How many pixels is enough for camera for bath IF?


Noel Goldsmith
 

Hi,
I am assembling my Bath, and am now looking at a suitable camera, I see that you can get 12 MP cameras to function on Raspberry Pi. And they allow the transmission of Raw images. Not sure yet about live. And you can fit different lenses, as the camera has a CS mount and a C to CS adapter. And the camera and lens is small. So for about $250 for the 8 Mb RASBERRY Pi and about $150 for the camera and lens, you could be good to go. But I am not sure about the real time viewing. Does anyone have experience in this niche?
I am also looking for suitable DSLR cameras secondhand.
Thank you
Noel


Bruce Griffiths
 

If you are dedicating a camera to interferometry (and Foucault or Ronchi testing) a monochrome camera is best.

Ideally a global electronic shutter is best particularly if significant turbulence is present.

If a colour camera is used one with (4x??) more pixels than a suitable monochrome one is required particularly if an antialiasing filter is used (often on chip).

Few DSLRs use an electronic global shutter most use a rolling/focal plane shutter.

There are methods of mitigating the drawbacks of a rolling shutter (at least for small system test airpath lengths) but using a global shutter is more flexible particulalrly for test setups with long air paths.

Bruce

On 06 June 2021 at 17:57 Noel Goldsmith <ngoldsmi@...> wrote:

Hi,
I am assembling my Bath, and am now looking at a suitable camera, I see that you can get 12 MP cameras to function on Raspberry Pi. And they allow the transmission of Raw images. Not sure yet about live. And you can fit different lenses, as the camera has a CS mount and a C to CS adapter. And the camera and lens is small. So for about $250 for the 8 Mb RASBERRY Pi and about $150 for the camera and lens, you could be good to go. But I am not sure about the real time viewing. Does anyone have experience in this niche?
I am also looking for suitable DSLR cameras secondhand.
Thank you
Noel


George Roberts (Boston)
 

12MP should be more than enough for 99% of amateur telescope mirrors.

So by default, I believe DFTFringe lowers the resolution of your igram such that there are about 600 pixels across the surface of the mirror.  So a 12 megapixel camera seems like overkill.  But this assumes you aren't cropping like crazy - if the image of your mirror in the igram is only taking up 1% of the field of view then you would need a lot more than 12MP.  But if you are testing a crazy difficult mirror (say one meter F/3 mirror) then 600 pixels is probably not enough.

Dale says you need 3 pixels per fringe.

You can put your mirror parameters into DFTFringe and see what your Z8 term comes out to and play with the igram simulator in DFTFringe - give the simulator your actual mirror parameters for Z8 (Spherical aberration) and then give it enough tilt to move the "bulls eye" off the edge of the mirror and count how many fringes there are.  If it's less than 200 fringes then 12MP is overkill.


George Roberts (Boston)
 

The DSLR I use is a Nikon D7000.  It's excellent for taking igrams and costs about $200 (used) for the body alone.  An F/1.8 50mm lens is an excellent choice for lens and used is about another $50.  There are many other similar cameras for a similar price.   One key feature to look for: nice big display on the back of the camera so you can see what you are doing in "live" mode and also for manual focusing reasons.


Bruce Griffiths
 

A 12MP colour image sensor is (in the absence of a built in low pass filter) equivalent to a 3MP monochrome image sensor if one is using a red laser source. I have several monochrome cameras with 4MP or greater image sensors.

Bruce

On 07 June 2021 at 00:09 "George Roberts (Boston)" <bb@...> wrote:

12MP should be more than enough for 99% of amateur telescope mirrors.

So by default, I believe DFTFringe lowers the resolution of your igram such that there are about 600 pixels across the surface of the mirror.  So a 12 megapixel camera seems like overkill.  But this assumes you aren't cropping like crazy - if the image of your mirror in the igram is only taking up 1% of the field of view then you would need a lot more than 12MP.  But if you are testing a crazy difficult mirror (say one meter F/3 mirror) then 600 pixels is probably not enough.

Dale says you need 3 pixels per fringe.

You can put your mirror parameters into DFTFringe and see what your Z8 term comes out to and play with the igram simulator in DFTFringe - give the simulator your actual mirror parameters for Z8 (Spherical aberration) and then give it enough tilt to move the "bulls eye" off the edge of the mirror and count how many fringes there are.  If it's less than 200 fringes then 12MP is overkill.


Dale Eason
 

I am familiar with the RPI and it's High Quality camera.   It probably will work but it is not a good choice as a camera.   Not because of the camera pixel quality but because of it's user interface.   That is how you need to write a program to take pictures and adjust camera variables like shutter speed and image resolution.    It has a preview mode but again you have to write a program to use it.   That can be as simple as writing a batch file or even a command line instruction.   But it is more complex and fiddly than just turning a knob and pressing a button.   
I use that system for other things but not to take interferograms.  It probably would work.

I prefer a real camera.  

In that distance past we use to use web cams with less the 300 pixels wide image for taking inteferograms.   As long as you can get the mirror's image to be over 300 pixels across you will have plenty of resolution.   I use a Nikon D50 but many others will do.  Many have used the Canon EOS versions.   It helps a lot to have a live view finder either optical or digital.  It is necessary to have manual control over focus, and exposure.

Dale


Noel Goldsmith
 

Thank you to all who responded.
I am now looking for a suitable second hand digital camera and a suitable lens to appear on the web or other suitable places. In Australia, as often postage or delivery from other countries is “eye watering”. It seems prices here are on par with prices in the USA, maybe a bit more. I am not going to dive into RPi or similar  for this purpose, as I should concentrate on the Bath and mirror making. There are only so many minutes in a day.
Thank you
Noel