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Afocal cam use - looking into an EP?

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#1 smee

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Posted 01 January 2013 - 08:15 PM

It seems all the 'cams, Mallins,Samsungs, Stellas, are used straight into the focuser of whatever telescope type.

Would it be possible and has anyone tried, to use them looking into an EP?

Just the point of having different FOVs with different EPs even if the hassle of taking off and tweaking focus. Of course when it comes to remote comfortable viewing kind of takes that away!

Or having a zoom EP, at least if all settings are parfocal.

And maybe too much - remote change of FOV for that zoom and focus would be the ticket, though the latter is doable I'm sure.

#2 ccs_hello

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Posted 01 January 2013 - 09:50 PM

Afocal is more popular in digiscoping and is mainly used by an imaging device which has a unremoveable lens.
Afocal has a few cons and just one or two pros. Without knowing your objectives and specifics, it's very hard to go down to details.

The first need-to-know thing on afocal is camera lens' entrance pupil should be reasonably close to eyepiece's exit pupil to avoid vignetting happens too soon. Bear in mind most eyepieces are designed to match human eye's entrance pupil (3-6mm).

Afocal can range from (by adjusting EP/camera lens focal lenth ratio)
- adding more magnification on OTA's native focal length, i.e., digiscoping, bird-watching, may be planetary imaging, etc. This is the main use case.
- 1:1 (neither magnifying nor reducing)
- adding more reduction on OTA's native focal length, i.e., focal reducing. HOWEVER, in most of the practical lens chosen scenario, vignetting kicks in very soon.

This is why when talking about afocal, people are mostly talking about digiscoping. When applied to astro applications, not many cases are there. In such case and in reality, (unless camera's lens is not removable), most will use a Barlow instead.

If you want to have a vari-focus telescope OTA, I would not recommend you to go for the afocal route but use a Barlow or a focal reducer. If you reeeeeally want to have a vari-focus OTA, buy a zoom lens designed for your imaging instrument.

Clear Skies!

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#3 Larry F

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Posted 01 January 2013 - 10:57 PM

For afocal, don't you need a lens on the camera? So what's the point of doing that with a Mallincam or Stellacam? You'd need to have an eyepiece on the scope, then a lens on the camera, then the camera. Eliminate all that glass and go prime focus with or without barlow or focal reducer.

I sometimes fool around with a Canon FD 50 or 135 mm lens on my Mallincam (I have an FD to C-mount adapter) to get wide angle views, but there would be no reason to use it in an afocal configuration.

#4 smee

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Posted 03 January 2013 - 04:03 PM

ok guys, I was just enquiering from that kind of idea that slips into the brain just before nodding off on the pillow.

I know that 'afocal' is applied more to the digiscoping or using fixed lens compacts or such with an adapter looking into the EP.

It was from that specific point of using an EP that prompted me to use the term- afocal.

I guess I should ask about where the focus point needs to be for the Mallincam types. Meaning, am I wrong in thinking that the Mallincam type should be able to be positioned such that it can "grasp" the focused EP image?

#5 ccs_hello

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Posted 03 January 2013 - 11:02 PM

Without knowing which C-mount or CS-mount CCTV lens you plan to use, it's very hard to predict. Usually such lens' entrance pupil typically goes from 3mm up to 20mm. While an EP's exit pupil is designed for human pupil (3-6mm max.) and often can be smaller.

So the answer is: it depends :).
We don't know what EP you plan to use either.
More often and most likely, result will look like looking through a straw (and do not forget there are many glasses in between :( ).

#6 rmollise

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Posted 04 January 2013 - 09:05 AM

It seems all the 'cams, Mallins,Samsungs, Stellas, are used straight into the focuser of whatever telescope type.

Would it be possible and has anyone tried, to use them looking into an EP?

Just the point of having different FOVs with different EPs even if the hassle of taking off and tweaking focus. Of course when it comes to remote comfortable viewing kind of takes that away!

Or having a zoom EP, at least if all settings are parfocal.

And maybe too much - remote change of FOV for that zoom and focus would be the ticket, though the latter is doable I'm sure.


There's a reason people don't do this. With a video camera more field is usually better, and shooting afocally reduced field and introduces vignetting. The way to change magnification is to start with a reducer, go to "barefoot" if more power is needed, and put a Barlow in the light path if that still isn't enough. ;)

#7 nytecam

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Posted 04 January 2013 - 11:06 AM

It seems all the 'cams, Mallins,Samsungs, Stellas, are used straight into the focuser of whatever telescope type. Would it be possible and has anyone tried, to use them looking into an EP? Just the point of having different FOVs with different EPs even if the hassle of taking off and tweaking focus. Of course when it comes to remote comfortable viewing kind of takes that away! Or having a zoom EP, at least if all settings are parfocal. And maybe too much - remote change of FOV for that zoom and focus would be the ticket, though the latter is doable I'm sure.

I can perhaps appreciate your logic - I've got a regular scope complete with eyepiece and the Mallincam etc is very sensitive so why not just apply it to the eyepiece [instead of the eye] to view deep.

If life were that simple. :grin: But we do something equally simple but much more effective by replacing the eyepiece directly with the camera. :o Thus we remove unnecessary, mismatched and distorting glass within the eyepiece and the lens before the camera that substitutes the eyeball.

Simple is often best. We see the result on a screen greatly magnified [via the scope's long focal length] and greatly intensified and made bright [via the camera's long integration] ;)

#8 Rat8bug

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Posted 07 January 2013 - 10:45 AM

I've used afocal for astrophotography. Useage includes P&S digital, camcorders, and webcams.

http://www.barrie-tao.com/afocal.html

Ciao...Barry

#9 smee

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Posted 08 January 2013 - 12:42 PM

To ccs_hello,

I'm aware of "c-mount" lenses generally but I haven't even considered their use in this matter.

Having seen a few other pages including Barry's a possibility is using a Hyperion EP which has a thread at the top to allow attachment of adapters.

To rmollise,

please correct me if I'm wrong in thinking that a camera straight onto the back of an SCT is seeing a _narrower_ field than if it was looking into a 32mm or wider EP?

To nytecam,

your italicised sentence neatly encapsulates my logic :)

if indeed it could be that simple! But the simplest thing was to ask if what appeared to me to be simple was at all viable, if you see what I mean.






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