DSLR and the Horsehead

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  • #573307

    Posted by Graham Relf at 12:05 on 2013 Dec 17

    I just want to correct an unfortunate impression given by one of my photos in the current BAA Journal (2013 Dec, p364). The photo has printed very dark so it implies that the Horsehead Nebula cannot really be photographed with an unmodified DSLR. Nothing could be further from the truth. A better version of my photo can be seen here.It was taken with an unmodified Canon 5D MkII at the prime focus of a 10" (250mm) f/4.8 Newtonian on an unguided HEQ5 mount, 81 exposures of 32s each, stacked and post-processed by my own software. An earlier attempt can also be found in the BAA Gallery.I am constantly trying to explain to beginners that, contrary to what they may read in Internet forums, they do not need a lot of expensive equipment to be able to photograph amazing things in the night sky. All you really need is a digital camera, something to fix it on, and some free stacking software. Oh, and get out of town for some darker skies. A motorised tripod is desirable but not essential if you take short enough exposures (see our field of view and trail calculator ) but more of them. Many interesting objects are too large to fit into a telescope’s field of view, so a telescope is optional. And if you balance ISO, exposure time, and number of exposures a guiding system is certainly not necessary (I haven’t yet felt any need to get one).I have a book in preparation which expands on that message, to try to encourage beginners to get out there and have a go. It will be available in the New Year. More details later.

    #576406

    Posted by Phillip Hudson at 13:25 on 2013 Dec 17

    Really nice picture and very encouraging to see it done with kit as you describe

    #576407

    Posted by Charles Taylor at 18:25 on 2013 Dec 17

    Many thanks for the superb image – you have inspired me to take courage and have a bash with my fairly basic DSLR (Olympus E410) and my 200mm Newtonian over the Christmas holidays. By then, if Santa is kind, my EQ5 should be motorised.

    #576408

    Posted by Grant Privett at 22:13 on 2013 Dec 18

    Could you tell us whether you dark subtracted and flat-fielded please?

    #576410

    Posted by Graham Relf at 09:49 on 2013 Dec 19

    Grant, A good question. I did NO flats or darks. I rarely do.This is another thing which I think puts beginners off. Yes, you must do those things for photometry but otherwise there is no need (with today’s cameras – and mine is 5 years old now). I know this attitude will be controversial.My method takes into account the inevitable fact that the unguided mount is inaccurate. That makes the camera’s fixed pattern noise and defective pixels wander around among the sequence of star images. So when they are stacked those problems are watered down by a factor equal to the reciprocal of the number of exposures taken. I find that is sufficient – except when doing photometry.Of course the mount must not be so inaccurate that a star image is elongated by more than the radius caused by atmospheric turbulence. That determines the maximum length of each exposure. My HEQ5 fits the bill fine for exposures of half a minute and sometimes 1 minute. In suburbia you cannot do exposures as long as that at high sensitivity anyway without complete fogging. (But the Horsehead cannot be photographed from suburbia because it is below the background level. I have tried.)

    #576412

    Posted by Grant Privett at 09:36 on 2013 Dec 20

    Yes, I know what you mean about the dithering provided by a mount’s PEC (and/or slight non-alignment) reducing noise – I used a Super Polaris for some years and found that after 50 or so frames the background could be very smooth indeed.RE: flats I wasnt thinking of photometry – it can be done with a DSLR, but its not the obvious choice. The main advantage of a flat in this instance is in reducing obvious vignetting. I feel dark corners in images detract from the overall appearance – especially with extended targets.It doesnt take long to create a flat (I put a plain fairly opaque plastic bag over the end of my ‘scope and point it at a nearby evenly illuminated wall) and with some scenes the resulting image is greatly improved. Not essential, but often worth it if you have the time.

    #576413

    Posted by Graham Relf at 11:51 on 2013 Dec 20

    Yes, I do have a flat box (cardboard box lined with white paper, 4 torch bulbs in the corners next to the circular aperture that goes over my telescope). I still do not often use it. I agree that it’s a simple way to fix vignetting but I tend to do that in post-processing (various methods).I would certainly use my flat box if I had any optics between my telescope and the camera body (to magnify the target*) because there is then a risk of out-of-focus dust spots and rings which a flat frame would correct.(* or perhaps a coma corrector but again I have one but don’t use it because I found it introduces other problems such as ghosting from bright stars – the coma is not conspicuous in my Horsehead photo). I think the golden rule (as in software development) is to keep things as simple as possible.

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