we wanted to get by using VE1 and VE2 filters.

CONs:
a) The shutter would be at the focal plane, instead of in the pupil. This could mean that fast exposures were not possible – insertion of a fixed ND filter woudl extend exposure times.
b) With CMOS chips at best being 14 bit (16 bit exist but cost a lot) we could probably not use the ‘direct imaging mode with BS and DS in same frame’.
c) Dark Frame issues appear murky for CMOS cameras. And what about cooling to get stability?

Neutral issues – i.e. same problem as before: We would still need an SKE to block the BS.

Suggestions:

1) Use an unmodified DSLR to check what the halo structure around the Moon – or Jupiter – looks like in R G and B. If these are the same – which is not the case with Johnson B V and not at all with VE2 – then we might be on to a system to get ‘same-halo images’ with known subtractive benefits! We could then go on and modify a DSLR camera and get the NDVI Index and see if halo issues are reduced.

This paper discusses the use of DSLRs in astronomy:
http://adsabs.harvard.edu/abs/2012JAVSO..40..815K

The Andor iXon-897 BV CCD camera had some properties we need to review

The RON was low (2 ADU/pixel in practise, 1ADU/pixel in the brochure …)

The bias pattern was strong, as it is on thinned CCDs. The level of the bias was temperature dependent and as the camera was cooled in a thermostat loop the mean strength of the bias pattern had to be adjusted for this. We did this by using only bias frames taken just before and after the science images and scaling a ‘superbias’ field. If the superbias was representative of the actual pattern we are probably doing well _ but this in itself should be tested. We certainly have the data! All shout: Student Project!

Linearity: We have data that suggest that the CCD was not ‘99% linear’ as all CCD brochures promise. As the evidence depends on the shutter being linear with exposure time we have to revisit this.

We had ‘dark bands‘ matching the width of the Moon, in the readout direction. While we may have compensated for this by doing ‘profile fitting’ also in the row direction of the image we would like to know what was going on, and choose a future CCD accordingly.

CMOS: they are available in 16-bits (Andor) and colour (in DSLRs: expensive!). Now, what was the benefit of using CMOS instead of CCD? Need to compare linearity and readout speed.

Some aspects of the expensive Andor were of no practical use for us: ability to have EM – that caused the bias average bias to flicker by +/-1 counts (not pixels – the average!). Faster readout modes were available, but never used – they cut into dynamic range or gave more noise. An internal shutter was a possible option but was not chosen – so we had to rely on the dodgy external one! A possible coating and enhancement of the blue-sensitivity was not chosen – choosing it would have left the red-sensitivity unaltered, so why not get it?

Cooling with water was possible but never used as it would be just so much more plumbing to worry about.