The next eshine telescope

Using modelling of the Sun-Earth-Moon system it is possible to show that there will be an airmass-dependency in the derived terrestrial albedo. This is due to extinction being colour-dependent and the presence of spectral slopes. The BS is more red than the DS so extinction will relatively influence the DS more than the BS. In monochromatic light the DS and BS would be affected identically by extinction. As our method depends on modelling the DS/BS flux ratio we must either model that colour dependency in the extinction, or correct for the effect with traditional Langley plots of -2.5 log10 (Albedo) vs airmass.

The challenge in modelling the effect is to make avilable wavelength dependent lunar albedo maps and apply them appropriately for the DS and BS individually. LRO maps are wavelength dependent, but correctly representing the colour of the DS and BS seems a bit on the difficult side since we are at the very least dealing with a variable-colour DS.

With narrow filters the problem is less.

This implies that a better future system ought to be based on narrow filters – or on spectroscopy?

It also seems to imply that there will be a signal from terrestrial albedo in the slopes of albedo against airmass. So, when all data are fitted, look for an Albedo vs slope-of-Albedo-vs-airmass relationship.

It appears possible to have a variable filter that has no moving parts. It is done with liquid crystals and commands over an rs232 cable:

http://www.dfisica.ubi.pt/~hgil/fotometria/cri/varispec2.pdf

The specs show that you can have a selectable filter from the blue into the NIR – but not in one device. The stability of the ability to dial up a specific filter needs to be investigated. Papers have used this device, such as this one.

The transmittance is low compared to glass, but with a moon that bright, who cares!!!

Scattered light in the filter could be an issue.

Highly sensitive to polarisation — one could make that a feature?

Filter stability would be the other issue — would they change transmission characteristics on long term (year) time scales?

Chris comments:

the transmittance is low compared to glass, but with a
moon that bright, who cares!!!

scattered light in the filter could be an issue.

highly sensitive to polarisation I see — one could make
that a feature?

filter stability would be the other issue — would they
change transmission characteristics on long term (year) time scales?

infrared and optical handled differently, so two would be
needed — could lead to calibration issues…

Many of the complexities and problems with the present SKE could be avoided by replacing it with a fixed KE. The orientation problem is solved by making the KE in the form of a plate with a sharp-edged hole in – behind some part of the edge of that hole the Moon could always be parked so that just the DS peeked out.

The present size of the Moon on the CCD is a bit too big for this to work. We need to understand which part of the system to rebuild in order to get the Moon to be smaller (perhaps 50% smaller) on the same CCD – or perhaps to get a bigger CCD. The present system is ‘short’ which means that the edges of the field and the centre have problems being in focus at the same time, so a larger CCD would have worse problems (but the worst affected areas would be behind the mask!)

We have received help with the software that extracts images from the Sigma SD10 X3F files. These are RAW files packaged in a special way. The software is ‘x3f_extract’, provided at proxel.se by Roland Karlsson. The code can be downloaded at http://proxel.se/x3f.html

A small modification of the code is needed to avoid ‘clipping’ of pixel values below the mean of the bias level. The modification must be introduced into the source file and then code must be recompiled. The code to modify is inside the ‘src’ directory that is built when you download all the source files from proxel. Inside the code ‘x3f_io.c’ go to the routine “huffman_decode_row” and find the line:

uint16_t c[3] = {0,0,0}, c_fix[3];

comment it out and exchange it with:

uint16_t c[3] = {100,100,100}, c_fix[3];

Then go one level up and run ‘make’.

I did the above and took some dark frames that I averaged in the half-median-half-mean way, in each pixel. I generated row and column sums of these dark frames and got this:

The structure is low, actually, at the 1 count level.

With this fix of the x3f_extract code we have no longer a ‘strange clipping’ problem and the SD10 can be used for astrophotography – the point being that when its internal IR filter is removed and a red filter i used over the lens we have in effect a camera able to perform VE1 and VE2 band photometry in one shot, without any moving parts. We have yet to do the image manipulations required to reduce the images obtained in the R G and B channels to R G and IR values – but that will happen.

It is probably necessary to have that SKE – it leaves a remnant halo, but the intensity of this is very much lower than the halo you get with our co-add technique. Our SKE never worked and I wonder how a potential future one could be constructed?

One alternative could be these ‘Digital Mirror Devices (DMDs)’ which are essentially a adressable semiconductor matrix of little mirrors. The whole things is like a CCD in size but each element can be tipped to one side or the other, thus deflecting light. If one of those devices sat in the prime focus of an objective we could first take an image of the Moon, find the pixels that needed their light deflected, and then address those only and send their light off to one side, thus obtaining a sort of dynamic SKE. Mechanical SKEs still have to be positioned, which I think caused the death of our system – too many motors with dodgy systems for measuring the turning angle, timeouts that never got their timeout and so on.

A DMD array could be fixed in place and operated with software only. Apparently DMDs are evolving fast and in 2014 some kits for experimenting, by Texas Instruments, will come out. The Kits are supposedly some hundreds of dollars.

http://www.ti.com/tool/dlpd4x00kit

They need to be driven by something, and here is a driver board (for $210):

http://www.ti.com/lsds/ti/analog/dlp/lightcrafter4500.page?DCMP=dlp-lightcrafter4500-en&HQS=dlplightcrafter4500

We need to get one of these – with some small pot of money to be raised by sale of stuff from the MLO system (when we decide to do that) – or a small proposal.

Such a DMD undoubtedly has its own PSF and that needs to be tested.

The DMDs have a ‘fill factor’ of about 90% – so an issue is what happens to the remaining 10%. Is it diffusely scattered or is it cast in another direction? Apparently there are three states to a DMD pixel – left, off and right, where the left and right states position the light at +/- 14 degrees. The Off state is to be understood.

An interesting paper

describes a new design for an earthshine telescope. They describe the essential instrument.
It seems to me that:

1) they only have 2 lenses (although both are doublets – we have one doublet and two singlets)
2) Their masks (like our SKE) sits outside the telescope. At least, that makes it easier to fix when it breaks.
3) Their telescope is a Cassegrain system – so lots of optics there…

Hmm. We should try to understand this design in case we want to try fresh approaches. Perhaps it is an option for a FW-less CMOS-based system?

It seems their ‘instrument’ sits on a conventional telescope – so many many optical surfaces in total…

In our very first eshine teleeescope (the 0’th) which went to La Palma for testing, we had placed the knife-edge on the surface of the CCD and did prime-focus imaging. We could control the pointing with software (clicks on a star atlas) and could rotate the camera in its ball-bearings so that the knife-edge was aligned as required.

With a somewhat larger CCD or CMOS, higher pixel count and an image scale that allowed the Moon to be well imaged in the half that was not covered by knife-edge we might be able to design a system with minimal movable parts – just the mount to point and a motor to rotate the camera. No secondary optics (we would not need a colour FW as we would be using a modified RGB CMOS). No shutter (provided CMOSs read out really fast so that there is no ‘dragging’).

We could go back and inspect our La Palma images – most of them were made with an attempt at the SKE.