D810A vs D850 for astrophotography

Started 2 months ago | Questions thread
Moonwalker Senior Member • Posts: 2,111
Re: D810A vs D850 for astrophotography

SoupOrPhoto wrote:

Moonwalker wrote:

I believe Thierry was referring to a filter that can block all but H-alpha light. In fact, I have seen pictures by people using this filter on an unmodified camera.

Certainly a modified camera will collect more H-alpha so you do not need as long exposures for nebulae.

That's something different: You can modify your camera as a full spectrum camera. All blocking filters are removed from the sensor. Then, a narrowband filter is placed in front of the sensor, e.g. a h-alpha line filter which only transmits wavelengths at 656 nanometers (plus or minus a few nanometers). You will need long exposures with such a solution since this filter will block most wavelengths. Such filters can be mounted as clip-in filters between lens & camera or in front of the lens.

With a full spectrum camera, you have a lot of different options: You can do IR or UV photography with a special infrared or ultraviolett pass through filter, or you can use line filters like the h-alpha, h-beta, oxygen III, sulfur II which only transmit a very narrow portion (typically a few nanometers) of the spectrum or you can use a visible band-pass filter which transforms the camera into a D810a type of camera. The visible band-pass filter blocks the IR and UV light and has a high transmission from 400nm to 700nm (numbers simplified). All these filters can be mounted between camera & lens as clip-in filters or in front of the lens. That's the freedom of a full spectrum camera. (But there is a small drawback: You will need some sort of filter, if you use a full spectrum modified camera without any filters, you will get star bloat due to IR and white balance issues).

Now a camera with a h-alpha modification has its IR filter removed and replaced with a similar filter as the visible band-pass filter mentioned above and this filter is mounted directly onto the sensor. You cannot take it out and replace it with a different filter since it's mounted onto the sensor. That's what is done with a D810a, but almost any camera can be transformed this way. Note that this modification doesn't use a h-alpha line filter which would only transmit at 656 nanometers. It has a high transmission from 400 nanometers to 700 nanometers (numbers simplified).

Best wishes,

Markus

I thought Thierry was referring to the filter that fits on the front of the lens and only allows H-a light through. These work whether your camera is modified or not (or modified at the factory as the 810a is). I misunderstood.

No, you are right. You can use a h-alpha line filter with any camera in front of the lens. It is more effective with a full spectrum camera or a h-alpha modified camera than with an unmodified camera and you will just get the light from that wavelength. Sorry, my English is not the best. Maybe Thierry has talked about that option as well. I thought you were all talking about the h-alpha modification type which is used in the D810a. My bad.

Markus

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