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MikeODay

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  1. MikeODay
    In an earlier post I examined the noise in single dark frames over a range of exposure times.  My conclusion was that the Nikon D7500 was a lower noise camera than the D5300.  This was also backed up by an examination of master bias frames that again strongly favoured the D7500.
    However ...
    The first image I attempted to produce with the D7500 exhibited very strong streaks in the noise.


    I had been in the habit of not using dark subtraction with the D5300 as it has very low thermal pattern noise.  Accordingly, I again only used bias and flat frame calibration in the workflow that produced the above images.  
    Whilst the streaks are due to patterns in the noise being spread across the image due to errors in the application of dithering during tracking, it did indicate that the D7500 did have significant thermal pattern noise.  I found this surprising because the noise in individual frames ( when looked at in isolation ) seems to be completely random.  I thought that perhaps my memory had failed me and maybe the D5300 has the same level of pattern noise but my memory was being tricked.  That is, all my recent images with the D5300 were taken at lowish air temperatures ( ~5 deg or so ) whereas the image above was captured on warm nights ( low 20s ) and so maybe the D5300 would be just as bad at higher temperatures.   
    To test this I produced bias corrected master darks for both the D7500 and D5300 from images all taken at around 20 deg or just over. 
    The images below have all been stretched using the same screenTransferFunction applied to the Pixinsight histogram tool.  The results are striking ...
    D5300 master dark ( 47 subs, bias corrected ) - red channel:

    D7500 master dark ( 281 subs, bias corrected ) - red channel:

    D5300 master dark 100% centre crop - red channel:

    D7500 master dark 100% centre crop - red channel:

    And the histograms of the full size images ( red channels ) ...
    D5300 master dark ( red channel ) histogram:

    D7500 master dark ( red channel ) histogram:

    The Pixinsight statistics tool calculates the following:
    D5300:  mean 2.3,  standard deviation 9.3
    D7500:  mean 7.5,  standard deviation 20.8
    -------------
    Analysis:
    The images and histograms clearly show that the D7500 has higher pattern noise than the D5300.  In particular, from the histograms, 0.1% ( 6,286 ) of D5300 pixels are more  than 44 ADU whereas, for the D7500 this figure is 27 times as great at 2.7% ( 141,305 pixels ).  Furthermore, the master dark for the D5300 was only produced using 47 images -v- 281 for the D7500 so I would expect that this difference would be even higher with more D5300 frames. 
    On the other hand, whilst not shown in the histograms above, my D5300 does have more 'very hot' pixels than the D7500 ( 579 pixels greater than 400 ADU -v- 10 pixels greater than 400 ADU ).  However, these hot pixels are very easily removed via dithering during tracking and sigma clipping when integrating.  The very large number of warm pixels however are very difficult to remove as dithering just places different warm pixels on top of each other.
    I went back and examined the 'random' noise seen in the individual D7500 dark frames ... and yes they do look random when seen individually, however, when flicking between a number of frames it is clear that the 'random' pattern is repeated in each frame! 
    Conclusion:
    My D7500 has very significant thermal pattern noise, albeit randomly distributed in a fixed pattern.
     
    ............
     
    Next steps (?)
    - I could use in dark subtraction during calibration to reduce the impact of pattern noise - however, as my camera is not cooled and the night's temperatures are constantly changing, any master dark will not closely align to the actual thermal pattern noise and as such dark subtraction may help but will not solve the problem
    - Using in-camera dark subtraction ( Nikon's long exposure noise reduction feature ) would almost completely remove the pattern noise from each frame.  However, due to the extra random noise being introduced by subtracting another noisy dark frame from each light frame, as well as the reduction in total light frames by 2, the resultant images will suffer from higher levels of random noise.  So whilst this would be an improvement with respect to the pattern noise, it is not a complete solution.
    - Third option, sell the D7500 and go back to using the D5300 ... 
  2. MikeODay
    Sir John Herschel at the Cape of Good Hope
    Having spent the years 1825 to 1833 cataloguing the double stars, nebulae and clusters of stars visible from Slough, in the south of England,  John Herschel, together with his family and telescopes,  set sail from Portsmouth on the 13th of November 1833 bound for Cape Town.   As detailed below, in an extract from his book, the family enjoyed a pleasant and uneventful voyage and arrived some 5 months later at Table Bay with all family and instruments in good condition.  Reading on however, one might very well think that it might not have ended so well had they but left shortly after ...
    “... (iii.) Accordingly, having- placed the instrument in question, as well as an equatorially mounted achromatic telescope of five inches aperture, and seven feet focal length, by Tulley, which had served me for the measurement of double stars in England; together with such other astronomical apparatus as I possessed, in a fitting condition for the work, and taken every precaution, by secure packing, to insure their safe arrival in an effective state, at their destination, they were conveyed (principally by water carriage) to London, and there shipped on board the Mount Stewart Elpliinstone, an East India Company's ship, Richardson,Esq. Commander, in which, having taken passage for myself and family for the Cape of Good Hope, we joined company at Portsmouth, and sailing thence on the13th November, 1833, arrived, by the blessing of Providence, safely in Table Bay, on the 15th January, 1834, and landed the next morning, after a pleasant voyage, diversified by few nautical incidents, and without seeing land in the interim. It was most fortunate that, availing himself of a very brief opportunity afforded by a favorable change of wind, our captain put to sea when he did, as we subsequently heard that, immediately after our leaving Portsmouth, and getting out to sea, an awful hurricane had occurred from the S. W. (of which we experienced nothing), followed by a series of south-west gales, which prevented any vessel sailing for six weeks. In effect, the first arrival from England, after our own, was that of the Claudine, on the 4th of April, with letters dated January 1st.(iv.) ...”

    “Result of Astronomical Observations, Made During the Years 1834, 5, 6, 7, 8, At the Cape of Good Hope ... “ by Sir John Herschel, 1847
    John Herschel rented a property and set up the twenty foot reflector near Table Mountain, at a site, that was then, just outside of Cape Town.

    The Twenty Feet Reflector at Feldhausen, Cape of Good Hope, South Africa, 1834
    This telescope was made by Herschel in England and transported, along with his other instruments, by ship to Cape Town and then inland to Feldhausen.  The telescope is a Newtonian reflector, built to William Herschel’s design, with a focal length of 20 feet and clear aperture of 18 1/4 inches ( f13 ).
    The location of the telescope was established by careful survey to be: lat 33d 55’ 56.55”,  long 22h 46’ 9.11” W ( or 18.462 deg E ). 

    The site of the great telescope was memorialised by the people of Cape Town by the erection of a granite column that is still there today.


     
    .............
    Observations of the Sculptor Galaxy
    Amongst his many thousands of observations made from Cape Town, of nebulae, clusters of stars, double stars, the sun, etc., Sir John Herschel records that he observed V.1 ( CH10 - Caroline’s Nebula - the Sculptor Galaxy ) during two different “sweeps” and gave it the number 2345 in his South African catalogue.

    Sweeps:    646 -  20th November 1835;   733 - 12th September 1836
    At the latitude of Feldhausen, and on these dates, the Sculptor galaxy would have been at an altitude around 80 degrees above the northern horizon when near the meridian ( which was where the telescope was pointed during Herschel’s “sweeps” ). The sight afforded from this location, with the Sculptor Galaxy almost at the zenith, must have been significantly brighter and clearer than the Herschels had thus far been granted from its location way down near the horizon south of Slough.
    ..........
    Other Obsevations by John Herschel from Cape Town
    Also observed by John Herschel in 1835 were the people and animals that inhabit the moon ...

    The Great Moon Hoax of 1825 - “Lunar Animals and other Objects, Discovered by Sir John Herschel in his Observatory at the Cape of Good Hope ... “
     
  3. MikeODay
    Below is a comparison between single dark frames taken with the Nikon D7500 and D5300 with exposure durations varying from 1 sec to 240 sec ( my usual main light frame exposure ) all at ISO400.
    Firstly a graph of the standard deviation of the noise in the dark frames versus exposure time:

    The standard deviation of the noise is a fairly constant 2 ADU less for the D7500 compared to the D5300 ( pretty much the difference in the read noise between the two )
    However, the difference is not just in absolute terms but also in the quality of the noise ...
    Below are the dark frames -  ranging from 240 sec exposures at the top to 1 sec at the bottom:
                                         D7500                                                                              D5300  



     
    The D5300 dark frames clearly show the pattern in the read noise ( banding down the bottom ) and also have far more chrominance noise compared to the D7500.
    At 240 seconds ( the main exposure I have been using ) the difference is starkly different; the D7500 produces images with much lower noise that is significantly more even and random and hence more likely to be reduced during integration.
  4. MikeODay
    It was love at first sight when I first laid eyes on the gorgeous noise curves of my dear Nikon D5300; young and beautiful with a great body and a large sensor, I was smitten.  Now, well, what can I say... my roving eye has spied a new beauty; younger with heaps of energy that should go all night and , whilst I do prefer larger sensors, I can’t stop thinking about the noise curves hidden beneath that lovely new body...  
    I struggled with my guilt for ages, I really did, but the time has come - I’m dumping the D5300 for the new love of my life; the Nikon D7500.

     
     
  5. MikeODay
    The Nikon D5300 has a well-earned reputation as one of the lowest noise DSLR cameras used for Astrophotography.  Now that I have a new Nikon D7500, I was keen to see how it compared to the D5300 in terms of the level of read noise and the extent to which that read noise is non-random ( and thus needs to be removed using a Master Bias frame to prevent it summing up during image integration).   
    So here goes ...
    .........
    A single bias frame Nikon D5300: ISO400, 1/4000th second:

    This may look pretty bad but really the extreme stretch is bringing out the very small variations across the frame:

    Standard Deviation:     3.53 ADU
    ( note: in this context,  3.53 ADU means 3.53 "units" on a real number scale ranging from 0 to 16,383 ( ie. a real conversion of a 14 bit digital scale ))
     
    The master bias frame looks like this:
    Nikon D5300: ISO400, 1000 x 1/4000th sec frames

    Standard deviation: 0.48 ADU
    The bands at the bottom are each 1 ADU brighter than the one above.
     
    Now for the D7500 ...
    A single bias frame from the Nikon D7500: ISO400, 1/4000th second:

    It is immediately clear that the single bias frame is cleaner.  The statistics confirm this:

    Standard Deviation:     1.37 ADU
     
    And the master bias ...
    Nikon D7500: ISO400, 1000 x 1/4000th sec frame

    Standard deviation: 0.07 ADU
    The band at the bottom is 1 ADU brighter than the background.
     
    The improvement is very obvious
    Std. Deviation single bias frame:  1.37 versus 3.53
    Std. Deviation master bias ( 1000 frames ):  0.07 versus 0.48
     
    In graphical form ...

     
    Conclusion:
    The read noise in a single frame from the D7500 is around 40% of that in one from the D5300.  This should give me greater flexibility to reduce exposure times and still ensure that the read noise is only an insignificant component of the overall noise.
    I will need to consider further the impact of the very low level of pattern noise in the Master Bias; it is so low that I will think about whether or not I still need to calibrate my lights with a Master Bias ( particularly for long exposures when the noise is dominated by light pollution and thermal noise ).
     
     
     
  6. MikeODay
    Observations of the Sculptor Galaxy ( NGC 253 ) by William and John Herschel
    The very large and bright 'nebula' discovered by Caroline Herschel in 1783, that we now know as the Sculptor Galaxy, was observed a number of times by her 'dear brother' Sir William Herschel and by her 'beloved nephew' Sir John Herschel, Baronet.  Some of these observations were recorded and published in the Philosophical Transactions of the Royal Society and, with respect to those by Sir John in South Africa, in the book of Astronomical Observations at the Cape of Good Hope.
    .........
    Part 1.  Observation of the 'class V nebula', H V.1, by Sir William Herschel, 1783

    In 1782, with the fresh patronage of King George III, William Herschel, together with his sister Caroline, undertook the not inconsiderable task of transferring his astronomical equipment from Bath to Datchet ( near Windsor ) in England.  Shortly afterwards, in 1983, Sir William began a "sweep of the heavens" with the very large Newtonian telescope of his design and construction.  With this mighty telescope's twenty foot focal length and clear aperture of a little over eighteen and half inches, William was able to see fainter objects and smaller detail than any other astronomer of that time.

    ( source: The Scientific Papers of Sir William Herschel, Vol.1 )

     

     

    ......
    On the 30th of October, 1783, in the course of one of his "sweeps" with the twenty-foot telescope, Sir William Herschel observed Caroline's 'nebula'  and noted down ( or perhaps more likely, dictated to Caroline )  a description of what he saw and a reference to its position relative to a 4th magnitude star in the Piscis Austrainus constellation, #18 Pis. Aust. ( with reference to Flamsteed's Catalogue (  or HD 214748 , HIP 111954 as we might call it )).  
    Over the course of the next three years, Sir William would go on to view the Sculptor Galaxy a total of seven more times; as recorded in his paper "Catalogue of One Thousand New Nebulae and Clusters of Stars", presented to the Royal Society on the 27th of April 1786.


    ( Source )
    Sir William's somewhat cryptic notes can be translated by reference to the key provide in his paper and doing so reveals the following:
    Class:  V. ( very large nebula )
    Number: 1
    Observed ( by WH ): 30 Oct 1784
    Reference star:  18 Piscis Austrainus ( Flamsteed's Catalogue; the best reference for the time - we might use epsilon Pis. Aust. or  HD 214748 / HIP 111954 )
    Sidereal direction rel. to star ( following or leading ):  following star
    Sidereal time rel. to star: 128 min 17 sec
    Declination direction rel. to star: north of star
    Declination amount rel. to star:  1deg 39min
    Observed: 8 times ( up until April 1786, the date of the paper )
    Description:
    - cB:     "confidently bright"
    - mE:    "much extended"
    - sp:     "south preceding"
    - nf:      "north following"
    - mbM:  "much brighter middle"
    - size:    50' x 7 or 8'
    " CH" denotes that it was discovered by his sister Caroline Herschel
    The note he refers to expands on details of Caroline's discovery ...

     
    ...............................................
     
  7. MikeODay
    Observations of the Sculptor Galaxy ( NGC 253 ) by William and John Herschel
    .........
    Part 2.  Observations of "Caroline's Galaxy" by Sir John Herschel,  1830's

    Sir John Herschel, the only child of Mary Baldwin and Sir William Herschel, was born in 1792 when his father was in middle age and already famous as one of world's leading astronomers.  Having excelled in school, and no doubt inspired by his famous elders, John Herschel decided upon a career as a 'man of science' and set out to pursue a wide range of interests; with one particular focus being a continuation of the study of the heavens commenced by his father and aunt, Caroline Herschel.   
    In 1820, with the assistance of his father, John Herschel supervised the construction of a new telescope at Slough in England.  As described in the extract below ( from a paper presented to the Royal Society in 1826, titled "Account of some observations made with a 20-feet reflecting telescope ... " ), the telescope had a polished metal mirror with clear aperture of 18 inches, focal length of 20 feet and was modelled on the same design created by his father.

    It is this telescope, in the 1820’s and early 30’s, following the death of his father and the return of his aunt Caroline to Hanover, that John Herschel used to 'sweep' the night sky and extend the catalogue of nebulae and clusters of stars that was published by his father ( see W. Herschel's  Catalogue of One Thousand new Nebulae and Clusters of Stars ).
    On the 1st of July 1833, having complied sufficient observations, John Herschel presented to the Royal Society an updated list of the positions and descriptions of the Nebulae and Clusters of Stars that he had thus far observed.  As noted in the introduction to the paper published in the Philosophical Transactions, he had planned to wait before publishing until he had complied a fully comprehensive general catalogue of objects visible from the south of England.  However, due to his expectation of “several more more years additional work” needed to complete the task and his assessment that he now was in a position to address, at least in part, the then current “... want of an extensive list of nebulae arranged in order of right ascension ...”, he elected to present his list, “ ... simply stating the individual results of such observations as I have hitherto made ... “.  It was not until October 16, 1863, some thirty years later, that Sir John would deliver to the Royal Society his General Catalogue of Nebulae and Clusters of Stars.

    As well as introducing many objects that had not previously been recorded, Sir John’s list of 1833 included a re-examination of, and in some cases a small correction to, the positions of many of the deep sky objects observed by his father and noted down by his aunt.   One of these re-visited objects was, unsurprisingly, the large and bright nebula discovered by Caroline Herschel in 1783 and recorded in Sir Williams’s catalogue as V.1 / CH 10 ( object number one, of class five ( very large nebulae ) / Caroline Herschel #10 ).
    In total, John Herschel records around 2500 observations of nebulae and clusters of stars in his 1833 paper; with observation #61 being V.1, the “ Sculptor Galaxy “
    .
    The measured position of V.1is given in RA and the angle from the north celestial pole ( all reduced to epoch 1830.0 ).  The description can be interpreted by reference to the legend in the paper.

    Thus, “ A vL mE vB neb “ becomes “ A very large, much extended, elliptic or elongated, very bright nebula “.  He also notes that in addition to this observation, #61, noted down from sweep #306, V.1 was also observed in sweep #292, “but no place was taken”.  
    The figure to which he refers , figure 52, is included towards the back of his paper and is a sketch he made of the Sculptor Galaxy.

     
     
     
    to be continued ...
     
  8. MikeODay
    The Discovery of the Sculptor Galaxy by Miss Caroline Herschel in 1783
    On the 23rd of September 1783, sitting before her telescope in the field behind the house she shared with her brother William in Datchet near Slough in the south of England, Miss Caroline Herschel "swept" the sky searching for new comets and never before seen star clusters and nebulae.   On this occasion, way down in the sky, not far above the Southern horizon, Miss Herschel saw and noted down a very bright and large nebula where one had never before been recorded and that was later recognised by her brother, Sir William, as the discovery by Caroline Herschel of the nebula he listed in his catalogue as H V.1.
     ( circ. 1825-33, Sir John Herschel, beloved nephew of Miss Caroline Herschel ) 
    Today we know this 'nebula' to be, not as some thought then, a swirling mass of stars and gases within our own galaxy, but rather, a galaxy not unlike our own but way more distant than the outer reaches of of own Milkyway galaxy.    Given various names, Silver Dollar Galaxy, Sliver Coin Galaxy or simply by its number in the New General Catalogue, NGC 253, it is most commonly called the Sculptor Galaxy and we owe its discovery to the first female professional astronomer.
     Caroline Herschel ( 1750 - 1848 )
     
    ...
     ( link )
     
    ( 1782 - 1783 )

    ...

    ...

     
    ...


    H V.1
    Observed ( by WH ): 30 Oct 1784
    128 minutes, 17 seconds following and 1 degree, 39 minutes north of  referenced star 
    Description:
    - cB: "confidently bright"
    - mE: "much extended:
    - sp: "south preceding"
    - nf: "north following"
    -mbF: "much brighter middle"
    - size: 50' x 7 or 8'
    from:
     ( link )
    ...............................
     
    The location reference to H V.1 ( NGC 253 ) in William Hershel's catalogue is in relation to a star found in Flamsteed's Catalogue, 18 Pis. Aust., which is #18 in Piscis Austrainus or Epsilon PsA, the 4th magnitude star HD214748 ( HIP111954 )  ( source )

    ( Plate from "Atlas Coelestis" by John Flamsteed, 1646-1719 )
     
    -------------------------------------
     
    William Herschel found favour with the King and was granted a position as Royal Astronomer to George III in 1782.  Shortly after, William and Caroline moved from Bath to Datchet ( near Windsor ) and took up residency in a rented house which, whilst somewhat delapadated and damp, had ample accommodation and fields for William to construct and deploy the large telescopes he wished to build.  It was in these grounds that Caroline set up her "Sweeper" to look for comets and doing so also discovered a number of 'nebulae' including ( in 1783 ) what was later to become known as the Sculptor Galaxy.

    ( The Herschel house at Datchet near Windsor )
     


    ( The Lawn, Horton Road, Slough ( Datchet ) - Google Maps )
     
    .............
     
    Caroline Herschel's "Sweeper" was a 27" focal length Newtonian telescope that was supported in a kind of altitude-azimuth mount consisting of a rotating table and a small gantry and pulley system that was used to effect altitude adjustments by sliding the tube up and down against a board used to provide stability.  There has been some conjecture as to the exact details of the construction, however the image below, even if perhaps not the actual instrument, gives an indication of the overal design philosophy. 

    Late in her life Caroline Herschel recorded details of her telescope in a booklet titled "My little Newtonian sweeper":

    In her memoir, Caroline Herschel describes the performance of her observations as the conducting of "horizontal sweeps"; from which one might assume the task consisted of setting the altitude in accordance with the plan for the night's observing and then slowing rotating the top of the table in azimuth as one observed and noted down the objects that passed across the view in the eyepiece.  However, with the arrival of this new "telescopic sweeper" in the middle of 1783, Caroline Herschel added the new method of sweeping in the vertical, as noted below in an extract from her observing book ( source for both extracts: "Caroline Herschel as observer", Michael Hoskin, Journal for the History of Astronomy, 2005 )

     
    ....
     
    The achievement of her discovery of the 'nebula' in the Sculptor constellation was remarkable in so many ways; not the least of which being the low path in the sky that the Sculptor galaxy follows when observed from Datchet in southern England - which on the night of her observation would not have exceeded 12 degrees or so above the horizon.
    Today, 234 years later, and blessed with 21st century luxuries and conveniences, I write on my IPAD and flip over to my planetarium application, SkySafari, and model the sky as it was seen by Caroline Herschel from near her house on the 23rd of September, 1783 ...
     



    ( SkySafari by Simulation Curriculum )
     
     
     
     
  15. MikeODay

    Cover Page

    By MikeODay,

    A blog by Mike O'Day
    -----------------
    When I show my astrophotography images to my friends and family they invariably want to know what they are looking at.  This led me to wonder if there was a way I could display my images on a single page together with a few notes on the target object as well as few technical details of the capture for those who might be interested. What I came up with a "scrapbook" like page that combines all of these three elements in a single PDF sheet ( or jpeg image) that ultimately I might combine together to form a PDF book that I can share online or send to friends and family. In the meantime, I thought I might create this blog to share each page of my work-in-progress towards volume 1 of my Astrophotography Scrapbook.
    Any and all comments, observations, suggestions and constructive criticisms will be warmly received.
    -----------------
     

     
    -----------------
    Higher resolution versions of the images in the scrapbook can be found in following galleries
    Mike's Images
    or
    flickr.com/photos/mike-oday       500px.com/MikeODay      photo.net/photos/MikeODay
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