2nd February 2011M31
The mount (ASA DDM60) is back from the upgrade in Austria and appears to be working very well. This is the first image with the 10" scope on the mount.
There is some distortion showing in the stars. Dim stars are OK but larger ones show an asymmetry. I suspect this is due to a problem with the position of the new larger secondary mirror (63mm). I fitted the new secondary in an attempt to get a larger flat field of view. Out of focus diffraction images showed some weird defects so I have removed the mirror and intend to make a more accurate placement.
Enlarged view showing distortion.
The main mirror - Foucault test image showing the classic 'Doughnut' shape.
There does not seem to be a problem here so the secondary may be the culprit.
The arrows show the fastening clip positions where most strain should occur, but none is seen.
The finger of suspicion is now firmly on the secondary...
Picture of the image of the pinhole formed by the main mirror. The pinhole is acting as an artificial star.
The pinhole was made with a fine needle pressed against aluminium foil and gentle rotated.
It is almost too small to see by eye.
A contour plot of the pinhole image. No sign of astigmatism.
The secondary, in situ on its holder, has been sent to Orion
Optics who have test facilities that can determine what is going on.
The left star contour is from the defective 10" scope. The right image shows well shaped stars, from the 6"
Breaking News: Orion Optics confirm that the
secondary is indeed badly distorted. Arrangements in progress for Orion
Negative image of M31 with the 6". (4x10 minutes).
Stars are pinpoint across most of the frame showing that collimation is good, guiding is working well and the coma corrector is doing its job.
The Foucault tester used to check the main mirror. The aluminium
foil wrapped around the light has the pinhole in it.
The offending secondary! Gluing the secondary with silicone glue
like this is obviously a bad idea.
Sheer decadence - two computers and two 24" monitors!
I found superb free software called Synergy which allows both to be controlled with one mouse and one keyboard.
Move the mouse to the edge of the left monitor and it appears on the right.
The keyboard controls whichever computer the mouse is on.
You can even copy and paste between the two!
M31 (6" scope) image from Feb 12th reprocessed with flats - severe vignetting removed.
The flat is back from Orion optics. Their method of attachment is three blobs of silicone adhesive with
a good sized gap between mirror and holder.
The secondary is back in the scope. This image shows the state of collimation. The camera was held against the
focuser tube and aimed at the centre of the primary. All circles are nicely concentric with the exception of the
outline of the secondary. This is acceptable, as the secondary is deliberately offset.
Clear skies allowed the secondary to be tested.
First target was Capella, and star images appear good. No uneven flare around Capella and dimmer stars small and round.
Next, the Double Cluster which will give a large number of stars...
Oh no! Odd shaped stars!
I am puzzled by the mishapen stars - they are very like the ones that caused me to investigate the secondary mirror.
I moved to M81. Most peculiar - excellent round star shapes!
Capella and M81 are both high in the sky at present whereas the Double Cluster is low over the house roof. Could warm air rising from the roof be the cause?
Philipp Keller, who wrote the Autoslew software, is of the opinion that the poorly shaped stars in Perseus are due to the phenomenon of 'tube seeing'.
In a telescope such as a Newtonian, currents of warm air can convect through the upper portion of the tube, and give the stars odd shapes.
This is a region near the Crab nebula, and the same shape is seen. The Crab nebula is also low down in the sky at this time of year.
This video was made by taking several 1 second exposures and linking them in sequence. One second
is such a short time that any movement of the mount can be ruled out. The video shows a 'flaring' off to the top-right of the brighter stars.
This does indeed look like a convection effect.
When the scope is pointing more vertically, the effects of tube seeing are reduced. The solution may be to take the back cover
off the scope tube an hour or two before imaging starts. This will give enough time for temperatures to equalise.
Positioning a small fan to blow air through the tube may also help.
The photo below shows the sealed end of the tube.
I opened the observatory hatch 2 hours before dark, and removed the plate at the back of the scope.
The first image of the Double cluster showed round stars. I put the plate back and the distorted stars returned.
Tube currents are definitely the problem.
The only problem now is that stray light from computer screens and various LEDs on computers
gets into the scope and finds its way to the camera.
This is with a thick black cloth over the computer screens!
Back cover removed.There is lots of space for light to get in. Some matt black paint in a few places will help, and perhaps some black card baffles...
Black flocking paper applied to the inside of the tube and the aluminium struts painted black.
A paper card circle with a 14cm diameter central hole should allow air to flow through, and keep out stray light.
An additional paper circle might be needed behind the struts...
The modification appears to be very successful. I imaged the Double Cluster through lots of cloud,
and the stars are perfectly round!
Perseus Double Cluster
25th JuneSolution to the open rear end of the scope - neodymium magnets hold the back plate securely in position.
It is very quick and easy to remove it when needed.