Limiting Magnitude

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    Posted by Eliot Hall at 15:53 on 2013 Dec 04

    I have question about limiting magnitude of my kit. Looking at the Minor Planets Center website most of the asteroids listed above 19th magnitude in the V-Band.Last weekend decided to hunt for some faint asteroids. Cartes du Ciel said that there would be 3 20th magnitude asteroids in the field of view of a galaxy I was imaging. I double checked the ephemeris on the MPC website none of the asteroids required further observations; therefore I assumed that they had stable orbits.I imaged the target area with my f/4.7 10inch Newtonian with a Starlight Xpress MX916 through a UV/IR cut filter. I look 10x60s and 10x120s exposures with darks and flats. The telescope was pointing above 50 degrees elevation. Using Astrometrica and Cartes Du Ciel I scanned the regions of the image where the asteroids should be. However I could not find any trace of them. Using the object selection tool in Astrometrica I measured at the V magnitudes of the faintest stars in the image. The faintest I could make out were between 18th-19th magnitudes.1.Am I being unrealistic expecting to find 20th magnitude asteroids using the equipment Ive got?2.Have I hit the skys magnitude limit? I live on the edge of a village and the seeing was OK but not brilliant. I also tried analysing a few 300s subs from the same night as well but couldnt see any extra stars.3.Do I need to stack the images to get a better S/N ratio? If so how can the images be aligned on something you cant see?Eliot


    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: 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 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 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 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: ‘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 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 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 Grant Privett at 16:37 on 2013 Dec 08

    Sounds about right. When detection is what you seek, Newtonians are better than refractors – as they bring all colours to focus at the same point and ensure you collect as many photons as you can.It is true that detecting objects that are just 1% brighter than the surrounding sky signal is difficult, but with care (good flats, clean optics, using the best dark nights only, many exposures) good results can be achieved. It has the added advantage that, having mastered the imaging of the very dim, you will have refined your techniques and will find your imaging of brighter targets also improves.Either way, its fun.


    Posted by Richard Miles at 18:24 on 2013 Dec 09

    Just one final point, Eliot. You write:"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)."The sky will have a certain brightness (usually in the range for a dark sky of magnitude 19.5-21.5 per square arcsecond – V mag equivalent). So imagine the seeing and sky brightness are fixed then the bigger the aperture, the greater the signal to noise and so the fainter the limiting magnitude. N.B. The plate limiting magnitude is often stated to be an SNR of 2.5.If my memory serves me correctly, the faintest object I have reported astrometry of using a Celestron 11 is V=21.5. I stacked quite a few images for that!Cheers,Richard

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