Focal length when using Webcam

Merlin66 (Ken) · #1 23-01-2006, 08:31 AM

What's the favourite set up when using the webcam for planetary images?
Eyepiece projection? Barlows? What focal lengths are normally used? Focal ratio?
I'm thinking of getting the Mogg adaptor for the Oly lenses I have (to use with the ep projector system I have).

[1ponders] (Paul) · #2 23-01-2006, 08:49 AM

Hi Merlin

For lunar and planetary imaging with webcams its normal to try to get as long a focal length as seeing will allow, so you can get at much magnification (ie image scale) as possible. f ratio is usually in the f/20 to f/50 range.

The majority of webcam imaging is done at prime focus, but for those very rare perfect nights then positive eyepiece projection will give you ridiculusly long FL for huge image scale.

Merlin66 (Ken) · #3 23-01-2006, 08:55 AM

Understood, but with a f4.5 dobbie; are you using a barlow to get to f20-50?
A x5 powermate? or what. What about other users?

iceman (Mike) · #4 23-01-2006, 09:04 AM

as ponders says, I usually use a barlow/powermate to get the desired image scale.

My 10" f/5 dob is FL1250mm, so using a 5x powermate gives me 6250mm FL @ f/25.

Dpending on the object, I won't be able to go too much larger because of the resolution of the ToUcam (640x480). Even Jupiter at opposition this year is going to be nudging close to the top and bottom of the capture box.

Some others do eyepiece projection, mainly Gary Beal. I've never tried it, though I can see the benefits.

[1ponders] (Paul) · #5 23-01-2006, 09:06 AM

If you can afford a X5 powermate, by all means go for it. That'll bump you up tho f/22.5 and give you 5 times your FL -> magnification. What's the FL of your Dob? Remember too, the higher the magnification the quicker the object is going to move out of the field of view, so the shorter the Avie you will get.

If a powermate is out of the question then you can get away with doubling up good quality barlows.

iceman (Mike) · #6 23-01-2006, 09:17 AM

Also, the higher the magnification (more focal length), the more you're subject to the seeing conditions.

Merlin66 (Ken) · #7 23-01-2006, 09:22 AM

With a pixel size of 0.0056mm in the ToUcam, the resolution/ pixel at 2000mm focal length is 0.5 sec arc. Which would represent very good seeing conditions. Going above this FL will just give better "over sampling" but no more planetary detail??????

RB (Andrew) · #8 23-01-2006, 10:37 AM

G'day Merl,

I usually use a powermate with my f/10 (3000mm)SCT and f/7.7 (1000mm) refractor for planetary.
With the SCT I've used 4x PM, with the refracr I was using the 5x PM the other night.
Like Paul and Mike said it also depends on the conditions.

Merlin66 (Ken) · #9 23-01-2006, 11:09 AM

Can someone take a couple of shots with a webcam.....
Firstly at about 2000mm EFL then a second at 4000+ EFL ( What ever you normally get with barlow) I'm not sure there will be any noticable difference!!!
Be interested to see the results!!

Merlin66 (Ken) · #10 23-01-2006, 11:33 AM

The attached spreadsheet may be of interest!!!!!

bird (Anthony Wesley) · #11 23-01-2006, 01:38 PM

Quote:

Originally Posted by Merlin66

With a pixel size of 0.0056mm in the ToUcam, the resolution/ pixel at 2000mm focal length is 0.5 sec arc. Which would represent very good seeing conditions. Going above this FL will just give better "over sampling" but no more planetary detail??????

Merlin, the top planetary imagers are using effective focal lengths around 10000mm or more (10m or more). It certainly seems to improve the end result - last season I was using around 6000mm and that wasn't enough. This year I'll be > 10000mm.

regards, Bird

Merlin66 (Ken) · #12 23-01-2006, 01:59 PM

Can you explain your comment that 6000 EFL " wasn't enough". I'm assuming you have images taken at this EFL and other at 10000 EFL. Can you post to allow us see the difference. Preferably each with the same processing.

I am really intrigued; the optical theory says it shouldn't work. Mind you they said the same about bees being able to fly!!

Merlin66 (Ken) · #13 06-02-2006, 09:57 AM

Trials at different EFL??? Anyone want to post examples? Interested in the results. I should have my cameras etc set up next week so I'll do the same.

davidpretorius · #14 06-02-2006, 10:33 AM

hey merlin,

I still think in magnifications, so for my 1250fl 10" newt, are you interested in basically if i take an image at 1250 x 5 (5xpowermate) and then at 1250 x 2.4 (2.4 x barlow).

I can get some extension with the powermate to 7.7x if i have the ccd 10cm away from the powermate, so i will test that as well. I mean to make an extension tube to see if i can go any larger.

Merlin66 (Ken) · #15 06-02-2006, 01:23 PM

Yes, that would an ideal starting point. I feel that depending on seeing that best resolution would be about 1 sec arc. Maybe, with the image manipulation we achieve a "smoother" outcome with the oversampling??? The text books say that if two pixels are covered by the smallest image ( resolution) then that's as good as it can get.

Robert_T · #16 06-02-2006, 03:08 PM

Bird might want to comments on this, but for webcam imaging using barlows and powermates I had the impression/belief (perhaps misguided?) that it is the effective Focal Ratio (eg f20, F25 etc) that determines image scale not the effective focal length. I'm pretty sure my 2150mm FL Mewlon 180 with base Focal Ratio of F12 provides a slightly larger image than my 2350mm FL C9.25 with its focal ratio of F10 when imaging with the 4x power mate on each?

regards,

iceman (Mike) · #17 06-02-2006, 03:50 PM

I wouldn't agree with that Robert.
To my knowledge, the focal length determine images scale, whereas the focal ratio is a measure of how much light is coming in.

Merlin66 (Ken) · #18 06-02-2006, 03:53 PM

Iceman's more right! The EFL gives the image scale and the EFR gives the "brightness"

Hitchhiker · #19 06-02-2006, 05:41 PM

Iceman and Merlin66 are correct. The size of your image increases as you increase your effective focal length.

The formula to calculate image size is:

Size of object in arsec X Focal length of Optical System in mm all divided by 206265.

For example, if Jupiter is 50 arsec in diameter and you are imaging with a system with a focal length of 2500mm, your image of Jupiter will be 0.6mm in diameter at the focal plane. Jupiter will cover about 107 pixels in a ToUCam with 5.6 micron pixels. For an LPI (with 8 micron pixels) Jupiter will cover about 75 pixels - this demonstrates why a ToUCam has a better image scale than an LPI for the same focal length!

If you increase the focal length of the above system to 5000mm (by adding a 2x barlow, for instance) the image of Jupiter will be 1.2mm in diameter at the focal plane. The image will cover about 214 pixels on a ToUCam and about 150 pixels on an LPI.

Robert_T, I think it's quite possible that your Mewlon may give you a larger image size than your SCT. The effective focal length of a 'compound' telescope ( eg an SCT or Dall-Kirkham Cassegrain) changes as you move the mirror to focus. It is possible that, even though your Mewlon has a stated FL of 2150mm it may have a very different value depending on where the mirror is. Your SCT may well have a different FL than that stated, again, depending on where the mirror is.

Hitchhiker · #20 06-02-2006, 06:05 PM

Hi Merlin66, on the subject of critical sampling for planetary images:

Quote:

The text books say that if two pixels are covered by the smallest image ( resolution) then that's as good as it can get.

But the new planetary imaging textbooks say that you should go further than covering two pixels with the smallest resolved 'unit' (the Nyquist theorem).

Have a look at the link below and you will see how it is possible to resolve such fine detail:

http://www.starizona.com/ccd/advtheorynyq.htm

I was in the same boat as you a couple of months ago. I couldn't believe the resolutions being achieved with amateur class instruments. The secret is in stacking many images together - it is effectively a statistical process where, even if a feature only just sticks its head above the noise, it can be captured.