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

From: Gavin Eadie (gavin@umich.edu)
Date: Fri Dec 14 2012 - 15:07:05 UTC

  • Next message: Marco Langbroek: "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 <fluorgutten@googlemail.com> wrote:
    
    > 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 <marco.langbroek@online.nl
    >> 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: sattrackcam@langbroek.org
    >> 
    >> 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: http://sattrackcam.blogspot.com
    >> Twitter: @Marco_Langbroek
    >> -----
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