Forum Replies Created
-
Posts
-
Posted by Martin Mobberley at 16:02 on 2013 Dec 08
You’ve got most of it right Eliot, yes.Seeing is basically down to turbulence….which determines how bloated the stars will appear in long exposures…. Of course, if your drive is poor and the stars are drifting around by arcseconds during the exposure, this will have a similar effect in limiting the magnitude. Bad light pollution will mean the signal-to-noise ratio gets worse….so 20th mag objects are sitting on a wall of light pollution… For example, you may find that objects on the limit of detection are only 1% brighter than the background sky, so given that the background sky is noisy and noise is random, the signal gets swamped by the noise. Without filters the stars can spread out, yes, but this rarely becomes a factor with reflectors, although for planetary imaging filtering is usually essential due to the spreading of light due to dispersion and the desire for sub-arcsecond resolution. Some telephoto lenses produce bloated stars and do need filtering to cut out violet haloes. Filters are desirable for photometric work on bright objects, but dim the light so much below mag 15 or so that they make the image very noisy…..OK if you have a huge telescope and like long exposures, but they do hammer the limiting magnitude.Clearly you are already getting to grips with letting Astrometrica allow for the object’s predicted motion…….Martin
Posted by Eliot Hall at 12:08 on 2013 Dec 08
So to summarise (let me know if I have any of this wrong).The sky’s limiting magnitude will be determined by seeing (particles, light pollution, and turbulence). Given perfect conditions mag. 20-22 may be possible, but around mag. 18-19 is possible average conditions. (I’m now starting to understand why they put all those multi-million pound observatories on top of dry mountains).Focus and accurate tracking/guiding is critical to maximise the signal received. Unfiltering the camera will allow more light onto the sensor. (Won’t this spread the light across more pixels degrading the definition of the star?)Using a bigger aperture or a more sensitive camera will only decrease the time to which you get to the sky’s limiting magnitude (however shorter exposures are useful for faster moving objects).Astrometrica will predict the path of the asteroid across the field of view and align the frames accordingly for stacking thus improving SNR. I’ve tried this on some other data I took a few days later and found an asteroids (a case of reading the manual for me).Eliot
Posted by Richard Miles at 22:27 on 2013 Dec 06
As you can see from the comprehensive responses of Martin and Grant that you certainly have the capacity to reach 20th magnitude using a 10" aperture but that there are quite a few things they mention to get the best out of your scope.Very good seeing can have a remarkable effect on going faint provided that you have an accurate focus. When doing a long time-series of a mover with the aim of tracking and stacking to go as deep as possible, you may have to refocus the instrument several times especially during the first 2-3 hours of use (i.e. when the temperature is dropping) to ensure the stars are reasonably pinpoint. So it’s often worth checking the turbulence in the upper atmosphere by looking at say the 300 mbar level pressure chart forecast, e.g. at:http://www.wunderground.com/wundermap/?zoom=4&rad=0&wxsn=0&svr=0&cams=0&sat=0&riv=0&mm=1&mm.mdl=GFS&mm.type=SURPRE&mm.hour=0&mm.opa=100&mm.clk=0&hur=0&fire=0&tor=0&ndfd=0&pix=0&dir=0&ads=0&tfk=0&fodors=0&ski=0&ls=0&rad2=0Select ‘Model data’then click on the ‘Tools’ symbolthen under ‘Map type’ select 300 mbthen move the ‘Forecast’ slider to the time and date you intend to observethe chart you obtain is colour coded: anything purple is good or very good seeing; anything green or yellow is poor seeingNo point in trying to go really faint if the weather and seeing is against you, despite it being a clear sky.Richard
Posted by Grant Privett at 23:13 on 2013 Dec 05
Think Martin nailed most of it. just a few comments as my scope is the same size as yours.A year or two back I had a bash at going deep from a fairly dark UK site (blue on the Phillips dark skies map). I took a couple of hundred 45 second images using a 10" f4.3 Newt and a Starlight MX7. By median stacking (though sigma clipped would have worked as well) the images in bunches of 10 (to get rid of artefacts and comic rays or satellites) and then stack-adding the resultant 20 or so frames I managed to get detection of stars at the mag 21 level with a SNR of about 5. So not spectacular, but real. Personally I would not use a UV/IR filter as many modern CCDs are surprisingly sensitive in the 650-950nm range and you are just throwing signal away for no gain – if it was a refractor it would be another matter all together. I reckon with my current set up, a good clear night, no moon, clean mirrors and exposures of 600s plus and a newer CCD like the 694, I might just hit 22.5. Similar to the photographic limit of the UK Schmidt survey, so respectable.One last thing, my old Polaris mount was pretty poor so I had to recentre the target often (I lost about 70% of images to trailing). In effect this dithered the hot/cold/dodgy pixels around the image and helped to supress background streaking which ruins so many attempts to go deep. Dithering long exposures is deeply tedious so, when I have mended my mount I shall be using the dither function of Nebulosity in cahoots with PHD autoguiding to help flatten the background. Worth a try.BTW Astroart can stack with images aligned with interframe offsets derived from the PA and speed of a candidate…Also remember a flat field really will help when looking for very small variations in the sky background….
Posted by Eliot Hall at 18:18 on 2013 Dec 05
Thanks Martin for such a comprehensive reply.You have given me a few things to muse over and research further. I’ll certainly go away and have a look at the Astrometrica Tutorial and stack the subs again.I thought that I might have hit the sky’s limit. It’s very interesting to hear what others have found possible in the UK.
Posted by Martin Mobberley at 13:13 on 2013 Dec 05
Eliot,As no-one else has answered your question, I will have a go…..I think you have sort of answered your own query as you are detecting stars of 18th to 19th mag near the limit and so any 20th mag asteroids will be just beyond the limit. I’ve imaged quite a few faint objects over the years with my Celestron 14 and other telescopes and while I have reached as faint as magnitude 22 under exceptional skies, with long exposures, it becomes much harder to extract objects from the noise beyond mag 19, when the signal is only a few percent above the noisy sky background…. Even with an unfiltered exposure and a 14 inch aperture, getting fainter than mag 19 or so becomes quite a problem unless the sky is really dark and crystal clear and the tracking and focusing are spot on. It may be worth you having a look at Peter Birtwhistle’s web site: http://www.birtwhistle.org/Peter is the undisputed master at tracking down faint asteroids and his website shows what can be done with a large aperture from the UK. Of course, any kind of filter will hammer the limiting mag…a V filter can knock a magnitude and a half off the limit quite easily. Also, while an ephemeris might say an object is mag 20, there is often considerable error in the values given by planetarium packages….the phase of the asteroid plays quite a role too. A lot of the keenest imagers of faint objects use Guide 9.0 to tell them where asteroids and comets are and how bright they are too….it is not a flashy graphics package, but it is accurate and very modestly priced too.So, yes, I would say you are close to the limit of your equipment on an average UK night, but mag 20 is certainly possible if the night is crystal clear and focusing is perfect and tracking excellent, with good seeing. An image scale of around 2 arcsecs per pixel is often quoted as the optimum sampling value for getting faint stars recorded without the sky background swamping them.You ask how can images be aligned on something you cannot see? If you have Astrometrica there is a Tutorial in the help section which explains how to track and stack images allowing for the object’s predicted motion….It is Tutorial number III in my version of Astrometrica…..Martin
Posted by TonyAngel at 20:53 on 2013 Dec 02
I tried this morning but the sky was too bright. I will be trying each morning this week – subject to weather – just to see if I can pick anything up, though towards the end of the week will be better. If there is I will follow it. I have captured it below the horizon just over a week ago with a declination of -1 degree – that is to say just the tail showing.As it was pointed out there are other comets around, especially Lovejoy which is nice and easy. Linear X1 is still there but dropped in magnitude, plus quite a few others worth capturing.Tony Angel
Posted by Neil Morrison at 16:13 on 2013 Nov 30
The Latest Lasco image 14.42 30th nov appears to show Ison vanishing like the proverbial Cheshire Cat. Will the smile remain long enough for us to see it in the early December Dawn!!
Posted by William Stewart at 08:39 on 2013 Nov 30
Thanks to Paul for reporting the observation and to Alex for the analysis … images and videos are now up on the http://www.nemetode.org website.William
Posted by A R Pratt at 00:39 on 2013 Nov 30
William alerted me to this forum posting and he forwarded his single station output from UFO Analyser.My Leeds South camera also captured the meteor, and a provisional dual-station analysis gives:-Sporadic meteorRadiant (corrected for zenith attraction) RA 56.0 = 3h 44m Dec 17.5Geocentric velocity 18.6 km/s (slow)Duration 4.2 sAbsolute magnitude -3.0Detection altitude 81.9km Extinction 47.6kmMoving from Gainsborough (Lincs) to LeedsOrbital data:a 2.6q 0.706e 0.730Peri 70.9Node 67.6Incl 1.2We can’t attach images here, so the plots of the ground track and orbit will be posted on the NEMETODE website. We will notify you when these are available.Clear skies, Alex.
Posted by James Fraser at 23:56 on 2013 Nov 29
In the words of a famous song..She comes out of the sun in a silk dress runningLike a watercolor in the rainDon’t bother asking for explanationsShe’ll just tell you that she cameIn the year of the cat(Al Stewart – Year of the cat)
Posted by Paul A Brierley at 22:57 on 2013 Nov 29
Thank you William.I thought it was brighter. Thank you for clearing that up.
Posted by William Stewart at 22:22 on 2013 Nov 29
Single Station Analysis from NEMETODE Ravensmoor NE Shows this was a sporadic with a maximum magnitude of -2.8This result is provisional as the fireball exited the field of view while still very bright. It may have been detected by other stations and if so, triangulation / orbit determination should be possible. Will update as additional information comes in. William
Posted by William Stewart at 21:37 on 2013 Nov 29
Fireball was picked up by NEMETODE Ravensmoor NE Camera and commenced at 21:00:04.2 UTC – working on the analysis.
Posted by Gary Poyner at 16:24 on 2013 Nov 29
Whilst everyone is going Comet crazy at the moment (and rightly so I guess), lets not forget this Nova in Delphinus. We are just past the first 100 days since it first appeared in our skies as a naked eye star, and it’s still visible around magnitude 11.0 in the evening sky. It’s usually around this brightness level that interest in Novae wanes amongst observers, so I’m appealing to all those observers with small telescopes, DSLR’s/CCD’s to continue monitoring the Nova for as long as possible in the evening sky – and importantly report your data to the VSS AND to Guy Hurst.The Nova is now in the nebular phase, and has been steady’ish around magnitude 11 since the middle of October. However some Novae develop a dusty environment which can manifest itself at this stage of the decline, so it’s quite possible that Nova Del (or V339 Del as it’s now known) just might take a dramatic fade. And remember…you don’t need to get up at Dawn to see it :-)Gary
Posted by Denis Buczynski at 13:10 on 2013 Nov 29
C/2012 S1 ISON will no doubt be remembered as one of the most unpredictable of comets. During the perhelion passage yesterday the first impressions were, as we watched via the SOHO spaceraft images, that the comet had totally disrupted and that there would be no coherent part of the comet’s nucleus left intact. The comet would be dead. However as time progressed we began to see a small but quite bright section of the tail reappear from behind the instrument’s occulting disc. Then this bright tail section seemed to become more compact and a short new tail seemed to start to reform. So it seems that a least a small part of the nucleus survived and has now begun to form a coma and shows some signs of emmisions taking place. Whether this means that the comet will continue to survive and be able to be seen in our skies in the next few weeks is still very uncertain. There may not be sufficient material left and the comet could fade away. However the posibilty of the comet remaining active and being observable,in some form,in the morning sky during the next few weeks is significant. This comet may yet suprise us again, we can only watch in amazment and some awe to observe what nature will offer us.If the nucleus does survive and remains coherent, there is one thing that is certain, it is a long way back to the Oort cloud from here!Denis Buczynski
Posted by Grant Privett at 00:07 on 2013 Nov 29
Will be interesting to see if, in a week or three, imagers can find anything near the predicted position.
Posted by Nick James at 18:21 on 2013 Nov 28
Like Monty Python’s parrot I think the only reason that this comet is still sitting on its perch is that it has been nailed there. It is now visible in the SOHO LASCO C2 field and there is no saturation blooming. Reports indicate that the nucleus no longer exists and we are just seeing a cloud of dust and debris.I’d love to be wrong though. Watching it on the SOHO images is fascinating, although quite hard at times since the site is clearly overloaded.
