Andy,
There are 2 reasons for reducing ISO on a DSLR for astrophotography.
1. Reduce noise
2. Increase dynamic range
The sweet spot referred to in some articles is the point where reducing ISO doesn't increase dynamic range. It says more about the dynamic range performance of the camera than the high ISO noise performance. The lower the number the better the dynamic range.
A company called DxO also measures the noise performance of sensors. For a given low light level, which they don't specify but I've always assumed is higher than astro imaging levels. They quote an ISO for each camera sensor that corresponds to the ISO where the noise levels are -30dBrelative to signal. -30dB is a signal to noise ratio of 31. They call this the sports rating. So I think of the light level as being like I might find at a night time illuminated sports match not astro levels.
I find I can do astrophotography that I am happy with at -
maximum ISO = 2 x (DxO sports ISO rating).
But I only use this sort of high ISO for static tripod photography or for a moving target such as aurorae when necessary. I use 1x or 0.5 times where possible.
See
http://joe-cali.com/eclipses/PAST/TS...ora/index.html
Others might not be happy with my ISO/noise tolerance. That's for each individual to decide. Of course you can go right down to the low ISO value and get better dynamic range but as Kens noted, you need to consider periodic error. I would add two other things -
1. polar alignment drift
2. an uncooled DSLR will suffer from thermal noise and it as not linear with time. The longer the exposure, the hotter the sensor gets, thermal noise quickly becomes a problem.
In Phil Hart's review of the Star Adventurer,
http://philhart.com/content/star-adventurer-review, he measured the PE at 50". If you do long exposures that span a full PE cycle, you will need to use lenses 25mm and shorter to keep the PE wobble < 1 pixel without autoguiding.
In his review, Phil stated,
"Once I moved up to 80mm focal length, I found I needed exposures of ~1 minute or less for pin-sharp stars and at 200mm I was using 30 second exposures. Two minute exposures at 200mm focal length were mostly trailed to varying degrees, but if you were just aiming for Facebook resolution then you'd still be fine. If you pixel peep and aim for perfection like I do then you'll need to stack lots of fairly short exposures for sharp images. But this 'track and stack' approach is well proven and is one I've used for a long time."
Your D600 camera has excellent noise characteristics.
DxO sports/low light ISO rating for D600 = ISO 2980
Highest ISO for highest dynamic range (sweet spot) ISO 200 Lower than this won't gain more dynamic range.
All astrophotography is a matter of compromises. The cheaper the gear, the more you need to compromise.
In your case,
The dynamic range ISO response of your camera results in improving dynamic range down to ISO 200 however polar alignment, large periodic error and thermal noise limit the length of exposures. Good high ISO noise performance allows you to experiment with high ISO's to shorten exposures to lengths that your gear can handle.
Lot's of good stuff on the web to self educate some aimed squarely at DLR's some more generally at noise :
References I used for info on the D600
http://dslr-astrophotography.com/iso...nikon-cameras/
https://www.dxomark.com/cameras/bran...taType=rankLln
Photographic Dynamic Range versus ISO Setting
http://www.photonstophotos.net/Chart...D,Nikon%20D800
Craig Stark of Nebulosity fame has an excellent youtube video of a talk
https://www.youtube.com/watch?v=EO4QFb3ydNM
and a 5 part series of PDF's on signal to noise under the 2009 & 2010 sections of his articles pages.
Another excellent youtube talk on DSLR astrophotography by Tony Hallas is this one :
https://www.youtube.com/watch?v=PZoCJBLAYEs
good luck
Joe