Blog Image

Earthshine blog

"Earthshine blog"

A blog about a telescopic system at the Mauna Loa Observatory on Hawaii to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Focus (3)

Mechanical design Posted on May 23, 2012 17:28

After completing the analysis of the best focus for all filters I am completely confused. It looks like all filters have the same focus near 24000:
Of the above only VE1 is a little different. ‘double rows’ of points imply there were two sets of images exposed at different times.

Note that if the minimum radius is near the same value, and the scale factor is the same then the ‘volume’ of the star image is the same – but the exposure times were the same .. usually there is a factor of 20 less photons in the B filter compared to V … so the above test has not selected different filters. And yet the command is there in the script:


The entire script is here:

I started noticing this ‘same exposure time in all filters’ problem some weeks ago – but it was intermittent.

I am going to ignore the new settings for focus for now and see if I get reasonable exposures next time I use the moon script.

What on EARTH is going on????

Focus (2)

Mechanical design Posted on May 23, 2012 13:34

Extending the lower limit on the search for a good focus we now have a preliminary result for the B filter:

The plots show various focus-related quantities against focus position. The first two panels shown (on lin-log and lin-lin scales) the radius of the fitted Moffat profile. The next shows the standard deviation of the image, divided by the mean value of the image, the fourth box shows the exponent of the fitted Moffat profile, and the last box shows the vertical scale (height, really) of the fitted Moffat profile.

The various properties – which should all be maximum (or minimum) at the best focus – indicate values of best focus from 24500 to 26000, with 25500 being perhaps the least uncertain.

The focus we use at the moment for the B filter is 28250 – outside the range found here. That value corresponds to almost a doubling of the width of the Moffat profile.

We do not understand how the setting can be that much off – possibilities include that something happened during transportation or that the addition of the heater has changed the focus position.

Perhaps Ahmad has an original INI file from the days in Lund and could check what value was found best a year ago?

Perhaps Torben could see Torbjörn, or whoever understands the mechanical design in detail, and ask whether the heater could be warping the optical bench? Temperatures to 60 degrees C is seen on the clip-on thermometer in the dome.

The good news is that if better focus positions can be found for all filters in the above way then we should be seeing less fuzzy images in the future – it seems we have a FWHM PSF now that is about 2 pixels.

JD2456070 – weather log

Observing log Posted on May 23, 2012 10:57

Clear. 15 m/s from 140 deg. RH is 20%. 4 deg C.


Mechanical design Posted on May 23, 2012 08:29

For some time there have been intermittent focus problems with the system – especially the VE2 filter would go in and out of focus for no discernable reason. The focus is supposedly set, in scripted mode, by referring to focus positions from a script. The currently used focus positions are:

; The names made of (Color filter) + (ND filter) + (Knife Edge DF)

IRCUT_AIR_SKE = “28750”
IRCUT_ND_SKE = “29000”
B_AIR_SKE = “28250”
B_ND_SKE = “29000”
V_AIR_SKE = “30750”
V_ND_SKE = “29000”
VE1_AIR_SKE = “28750”
VE1_ND_SKE = “29000”
VE2_AIR_SKE = “24000”
VE2_ND_SKE = “29000”
AIR_AIR_SKE = “29000”
AIR_ND_SKE = “29000”

As the focus seemed more and more unreliable I defined a new set of positions in order to be able to test on stars from 28000 to 31000:

FOCUS090 = “28000”
FOCUS091 = “28100”
FOCUS092 = “28200”
FOCUS093 = “28300”
FOCUS094 = “28400”
FOCUS095 = “28500”
FOCUS096 = “28600”
FOCUS097 = “28700”
FOCUS098 = “28800”
FOCUS099 = “28900”
FOCUS100 = “29000”
FOCUS101 = “29100”
FOCUS102 = “29200”
FOCUS103 = “29300”
FOCUS104 = “29400”
FOCUS105 = “29500”
FOCUS106 = “29600”
FOCUS107 = “29700”
FOCUS108 = “29800”
FOCUS109 = “29900”
FOCUS110 = “30000”
FOCUS111 = “30100”
FOCUS112 = “30200”
FOCUS113 = “30300”
FOCUS114 = “30400”
FOCUS115 = “30500”
FOCUS116 = “30600”
FOCUS117 = “30700”
FOCUS118 = “30800”
FOCUS119 = “30900”
FOCUS120 = “31000”

I then ran, in steps of 100, over all these positions in all filters for 5-15 images in each position. I fitted a Moffat profile to the brightest source in each image and plot here the radius (sqrt(rx² + ry²)) of the profile:

[Sorry for the poor quality of that plot – a downloadable pdf of that plot is here:

The width of the distribution of points is due to repeated trials in some cases. Scattered points far from the general relationship are shutter failures.].

The result is, apparently, that all minimum-radius occurs at the leftmost side of the plot – i.e. at or below 28000 – this is not at all close to what we use from the scripts. What is going on?

Is the fitting of a profile a bad way to estimate best focus?

Is the setting of the focus from the System INI file misunderstood somehow?

The start of the script used to run these tests is here:


etc etc and then next filter.

Anyone? Help?