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One of the advantages of an SCT is that you can move the focus in and out by a lot through small shifts of the primary mirror. You should be able to solve your problem by releasing the mirror lock and turning the focus knob. Given that this is an old C14 any mirror lock will have been added afterwards so you will need to work out how to release it.
Anticlockwise rotation of the focus knob (as seen from the back) moves the focal point away from the telescope (at least in all the SCTs I have) but you can check how the focal plane is moving by looking at your projected moon image. It is a characteristic of this method of focussing that the effective focal length changes a bit as you shift the primary back and forth. Don’t worry about the mirror falling off the central baffle. The focus knob will stop moving at the end of its travel.
Once you’ve got a decent back focus distance you can lock the mirror again and use your Moonlite focusser for fine adjustments.
And here it is this evening (Jan 5). It is the short streak in the centre near the bottom edge of the frame.
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Here’s a quick snapshot of it approaching M33 taken as I was waiting for it to get dark tonight (Jan 4).
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Clear through the night in Chelmsford but cold (-4C min). A very bright Moon but dry transparent polar air. I got 580 Quads on 6 cameras including this bright one (apparent mag -2.1) on my SE pointing camera just below the Moon at 02:33:533 on the 4th.
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James,
Nice to see Mercury being treated as a comet!
Indeed, as Dominic says, Python is a great way of doing this. Once you’ve understood the basics of the language you can do almost anything and there is a huge amount of existing code in libraries that make even complex tasks very simple. It is definitely worth the time spent learning it and it will open up many interesting areas of study on digital images.
With regard to your questions, it depends a bit on how flat your sky background is (or how flat you can make it). I generally estimate the sky background using an initial median to reject outliers and then a mean but this only works when (a) the sky is reasonably flat and (b) you have a lot more sky pixels than non-sky ones. That is usually the case with 2D images containing stars but may not be for a 1D slice depending on how you do things. For one thing I would make your slice quite wide and integrate up all of the pixels in the PA of the tail. This will improve the SNR significantly. This is a bit like the way that spectroscopists integrate pixels normal to the slit direction.
Regarding thresholds I compute the RMS offset of “sky” pixels relative to the calculated mean sky value and then set the detection threshold at something reasonable like 1 or 2 sigma.
My comphot software does this for comet magnitudes as described on page 23 of Comet’s Tale 40:
That is an impressive Scotch mount.
I think that the Scotch mount (aka barn-door mount) was first described by G.Y. Haig in JBAA Vol 85, No 5. I’d only just joined the BAA in 1975 and this was the first paper that I found really interesting. I built a basic copy and used it for many years to take night-sky photos using a manually driven screw and a stop watch (things were hard back then). In the 1980s I built an advanced one which had a crystal controlled stepper motor drive and took this to Tenerife to photograph C/1996 B2 (Hyakutake).
Indeed. LLMs like Chat GPT are pretty hopeless at logic and arithmetic since they can only regurgitate stuff that they have seen in their training. Their natural language abilities are pretty amazing though.
You can work out the miss distance of 3I assuming no gravitational accelerations by linearly extrapolating the “infinity” state vector to closest approach. JPL Horizons will give you the barycentric position and velocity vectors at any past time if you select “vector table” as the ephemeris type. The state vector for 1900-01-01 is:
2415020.500000000 = A.D. 1900-Jan-01 00:00:00.0000 TDB
X = 6.146692847218195E+02 Y =-1.410989916978468E+03 Z = 6.302147691517926E+01
VX=-1.339387085953456E-02 VY= 3.065893078363736E-02 VZ=-1.367393155520660E-03(position in au, velocity in au/day). The position at any future time (if you ignore gravity) is just P + Vt where P and V are the initial position and velocity vectors. Differentiate the magnitude of that and find the minimum to get the closest approach time (JD 2461021) and you get a miss distance of 1.60au so you did eventually get the correct answer! You should ask it how it knew this? It will have found it on a website somewhere. I’ll be more impressed when these “AI” sites can actually calculate stuff from first principles.
Thanks to all of the observers who have sent in images and measurements of this comet we have almost daily coverage since the initial fragmentation in early November. The comet is now moving away from the Sun and Earth and seems fairly stable with two fragments visible (A & C). Astrometry from BAA observers indicates that fragment A is probably the original nucleus although it is now much fainter than fragment C. These two fragments are currently around 70,000 km apart and their slightly different orbits can be modelled quite well using a relative velocity change at separation of 30 m/s.
The comet is well placed in the evening sky so please keep imaging it in the highest resolution that you can manage.
It is interesting to compare the Gemini image with our recent images here:
https://britastro.org/cometobs/2025k1/thumbnails.html
We have the advantage of a much better time resolution and so can see that the fragments have been changing rapidly over the last week. The Gemini image was taken on Dec 6.37 and it looks similar to the arrangement in this image from Dec 7.05:
https://britastro.org/cometobs/2025k1/2025k1_20251207_011026_ndj.html
Gemini shows fragment A as being double and fragment D as very faint and diffuse. Fragment C is the brightest object. Only two days before (on Dec 4.99) fragment D was brighter than fragment C:
https://britastro.org/cometobs/2025k1/2025k1_20251204_234701_nquinn.html
I’m preparing a report for the February Journal and it has been fascinating tracking the daily variations in this comet over the last month.
Taking the recent astrometry I think the attached image from I79 this morning has correct identification of the components.
It looks like the main component and component B have pretty much gone now. Component D appeared recently and is faint. Component C has been around for a while and has recently brightened to the point that it is the most prominent part.
The comet doesn’t look very healthy though and I doubt if there will be much remaining to measure soon.
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The brightest component as of last night (Dec 6.95) seems to be close to the predicted position of the C fragment.
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As of tonight there is not much left. The brightest fragment is the one furthest down the tail. It looks as if this comet doesn’t have much life left in it.
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Yes, it is still there but it is getting a lot fainter and the sky tonight was very bright. My image from earlier attached. The fragments are still visible but the main component is around 3 mags fainter than it was a week ago.
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The weather at the Alnitak telescope in Spain has not been too good the last few nights but I did get an image of the comet from Chelmsford yesterday morning. The comet is now moving away from the Earth so the physical scale is getting smaller but fragments are still visible. I’ll be preparing a report for the February Journal so many thanks to everyone who has submitted images.
The latest images in the section archive are here:
https://britastro.org/cometobs/2025k1/thumbnails.html
Most of these are north up so you can see how the tail has rotated rapidly over the past few weeks as our viewing angle has changed.
C/2025 K1 continues to change night after night. Tonight’s (Nov 30.7) image attached. Our viewing angle has been changing rapidly and the tail has now rotated so that it is almost pointing due south. The fragment we saw a few days ago south and a little east of the brightest component is still there but much fainter and there is a definite extension north of the bight component.
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Here’s an image just taken from Chelmsford. It shows the nova at 18.3 unfiltered (ref Gaia G) in the blue circle.
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I’ve been away so I haven’t had a chance to do my nightly M31 patrols. I did get the 2023 eruption just after it was discovered though:
https://britastro.org/forums/topic/m31n-2008-12a-call-to-arms
so it is definitely available to small telescopes although it does fade very quickly. I’m back home now and it is clear here at the moment.
Sheridan – It depends on exactly what you want to do and what focal length of optics you’ve got. A lot of current CMOS cameras have small pixels so that might not be ideal if you have a very long FL although you can always bin them. Most are now based on Sony Starvis sensors which are very good. I use ZWO cameras so I would suggest having a look at what they have available. The 585MC would be a good place to start. It can be used for deep sky and planetary, has pretty high frame rates but is also able to take the longer exposures you would need to show deep sky objects in real time. The ZWO software suite includes software that can real-time stack images too.
As of this morning (Nov 27.2) the C fragment is no longer measurable in the I79 images.
Steve – certainly looks like you got A and C with the Seestar even though you needed 3 hours exposure to get a reasonable SNR. It is a 50mm aperture refractor after all!
The HST image in the paper with Helen as co-author mentioned above is very impressive too.
