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Thank you all. I am learning a lot of information which will doubtless come in useful one day. Although the SX 814 now used has a CCD chip it seems inevitable that I will need to use a CMOS device sooner or later.
Actually, I have already used CMOS sensors in extremely elderly Canon DSLRs. I have not yet attempted to do photometry at anything better than 100mmg accuracy or precision. 2mmag is possible with the SX camera.
Paul
Not having a CMoS camera to play with, I throw this suggestion out for those who do for them to consider.
Take a series of dark exposures under conditions as equal as possible, especially sensor temperature. Fit a smooth curve to the intensity readings at each pixel. Extrapolate back to zero exposure. That is your bias frame.
Rather tedious but I don’t see why it shouldn’t work. Whether bias frames are useful to you I couldn’t say.
I can’t answer your question authoritatively but will point out that the precision of 10:47.7 and 10:47:42 differ six-fold.
If a calculated value is known only to 0.1 minutes it is often not a good idea to specify it to a precision of 1 second as that would give a false impression as to how well determined is the true value.
Thanks Ian,
Very useful starting point for me.
Some imagers have advised to cool to only zero, but I believe that there is some advantage in going cooler with CMOS cameras, but I’m not sure about this.
Regards
KevinYou should cool to the point where the thermal noise is negligible compared with other sources of noise, such as sky background, Poisson noise, and so on.
Where that point lies depends strongly on the camera and, to some extent, on the sky background. For my present camera (a SX-814 CCD) -10C is easily cold enough and well within the reach of its Peltier cooler except in the very hottest of summer nights. Air temperatures of 25C are not unknown in La Palma and were reached this last summer on a few nights, but they are not common.
It’s easy enough to measure the thermal noise of your camera as a function of temperature. Take some (say) 60 second darks at a variety of temperatures and look at the variance of the pixel values.
Thanks Paul,
How do you calibrate defocussed images? Presumably you have to take all the flat frames at that the same defocus?Not really. All flats are completely out of focus anyway!
That is the whole point of flats: they should have no extrinsic structure whatsoever and all non-uniformity over the flat image arises from sensor sensitivity variation, dust rings, vignetting, etc, and not from anything outside the optical chain.
Richard: I have just checked again. In the text which follows I have replace square brackets with curly brackets and put a space each side of the : characters. This is to defeat any attempt by the forum software here to make my text into hyperlinks.
If one goes to the end of the page https://ukdarkskies.org.uk/dark-sky-places then down at the bottom one finds:
Planispheres (https : //britastro.org/node/12028) have rotating discs that allow you to set the date against the time and display the current night sky. See also https : //www.youtube.com/watch?v=UM8fH_NmtU4
The Youtube link is active and is clickable. The one intended to reach britastro is not clickable, for me anyway. I am using the Firefox browser.
If one examines the HTML source, the clickable version reads
{a href=”https : //www.youtube.com/watch?v=UM8fH_NmtU4″}https : //www.youtube.com/watch?v=UM8fH_NmtU4{/a}
but the other only
Planispheres (https : //britastro.org/node/12028)
There should be {a} {/{a} tags wrapped around the latter.
A small point, but on that website the link https://britastro.org/node/12028 is not active. It is correct but I had to copy and paste it into a browser.
Do you have contacts at the organization which may be able to activate the link? It seems a shame to make life harder for interested parties and to reduce the visibility of the BAA.
I’m in LP for all of February and all of August every year. How many weeks before or after those depends on what I feel like closer to the dates.
Anyone who happens to be in the district is welcome to drop by. Just let me know at least a day in advance …
(Sorry, that this is off-topic but I don’t know where else to make the offer.)
By coincidence I’m on La Palma at the moment.
What a shame. I flew out of La Palma on the 16th. We could have met up at my place and have shown you my observatory if the timing had been more convenient.
I am also on the astrometry.net mailing list and have been following your communications there with interest.
At present I’ve no real use for precision astrometry, using the plate solver only to find stars to within a few arcseconds which is plenty good enough for pointing a telescope and to enable APT to find a VS and its comparison sequence, then to do precision photometry.
On the odd occasion I do need good astrometry I use either Astrometrica, if it can be persuaded to work at all, or the IRAF tools.
Ah, yes. I remember ansvr. Its a cygwin thing isn’t it? Surprised the guy who did the conversion job has not updated it Apparently, the absence of a true Windows version is due to the way AN maps the Index files to memory.
I was forced over to WSL (or Linux on a Rpi) because the ansvr version of AN is quite old and it was failing to solve some sparsely populated fields, while the latest AN did.
Ansvr certainly worked okay most the time though.
Thanks Grant.
Perhaps I should run the latest server on a Linux box which shares the control room with the Windoze TCS. It may well be faster and more effective than ANSVR (apologies for my typo earlier) but I’ve not yet done it on the grounds that if it aint broke, don’t fix it.
Both systems use the Gaia index files so images almost always solve because there is a large number of stars in those files. The Linux local astrometry.net installations (another one lives on my laptop) are not yet configured as servers so I will need to find out how to do that.
A local astrometry.net will also run under Windoze. It is what I use on my TCS. The search term “ANSRV” will find it for you if you are interested.
Thanks to everyone for great suggestions
Seconded.
I have a rather, err, crappy Meade here which was shipped almost 5 years ago and never put into use. When the next delivery of round tuits arrive I will put it into a roll-off shed under software control
Until then, I have now decided to buy a BBQ or motorbike cover pro tem.
This may or may not be applicable for you but I use a standard small car cover (a cover for small cars, rather than a small cover for cars though there is little difference between the two concepts in practice). A motorcycle cover may be more suitable for a smaller scope.
My cover is too big but I don’t care. Rope and/or bungee cords take good care of any excess.
I’d love to attend but I have to stay in La Palma until at least mid-September.
Will the meeting be recorded for later viewing? Recording is generally much easier than live streaming over Zoom or whatever.
