M104 Galaxy

[Bassnut (Fred)]

AND, Peter, am I right or wrong, OTA optical performance is generally measured by image quality over the whole illumination circle (I might have the terminology wrong, sorry), which is important with a large chip that is fully illuminated. If a small chip only captures a small proportion of a large aperture OTA illumination, then edge performance is far less critical?.

[multiweb (Marc)]

I see your Schwartz is as big as mine..... thread's gone waayyy off topic from M104.

[Bassnut (Fred)]

Opps, OK, yes, thats a nice M104 too.

Oh, and welcome to GRAS Peter

[strongmanmike (Michael)]

Originally Posted by Bassnut

One thing ive never understood. Bear with me, this gets to a puzzle to the need for megapixel sensors for long FL, other than oversampling, and convienient framing as Mike has picked on me for.

It seems intuative that a larger chip with the same pixel size as a smaller chip just allows a bigger FOV, and with croping to the same FOV, would render essentially the same image quality generally.

But no, except for a few fine examples, generally, croping, or zooming in it seems does not mean the same sort of image you would get with a small chip with the same FOV as the crop. I have seen many many examples from the best that just simply dont cut it, zoomed in.

So I wonder why?. When processing a wider view, colour balance and dynamic range especially must suit the image as a whole, so the core of an object is often overexposed (ie, individual parts of the subject are not processed individually, which I guess would be very difficult to blend in with an overall pleasing result). Or, maybe, the lower QE of megapixel cams gives a different result alone.

Im not convinced at all that a bigger chip always gives a subjectively "better" or even equal result (with the same image scale) at long focal lengths, despite the advantages of convienient framing. Croping on a small object is just a waste of money spent on the cam.

Mike, I like narrow field high QE and cheaper imaging for the same or arguably better results than a croped megacam. Bite me .

....?

I think some confusion has crept in here me hearties and I cana be bothered typing anymore (said in Scottish accent).. suffice to say.....I wish I could just image more often

Mike

[Peter Ward]

Originally Posted by Bassnut

AND, Peter, am I right or wrong, OTA optical performance is generally measured by image quality over the whole illumination circle (I might have the terminology wrong, sorry), which is important with a large chip that is fully illuminated. If a small chip only captures a small proportion of a large aperture OTA illumination, then edge performance is far less critical?.

Que? You asking me?

I suppose by definition Strehl looks at the whole wavefront.

But this is a bit moot, as is field curvature, if you just use the say centre 1/3 of the available field. That said, I suspect visual observers would not be happy with an optic that was only sharp in the centre 30 degrees of a 100 degree Ethos field!

But with imaging provided your optic pumps most of the available flux into the airy disks at the centre third..or what ever...and if your CCD is too small and can't "see" the rest you will have a high resolution solution of sorts. But wait there's more.

If you happen to image from within the atmosphere you are sky limited.

Lets say at 500mm you can get 1000 counts per pixel from the sky, plus 100 counts from the star sampled at 4 arc sec per 1 pixel. Total flux 1100 but less than 1/10 is signal

Let's now increase the FL to 2000mm . Our sky contribution over the same pixel has reduced by 4x. The object flux remains the same, Total flux 350, but now over 1/3 is signal!

Having more signal vs noise is pretty handy....de-convolution works better
plus when noise is pushed down to trivial levels the signal clearly and cleanly resolves above it.

Yes, narrow field rules

[Bassnut (Fred)]

Originally Posted by strongmanmike

....?

I think some confusion has crept in here me hearties and I cana be bothered typing anymore (said in Scottish accent).. suffice to say.....I wish I could just image more often

Mike

Fair enough, but I think I do have a point anyway (albiet not expressed very well), despite the math and $, there is more than meets the eye (no, begum, its what meets the eye ). Horses for courses. When it comes to Narrowfield, megapixels and mega $ is not always the obvious answer.

[Bassnut (Fred)]

Thanks Peter. high QE will do me over low QE big FOV anyday......... for narrow field anyway

[Peter Ward]

Originally Posted by Bassnut

Thanks Peter. high QE will do me over low QE big FOV anyday......... for narrow field anyway

Well....You can sort of have your cake and eat it too...I bit the bullet (instead of the cake) and sent off the deposit for a 16803 based camera yesterday...... Good QE and a field that makes 2" accessories way too small

[Bassnut (Fred)]

I guess you mean the $11,875 Sbig model (well, less for you, natch ). At 60% QE, no big shakes, but huge FOV (what happened to NF rules ), and fancy guiding. and oh those filters, there goes the one-to-be-obeyed funding permission (for a piece of coloured glass, you have to be KIDDING )

[Peter Ward]

Originally Posted by Bassnut

I guess you mean the $11,875 Sbig model (well, less for you, natch ). At 60% QE, no big shakes, but huge FOV (what happened to NF rules ), and fancy guiding. and oh those filters, there goes the one-to-be-obeyed funding permission (for a piece of coloured glass, you have to be KIDDING )

Yup about $19K in Pacific Pesos with the fancy coloured glass wheel......
second thoughts I'm probably mad given the weather to date!