Re: Takumar Pentax 6x7 tests RE: [ap-gto] [ap-ug] A colorful Southern Sky Beauty



What you describe below is precisely what one observes with cooled CCD images. With my ST-10XME, I recall thermal noise being 2-3 ADU whereas read noise was around 100 ADU (from memory). As for driving down noise by increasing the number of images in the stack, I agree that one gets limited returns as the sample size (ie stacked images) increases. Again, using my own personal experience, anything around 16 to 20 images in the stack is optimal, for additional images in the stack have a minor further effect on the S/N.


On 01-May-21 16:55, Richard Crisp wrote:
SNR can be increased by taking more and longer exposures as long as the exposure doesn’t saturate

You continue to win using longer exposures if your dark signal from the longer exposure has less shot noise than the single exposure’s read noise: once the dark shot noise exceeds the read noise, longer exposures will reduce the SNR. Cooling controls this.

Multiple images stacked imposes no limit to how low you can drive the noise in the stack, thereby increasing the SNR up to the point of zero noise (unachievable)

Sorry but that’s how it works

“Corrected” by my iPhone

On May 1, 2021, at 1:18 AM, ayiomamitis <anthony@...> wrote:


I am not sure I agree here. We both know that an easy and effective method to increase S/N is to stack multiple images. As for aperture impacting light flux, I would agree here for objects with a low surface brightness but, again, we have many targets where aperture (and absolute light flux) is not necessarily the only solution.


On 01-May-21 02:23, Roland Christen via wrote:
F-ratio means nothing. Aperture is the deciding parameter that determines light flux arriving at the detector. More light = more signal to noise.


-----Original Message-----
From: ayiomamitis <anthony@...>
Sent: Fri, Apr 30, 2021 5:05 pm
Subject: Re: Takumar Pentax 6x7 tests RE: [ap-gto] [ap-ug] A colorful Southern Sky Beauty

Why shoot wide open (say f2.8) when our astrographs are f4 or slightly slower? I would rather see results at such f-ratios (ex f4 to f6.3) so that results can be that much more comparable.
On 01-May-21 00:48, Roland Christen via wrote:
Try shooting stars wide open with a color digital chip. You will see the limitations. I wish I had kept the result of my 90mm F2.8 Pentax lens - you would not believe how bad it is wide open.


-----Original Message-----
From: Richard Crisp <rdcrisp@...>
To:; robert-wynne@...
Sent: Fri, Apr 30, 2021 3:28 pm
Subject: Takumar Pentax 6x7 tests RE: [ap-gto] [ap-ug] A colorful Southern Sky Beauty

Some years back I did some tests of a handful of Pentax6x7 lenses I had picked up on the cheap
Even though they aren’t very well corrected over an electronic imager’s broad spectral responsivity, it seemed if you stopped them down, used a single wavelength filter with a mono sensor, you could get pretty decent results
You might find the slide deck useful that I made
I was using an FLI ML4022 camera (KAI series sensor w/7.4 micron interline pixels).

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