Re: Using a DSLR (was Re: North Korea satellite observed)

From: George Roberts (
Date: Fri Dec 14 2012 - 15:19:06 UTC

  • Next message: Ted Molczan: "North Korea satellite: identity of catalogued objects"

    What kind of camera do you have exactly as the answer depends on that?
    As Marco explained, ideally you want a camera with "live view" mode which 
    lets you zoom in to just a tiny part of the image live on the camera's 
    display screen and focus on a bright star live and then don't touch the 
    focus.  If your lens is "loose" you might have to keep it from moving with 
    rubber bands or velcro or something so you don't lose focus every time you 
    take a picture.
    I have an older DSLR that doesn't have this feature and so I put it in the 
    lowest quality mode (for fast transfer to laptop) and hooked it up to some 
    special software on the laptop that downloaded pictures as you took them 
    "live" (delayed by 4 seconds).  It was very frustrating and I would memorize 
    the location of the focus within about 0.1mm using scratches and dust and 
    take 30 pictures at different focuses and pick the best picture and take 
    good notes.
    On older SLR cameras (before digital) you could buy a magnifying lupe that 
    sticks out of your eyepiece viewfinder that zooms into just part of your 
    focusing screen - that's what astrophotographers used to use.
    Also you should definitely get an f1.8 or lower lens.  You should be able to 
    get a 50mm F/1.8 lens for $50.
    - George Roberts
    -----Original Message----- 
    From: Gavin Eadie
    Sent: Friday, December 14, 2012 10:07 AM
    To: Petter Aslaksen
    Subject: Re: Using a DSLR (was Re: North Korea satellite observed)
    I've an associated question regarding focusing.  I've never been able to 
    focus really well on star fields, and seem to often catch stars as little 
    blobs of light [a recent posting here, into Orion, also exhibited a little 
    of this].  Autofocus doesn't work well (at all?) in the dark of course, and 
    manual focus is tricky -- visual focus throughout the lens doesn't have 
    enough light or precision; or I can set focus under lighter conditions and 
    carefully not touch the lens, though I suspect temperature changes cause 
    some drift.  Also, I can't just rotate my lens to the physical limit and 
    expect that to focus at infinity because it travels a tiny bit past 
    infinity.  Obviously many people in this group have solved this problem and 
    I'd much appreciate some suggestions.
    On Dec 14, 2012, at 7:04 AM, Petter Aslaksen <> 
    > Hello
    > I have a follow up question here, making my comeback on the mailing list
    > after about 10 years
    > I am stuck with my 18-200mm 3.5-5.6f for now, I have a 50mm 1.4f at home,
    > which I guess would be a good lens for this.
    > I want to shoot satellites tonight. What should I do, zoom in all the way
    > and keep it at 5.6f, or should I zoom out and have it at 3.5? Any ISO
    > suggestions for 30 sec exposures.
    > By pure accident I photographed Lacrosse 4 two days ago when I tried to
    > shoot Geminid meteorites. Coincidentally, Lacrosse 4 passed through the
    > Geminid radiant, and at first I thought it was a meteorite.
    > Petter
    > On Fri, Dec 14, 2012 at 12:11 PM, Marco Langbroek 
    > <
    >> wrote:
    >> Op 13-12-2012 1:09, Mal Ninnes schreef:
    >>> Hi Marco,
    >>> It's a Canon 600D with EF-S 18-55 IS II lens, which I only got just
    >> recently.
    >>> Still getting the hang of night-time shots, as I'm not an expert
    >>> photographer. I read the messages from yourself and Greg the other day
    >> and
    >>> took photos of my GPS app (on android) at the start and end of my
    >> session,
    >>> also taking into account the 16 leap seconds, and I've previously 
    >>> checked
    >>> this against the US navy time servers on the net. The Canon time was off
    >>> during last nights session by 4 seconds, which I corrected for as well.
    >>> Obviously with this setup, I can't get sub-second accuracy. But for a
    >> start,
    >>> I'm ok with it.
    >> Hi Mal,
    >> It is perfectly possible to get subsecond accuracy with a DSLR (it is 
    >> what
    >> I
    >> do), but it involves carefull calibration.
    >> Note that the time display of a GPS device is seldom quite accurate 
    >> (while
    >> GPS
    >> time in itself is very accurate, this is not the case for the time in the
    >> display on most GPS devices. Unless they are specifically build for 
    >> timing
    >> accuracy, such as GPS video time inserters). The display of my Garmin GPS
    >> can be
    >> off by more than a second. This is because sending positional data to the
    >> display gets priority over time information in internal processing in
    >> these devices.
    >> The best time source to use with a DSLR actually is a radio-controlled
    >> clock, at
    >> least if you avoid the Cresta brand clocks (they are inaccurate, I have
    >> found).
    >> Oregon Scientific is a good brand. Avoid too fancy clocks with many bells
    >> and
    >> whistles, as you never know how detrimental those extra bells and 
    >> whistles
    >> are
    >> on the actual display accuracy. Force the radioclock to synchronise 
    >> shortly
    >> before your observing session (e.g. by taking the batteries out and then
    >> put
    >> them in agan).
    >> Don't bother with your camera EXIF time: use the radio-controlled clock 
    >> to
    >> try
    >> to trigger your camera at an exact time and write those times down. 
    >> Target
    >> a
    >> number of unclassified satellites in a controlled, not too low orbit 
    >> (e.g.
    >> Iridiums) and map the offsets in delta T of your obtained positions to
    >> predicted
    >> positions (Scott Campbell's software is very useful for that). That will
    >> give
    >> you your calibration values.
    >> For satellite photography, ideally you would want a fast prime (= fixed
    >> focal
    >> length) lens rather than a slow and optically mediocre zoom like your 
    >> EF-S
    >> 18-55
    >> (if your lens is the kit-lens, you'd want to replace that one anyway). On
    >> the
    >> second hand market you can get a fast EF 50mm for very reasonable prices:
    >> try to
    >> get one and use it with an F settings no larger than 2.8. The larger your
    >> lens
    >> opening, the fainter objects you will be able to capture. For a given 
    >> focal
    >> length, an F2.8 or F1.8 hence is advantageous over an F4.5 or F5.3: your
    >> object
    >> will appear brighter on the image.
    >> I noted that your picture was slightly out of focus. The best way to 
    >> focus
    >> is to
    >> put your lens on "manual", then put the "live view" of your camera 
    >> display
    >> on.
    >> Point to a bright star, zoom in on it on the display (not with the lens
    >> itself!
    >> Just on the display with the "+" button) and focus manually untill the
    >> star is
    >> pinpoint. Take a test image to see whether focus is indeed sharp.
    >> Hope these hints are helpful!
    >> - Marco
    >> -----
    >> Dr Marco Langbroek  -  SatTrackCam Leiden, the Netherlands.
    >> e-mail:
    >> Cospar 4353 (Leiden):   52.15412 N, 4.49081 E (WGS84), +0 m ASL
    >> Cospar 4354 (De Wilck): 52.11685 N, 4.56016 E (WGS84), -2 m ASL
    >> Station (b)log:
    >> Twitter: @Marco_Langbroek
    >> -----
    >> _______________________________________________
    >> Seesat-l mailing list
    > -------------- next part --------------
    > An HTML attachment was scrubbed...
    > URL: 
    > _______________________________________________
    > Seesat-l mailing list
    Seesat-l mailing list 
    Seesat-l mailing list

    This archive was generated by hypermail 2b29 : Fri Dec 14 2012 - 15:19:47 UTC