Astro-modded A7s or monochrome ASI1600mm-cool?

[GTom]

I have been wondering, which is the better solution? The A7s and the dedicated ZWO ASI1600mm-cool astrocamera costs roughly the same.

Leaving the A7s' Bayer array in place (of course removing the ICF) results that the effective light collecting area for H-alpha is ~1/4th of the total sensor area (~860/4=215mm2). The lucky thing is,
that both the Bayer transmission and the cmos sensitivity is great at 656nm.
Thanks to the unlucky transmission curves of the green and blue pixels, O-III at 500nm also takes a toll: here we are blessed with 3 pixels of the array (G+B) but both G and B filter components
absorb around 50% here! Effectively in O-III, the a7s acts as a 322mm2 area monochrome camera.

Considering that the Zwo has better cooling than the a7s can practically ever have (unless one completely disassembles it and sticks a dual-stage peltier on the sensors back), less-than-ideal
transmission of the bayer filters AND camera-independent factors, like vignetting (depends on your scope and optical train), I would say the Zwo is at least equivalent if not better than the fullframe
a7s. Meaning, that if you want to cover a certain LARGE area, you get there at the same time with mosaicing. However, if you target one object which fits the Zwo's field than it should be clearly
superior, delivering the same results at ~1/3 exposure times.

 

Does my line of thinking make sense? Can someone support it with experience?

 

Monochrome modding of the A7s could be another step, however that kills the microlenses and results in a 20-30% light loss + costs a fortune if done professionally or plays Russian roulette with
your wallet if you try scraping off the Bayer yourself.

[DaveS]

The A7s is a 35mm format camera, which means a 43mm imaging circle. Getting a 'scope that will cover that won't be cheap, even the Baby Q won't do it.

Given the choice I'd go with the ASI every time, it's just so much more versatile, while putting less severe requirements on the rest of the optical train. And it is designed for the job, not something bodged about like any kind of modded DSLR / CSC.

[GTom]

Scope is not an issue, I have a TS 90/600 Photoline and a TSFLAT2.5 flattener is on its way, this will cover the full frame. I know, everything has to be larger, I started to pick up 50mm filters and a matching filter drawer. In the moment it is a bit more investment, but sensors are growing, I believe in a couple of years most of us will be on fullframe stuff... One day the CMOS sensor in the a7s will hopefully find it's way to astro-cameras...

[DaveS]

Well, it *says* it will.... BTW I have one of those on my 130 f/7 Photoline. In email with TS over spacing they said +-1mm or so with a 694 sensor, the implication being that it will be more critical as the sensor gets bigger.

I would *still* go with the ASI for the moment to keep things simple. Have you seen how much 3nm Astrodons are in 50mm size  ? Besides which, my view is that modded cameras are OK if you're strapped for cash, but as soon as you can afford a cooled mono astrocam that's where you should go.

[GTom]

Quote

" Have you seen how much 3nm Astrodons are in 50mm size  ?  "

Recently picked up a Baader 3.5nm, in 50mm size :)). And the price was about the same as you pay for a 1.25" Astrodon.

Budget is indifferent here, actually the ASI1600 being cheaper :D. For the price of a modded, cooled (even if I discount DIY at scrap price) A7s I could buy an Atik 460ex, or a second hand Atik 4000 (16x16mm sensor).