Canon vs. the competition - time to switch ?

[RobertM]

Canon seems to now be laging behind the competition sensor wise with both Nikon and Pentax producing very low noise camera with wide dynamic range.

The question is whether it's time to look at the alternatives to Canon for an Ha modded camera ?

I think this is a topic that probably needs a bit of discussion.   Even though many of us use Canons, that's really because there hasn't been a viable alternative, perhaps there are now.

Thoughts...

Robert

[mickw]

Doesn't the Nikon use an internal IR detector to monitor the shutter speed ?

I think that Canon will stay in the lead for some time because it's a relatively easy conversion (not counting my attempt  ) and the availability of control software/hardware.

[Mike]

Doesn't the Nikon use an internal IR detector to monitor the shutter speed ?

No not on the new models.

Robert - The D7000 I have has a steadily growing fanbase for astro-photography due to its low noise.

[MarkS]

Yes, you're right Robert.  My understanding is that Nikon, Pentax and Sony have definitely overtaken Canon in terms of Quantum Efficiency.

However, as far as I know, for astrophotographic use Nikon is still hampered by the fact that you can't get a true raw image from the camera - the raw image is first passed through their infamous non-linear "remove stars" digital filter.  In practice I'm not sure how serious this effect is - it probably depends on how oversampled your image is.  Even if the filter doesn't remove your stars, it will affect the calibration with darks, offset and flats to a greater or lesser extent

Also, how easy are Nikon and Pentax to modify (in terms of removing/replacing the internal filter for H-alpha response)?

There are some useful comparisons here for QE (for green light) and read noise:
http://www.sensorgen.info/  (click on the indiviual camera links to get additional useful info)
http://forums.dpreview.com/forums/thread/2750802?page=5  (mainly Canons)
The figures in the links above have all been "reverse engineered" from the figures produced by
http://www.dxomark.com
In my mind, there is a question mark over how accurate this approach (of curve fitting to DXO measurements) is.  I would prefer an approach that directly measures gain and read noise for each ISO and then points the camera at a calibrated light source to determine the QE (which I would like to see at green and at other wavelengths).

One other thing to be wary of is the dark current.  For astrophotography we do very long exposures - which means that the typical quoted "low light" performance of the camera is not necessarily a useful indicator of what to expect.  Worse, we tend to do many hours of long exposures continuously in one sitting - which heats up the sensor and increases the dark current.  This is why I think it is crucial to measure dark current over a long period (e.g. 2 hours) as I have done here:  http://forum.orpington-astronomy.org.uk/index.php?topic=8742.0

The important measures that ultimately determine how well the sensor will perform for astrophotography are:
1) Quantum Efficiency
2) Read Noise
3) Dark Current

It would also be interesting to know the H-alpha response for a modified camera.  It could well be that some ranges of sensors have a much better H-alpha response than others.  But I have yet to see any data on this issue.

Mark

P.S.  One interesting thing from the sensorgen site.  Click on the Nikon D7000 and the Nikon D3s.  Why is it that the D7000 has constant low read noise right across all ISOs whilst the D3s has ridiculously high read noise at ISO 100?  This is also something that has always annoyed me about the Canons - to take advantage of the low read noise you have to increase the ISO which decreases the dynamic range.

[Mike]

Mark, again that "star eater" algorithm only affected older Nikon models.

[MarkS]

Mark, again that "star eater" algorithm only affected older Nikon models.

It was still affecting the D3:
http://www.astrosurf.com/buil/nikon_test/test.htm

I'll be very interested in any info you have regarding more recent models.  Are you saying that a Nikon raw image from a recent camera is identical to a Nikon "Mode 3" image? (Mode 3 was the trick where you had to switch off the camera before it had a chance to apply its hot pixel reduction algortihm aka "star eater" - this left the true raw data on the memory card)

[Mike]

The Nikon D5100 and D7000 are not affected.

There are also Firmware patches you can apply for the D5100 to give full lossless uncompressed NEF files:-

http://nikonhacker.com/viewtopic.php?f=2&t=606

Also see - http://forums.dpreview.com/forums/post/37071846

BTW a great place to find out technical specs on just about any camera is Snapsort: http://snapsort.com/

[MarkS]

Also see - http://forums.dpreview.com/forums/post/37071846

Thanks - I'll read this carefully tonight.  This fact, together with the improved QE and the low read noise across all ISOs could mean the Nikon D7000 is a DSLR astrophotography game changer!

So how easy are they to mod?

Mark

[RobertM]

Mike,

Would you have time to characterise your camera in the same way Mark has ?

There are also the Pentax K5 II and the Nikon D600 to consider as 'affordable' alternatives.

The only downer I can see it that Canon overall has the best glass and that can't be used on other cameras.

Cheers
Robert

[MarkS]

Thanks - I'll read this carefully tonight.  

Extremely interesting read.  Also you should take a look at this one where the author describes her approach (taking images of a carefully designed test chart at two different exposures - a slower exposure where the hot pixel removal algoithm is triggered and a faster exposure where it is not).
http://forums.dpreview.com/forums/post/34309201

Looks like Nikon have definitely improved the algorithm, although to be honest, my preference would be for an option that switches it off entirely.

Mark

[MarkS]

... my preference would be for an option that switches it off entirely.

The reason being that if you do wide angle shots of the night sky - e.g. using a 50mm lens, then depending on where the star image falls on the sensor grid, many stars could end up as single pixels (if your lens is sharp enough and focusing acurate enough) and will still be removed.  Although you can take many subs and use dithering to mitigate this problem, it is still not ideal.

[RobertM]

It doesn't look like many are using the D7000 for astrophotography but I did find a link to a page describing the filter removal process http://www.lifepixel.com/tutorials/infrared-diy-tutorials/nikon-d7000-ir

I wonder whether the Pentax K 5 would be any different.  It has the same sensor as the D7000 but that camera is promoted for astrophotography (with the O-GPS1 tracker).

Robert

[MarkS]

I wonder whether the Pentax K 5 would be any different.  It has the same sensor as the D7000 but that camera is promoted for astrophotography (with the O-GPS1 tracker).

Is that also the same Sony sensor used by Sony's own A580?  All 3 cameras seem to have very similar performance.
I wonder how much the dark signal changes form camera to camera (after 2 hours solid use).  It might be very dependent on the heat generated by the rest of the camera and could easily vary between manufacturers.

D7000 certainly looks easy enough to mod.