5 replies to this topic

[drmikevt]

I'm trying to make final decision on the eclipse imaging run and have read various things about how to translate DSLR recommendations to the 6200.  In order to do that, I need to have a sense of what the ISO equivalent is of the camera at gain zero and/or gain 100.  

 

I've read that gain 0 equals about ISO 6 or 12 - can anyone comment on this?

 

Many Thanks!

[WadeH237]

That doesn't sound right to me at all.

 

From what I understand, the brightness of a total eclipse is similar to a full moon.  I've imaged the full moon with one of my older CCD cameras (it was an ST10).  To avoid saturation, I had to take very, very short exposures, like .001 seconds or less.  I've never imaged the moon with my ASI2600, but I assume that it would similar in this regard to the older camera.

 

I was at Disneyland one time, waiting in a long line after dark.  I noticed the full moon overhead and started snapping pictures with my DSLR.  I believe that I was at ISO 800 and was getting the best results with something like 1/4000th of a second exposures (I was shooting in manual mode).

 

Unfortunately, I am on the road, hoping that skies will be clear at the right time tomorrow, so I can't review my records for either of the above situations to get the actual numbers.

 

So my suggestion would be to shoot very short exposures.  If you under expose, there are things you can do in post processing to clean it up.  If you over expose and saturate things, you can't salvage it.

 

Please consider this to be qualitative, rather than quantitative.  I'm explicitly not going to suggest specific settings, since I could be wildly off...

 

One thing that you could do if you use automation, is you could create a session that uses many different exposure times, kind of like building your own bracketing.  The more that I think about it, this is probably what I would do if I were using my deep sky rig to image the eclipse.

 

To be honest, though, I would never attempt using my deep sky rig for a total solar eclipse.  The personal experience of observing it naked eye is far too important to me.  So I left all that stuff at home (2200 miles away) and only brought my DSLR.

 

If you've never witnessed a total solar eclipse, the experience at totality is almost spiritual (and I've been accused many times of being as emotionless as a Vulcan from Star Trek).

 

If you have already witnessed a total solar eclipse, and your interest is solely in imaging, then take everything that I say with a large grain of salt.

 

Here is my plan:

 

I am going to set up the camera today with exposure settings, ISO setting, exposure bracketing, etc.  I'll use autofocus on a very distant object - perhaps an airliner at cruise altitude - and then flip the switch to manual focus so it doesn't change.  I'll have the camera on a neck strap.  Right after totality, I'll point the camera by hand and go click-click-click-click-click, and then just drop the camera and watch totality.  If the focus looks off in the view finder, I'll switch to autofocus and try.  If that doesn't work, I won't spend more than a couple of seconds on it and give up on the camera.  There will be plenty of images taken by others that are far better than anything I could do.

[SilverLitz]

Go to DXOMark website to get some data by stated ISO for your camera and compare to the ZWO's data curves by gain.  There is also another website with good DSLR info, Clarkvision???.

 

I look at saturation limits by ISO/Gain and put everything into a standardized per micron^2 basis and map in your OTAs FL, t-ratio (taking into account transmission losses).  That way you can compare differing cameras, ISO/Gains, and OTAs.  I start from a known combo (for me it was my 7Dmk2 w/ 70-200 f/2.8L + 2x teleconverter at 400mm and f/8).  From my 7 exposure brackets from 2017 eclipse, I have an idea at the good exposure limits.  You will want to build from the longest exposure combo that has a decent histogram for the faint end.  For the fast exposure, use the fastest your camera can allow (needed for prominences in totality).  Your settings for partial and totality will be significantly different.

 

Here is info from DXO for a Canon 7Dmk2:

 

 

Here are my settings for tomorrow:

 

 

Here is some of how I arrived at these, as well as showing the SVX090T with either a 5Dmk4 DSLR or QHY268C:

 

 

 

[drmikevt]

After doing some experiments today...

 

On a full disk image with a Baader solar film on (ND 5.0), at about f/7, with the 6200MM at gain zero:  .001 (1ms) seemed to be a good exposure to slightly underexpose (to be fixed in post)

 

Assuming the full disk is about the same intensity as the early partial phases, this translates to an ISO of 100 according to http://xjubier.free....seExposure.html

[sharkmelley]

I've read that gain 0 equals about ISO 6 or 12 - can anyone comment on this?

It would be interesting to know the methodology used to arrive at that figure.

 

My starting point would be the full-well depth.  At gain 0 the 60 megapixel ASI6200 has a full-well of around 51,000 electrons.   The Sony A7RIV has a 60 megapixel sensor that is probably quite similar and has a full-well of 36,000 which is probably achieved at the base ISO of 100.

 

I would therefore suggest that using full-well as the equivalence measure, gain 0 for this sensor equates to approximately ISO 100.

[drmikevt]

I would therefore suggest that using full-well as the equivalence measure, gain 0 for this sensor equates to approximately ISO 100.

It's nice to know that a totally different approach to an answer came up with the same one as me.  I'm going with it, but taking a wide variety of exposures just to be safe.