First Light New Camera

Maybe its an issue if the CCD uses a cover slip and/or glass/quartz etc window...

But if the glass/quartz and the chip are orthogonal in the CCD unit and the CCD unit is orthogonal to the scope, then sure even that couldn't make a difference - the focused light would be hitting the CDD at a 90 deg angle... Or is it differentially refracted in the same way the optics refract it?

Or is it he microlenses on the chip surface messing up the refraction of the different wavelengths?

All I know (and I don't know much) is that I assume the chip itself (ie the silicon) once it is positioned in the focal plane of the incoming light cannot have an effect on focus. Its either in the focal plane or it isn't. I can't see how a (flat) filter could change the focal position... All it can do is filter out selective wavelengths and then perhaps tighten up the image - but I don't see that that has anything to do with focus.

Am I going around in circles here? :silly:

The CCD chips are more sensitive to regions beyond what the eye sees. The difference in focus is there anyway (whether you had a CCD or not). The eye can't see, for example, the near infra red - but the CCD can.

The difference in focus is such that when blue is brought to focus at the focal plane the red - and especially the near infra red - is brought to focus behind the focal plane (because the glass bends/refracts red less). Hence at the focal plane the near infra-red is slightly out of focus and will be more disc-like. If one was to focus on the near infra red then blue wil be brought to focus slighty ahead of the focal plane (it is refracted more so comes to focus sooner).

As a sanity check look around for any old film SLR lenses that you might have. The lenses have manual focus (with distance) and stop-down ring. It shouldnt be hard to find one that also has a little red indicator offset from the main focus line. This is to compensate for focus in and around near infra red (or allows user to select an f-stop that has sufficient depth of focus to accomodate this position). Might be true of DLSR's too but it is an independent phenomenon of the recording medium.

I'm looking at a Mamiya 55mm f1.4 and from that I can see that if I choose f/8 I will have NIR and visible "in focus at the same time". The red indicator is offset slightly in the direction of infinity.

That "depth of focus" thing in reality is a compromise and accepting a common "circle of confusion" (read PSF) that is acceptable at both visible and the near infra-red

Of course the depth of focus thing is rarely used for that purpose - it's main purpose is to find an acceptable comprise of circle of confusion (PSF) that all points between two selected distance will not exceed.

I hope I've not made understanding more difficult by introducing the film-SLR example. Its really just to emphasize that it is a phenomenon independent of the CCD chip.

If I find anything on the web that describes this clearly and succintly I'll post follow-up. Particuarly as I know I can sometimes make a meal out of things which is nt always as helpful as intended.

But feel free to ask any further questions.

Mark

OK, I think I have it now...

Premium apo's can focus visible light such that the human eye can't see any chromatic abberation. However, the CCD *can* because it "sees" wavelengths the eye cannot.

So, the blue is in focus, but the infra red isn't. Presumably this effects (bloats) objects that emit in infra-red - e.g. everything (right?).

So Michael's filter is simply blocking out the IR wavelengths and not smearing the light across the CCD and giving it a general caste. For objects emiting significantly in IR (stars) they appear "tighter" (less "bloated") because it is only (mostly) visible light being focused on the chip.

If that is a correct sumamry, then I have it.

Sorry Michael - I thought you were saying the chip itself has an impact on focus. Sure, I can see that chips with a degree of sensitivity in the IR would perform less well than those with less of an IR sensitivity (without the filter). And sure, I recall the Meade DSI having an IR blocking filter. But I'd imagine all chips must have *some* IR sensitivity unless they are specially coated or have special cover slips (acting like filters) etc.

Thanks for that Mark

Dave

Bang on Dave!

Mark

Phew! And I thought I was too old to learn anything new

Thanks for the mentoring Mark

Hi Mike - sure - I'll be there. We'll meet up and hatch a cunning plan to beat WISE and Pan Starrs

The slacker from J41 definately won't be there but I'm not sure about J16 or J09...

Been a lot of de-commissioning of arms up this end of the country over the weekend so if we put our heads together maybe we could get them into polar orbit to take out WISE and get it to drop on Pan Starrs.

The slacker from J41

Hello calling J65 check your stats against J41

Back to topic:

I always understood that the IR/UV cut ccd filters were purely to allow best focus with the luminance filter. The RGB filter data even using a matched filter set usually gets a slight Gaussian blur before combining with the luminance image. Narrow band focus is a completely other pain in the A

J16 will not be at the Galway event since it came as a shock that all the money I had been saving this year was not for the CDK17 but the Tax Man instead. :pinch: