Webcam and Baader barlow

[Vox45]

Hi all,

I have a toucam webcam that I have fitted with a 1.25 nosepiece like this:

Right now I am lookin at the Baader click zoom and I can buy it with or without the barlow combo.

Question: is it possible to attach the barlow on the nosepiece of my webcam out of the box or do I need to buy an adapter of some sort ?

I know there is a way to connect the EP zoom to a webcam with a bunch of adapters but in my case I just want to do prime focus with a barlow not eyepiece projection...

Also: would it be possible to thread an IR filter to it all ? So I would have barlow+IR filter+webcam in one piece of hardware (I may be pushing my luck a bit there...)

[Vox45]

Do not hesitate to post a pic of your setup I am more of a visual guy...

[JamesF]

I think it makes more sense to use separate components to allow you to better control the focal length at which you're imaging.  An IR filter may well fit into the end of the nosepiece if it is threaded, but other than that I'd personally want the flexibility to change barlow multipliers and to introduce extensions between the camera and barlow to achieve the optical focal ratio.

James

[Vox45]

I think it makes more sense to use separate components to allow you to better control the focal length at which you're imaging.  An IR filter may well fit into the end of the nosepiece if it is threaded, but other than that I'd personally want the flexibility to change barlow multipliers and to introduce extensions between the camera and barlow to achieve the optical focal ratio.

James

Thanks James.

This is were it gets quite complicated for me. I read that to find out what is the magnification when you are at prime focus you can use this formula:

(Work out the diagonal measurement of your sensor in mm, divide the focal length of the telescope by this figure and this will give you a good approximation of the magnification)

For example: a Philips 750k has a sensor 4.60mm (H) ×3.97mm (V) The diagonal is 4.5mm

So for my C6 SCT I get: 1500/ 4.5 = 333

So the magnification at prime focus is 333x (!) very close to the maximum magnification for my scope. A 2x barlow would give me 666x magnification ...

Does this mean that I cannot use a Barlow when I use my webcam ? and the extension tube modifies something depending on it's length but I am not clear on what is the effect of having a longer or shorter tube...

That being said, I may be mistaking about the formula and have this all wrong !

If someone has the patience to clarify those concepts, I would appreciate.

I may be better off doing eyepiece projection ... but I feel it's just gonna a different set of problems

[JamesF]

Magnification not relevant to imaging.  It's a visual-only thing.  Don't worry about it.  The issue of magnification seems to cause much confusion when starting imaging, to absolutely no benefit whatsoever.

What really matters for imaging is what's called "image scale" or "plate scale", which is basically a measure of how much of the sky corresponds to how much of the focal plane (where the camera sensor is, for prime focus).  Commonly you'll see it referred to as arcseconds (of sky) per mm (of sensor) or arcseconds per pixel.  Ideally what we want for imaging is for one pixel on the sensor to be equivalent to the smallest detail your telescope can resolve.  If one pixel is larger than the smallest detail then you're losing data and if it's smaller then you just have more pixels with effectively the same data.  It's not hard to work this out if you're comfortable with trigonometry (that's the way I've always done it), but someone pointed out the other day that it actually turns into a nice little rule of thumb that can be used as a starting point which is that the focal ratio of the entire optical train needs to be five to six times the numerical value of the pixel size in thousandths of a millimetre.  This isn't a hard and fast rule, but it's pretty good unless you're seriously hardcore.

So, for example, with your camera which I believe has a pixel size of 5.6um, ideally you want a focal ratio of somewhere in the region of 28 to 34.  As the native focal length of the telescope gives a focal ratio of f/10 that means a 3x barlow ought to work quite nicely, but because the multiplicative effect of the barlow increases based on the distance between the lenses and the focal plane, you can use one with a lower multiplier and add extensions between the barlow and camera to increase its effect.  I'm now using a C9.25, but with the ASI120 camera with a pixel size of 3.75um.  I try to use a focal ratio in the low 20s (five to six times gives a range from 18.75 to 22.5).

There's one more little wrinkle that's handy to know about when using SCTs (and Maks).  They achieve their focal length by combining the effects of multiple optical surfaces (the primary and secondary mirrors, both of which are curved unlike a newt, say).  The focal length of the combined system is in part dependent on the distance between the two.  But they are focused by moving the primary mirror and changing the distance between the two mirrors, thus changing the focal length.  In fact, there is only one position in which the telescope actually has its stated focal length.  I don't have any specific evidence, but I suspect that is when the scope is in focus about 10cm behind the visual back (about where an eyepiece would be, accounting for the additional length in the optical path due to the diagonal).

What that means is that when using an SCT or Mak for imaging, you can't actually know what the exact focal ratio is with any given arrangement of kit without capturing some images and working backwards to see what focal length would have given the size of image you've captured.

All that said, I'd strongly recommend staying with prime focus imaging.  I think you'll probably achieve far better and far more controlled results than if you went afocal.

James

[Vox45]

I did not expect such a detailled answer ! Thanks mate

I've read before on some of the things you mention but it was never clear to me and was just bits and pieces of information. Now with your explanation, everything is falling into place ! Bottom line is that there is more to it than it seems and I need to dig further and learn a bit more on this topic. That also explain why some people here are able to take such beautiful pictures and some others (like me) struggle. I'm fine with that, it keeps things interesting.

I'll keep your post in my archives, best explanation on the topic I've had so far.

[JamesF]

No problem

I've no idea what the weather is like around Paris at the moment, but hopefully you'll be getting more chances to use your camera than I am right now...

James

[Vox45]

Bleh ... same here

Last time I had the chance to look at something  (M42) was a month ago ... Last imaging session was in october :/

[Vox45]

I think it makes more sense to use separate components to allow you to better control the focal length at which you're imaging.  An IR filter may well fit into the end of the nosepiece if it is threaded, but other than that I'd personally want the flexibility to change barlow multipliers and to introduce extensions between the camera and barlow to achieve the optical focal ratio.

James

I got the Baader Barlow and started to experiment with it ... So far I passed the POC part (another thread) I can screw the Baader Barlow onto the nosepiece of the webcam, I even have a 1.25" thread on the barlow were I could screw an UV/IR filter. The filter would be in front of the barlow which I guess is not an issue.

I just need to test with and without the barlow and see what gives the best result

[Vox45]

Magnification not relevant to imaging.  It's a visual-only thing.  Don't worry about it.  The issue of magnification seems to cause much confusion when starting imaging, to absolutely no benefit whatsoever.

What really matters for imaging is what's called "image scale" or "plate scale", which is basically a measure of how much of the sky corresponds to how much of the focal plane (where the camera sensor is, for prime focus).  Commonly you'll see it referred to as arcseconds (of sky) per mm (of sensor) or arcseconds per pixel.  Ideally what we want for imaging is for one pixel on the sensor to be equivalent to the smallest detail your telescope can resolve.  If one pixel is larger than the smallest detail then you're losing data and if it's smaller then you just have more pixels with effectively the same data.  It's not hard to work this out if you're comfortable with trigonometry (that's the way I've always done it), but someone pointed out the other day that it actually turns into a nice little rule of thumb that can be used as a starting point which is that the focal ratio of the entire optical train needs to be five to six times the numerical value of the pixel size in thousandths of a millimetre.  This isn't a hard and fast rule, but it's pretty good unless you're seriously hardcore.

So, for example, with your camera which I believe has a pixel size of 5.6um, ideally you want a focal ratio of somewhere in the region of 28 to 34.  As the native focal length of the telescope gives a focal ratio of f/10 that means a 3x barlow ought to work quite nicely, but because the multiplicative effect of the barlow increases based on the distance between the lenses and the focal plane, you can use one with a lower multiplier and add extensions between the barlow and camera to increase its effect.  I'm now using a C9.25, but with the ASI120 camera with a pixel size of 3.75um.  I try to use a focal ratio in the low 20s (five to six times gives a range from 18.75 to 22.5).

There's one more little wrinkle that's handy to know about when using SCTs (and Maks).  They achieve their focal length by combining the effects of multiple optical surfaces (the primary and secondary mirrors, both of which are curved unlike a newt, say).  The focal length of the combined system is in part dependent on the distance between the two.  But they are focused by moving the primary mirror and changing the distance between the two mirrors, thus changing the focal length.  In fact, there is only one position in which the telescope actually has its stated focal length.  I don't have any specific evidence, but I suspect that is when the scope is in focus about 10cm behind the visual back (about where an eyepiece would be, accounting for the additional length in the optical path due to the diagonal).

What that means is that when using an SCT or Mak for imaging, you can't actually know what the exact focal ratio is with any given arrangement of kit without capturing some images and working backwards to see what focal length would have given the size of image you've captured.

All that said, I'd strongly recommend staying with prime focus imaging.  I think you'll probably achieve far better and far more controlled results than if you went afocal.

James

So; thanks to you in part, I was able to take my first planetary image

I did so by just inserting my cam in the diagonal as it looked a bit bigger (directly in the visual back Saturn seemed too small on the sensor). I did not use a barlow yet, I am still waiting to get the 3x barlow I ordered.

I know you said before that I should get a Focal ration of around 30 therefore using a 3x barlow would give me the correct F/D, but then I get a 4500 FL for my 150mm scope.

Focal Ratio being F (focal) / D (diameter) : 30*150 = 4500 focal lenght

With that focal lenght I calculated the sampling:

Resolution = (CCD Pixel Size / Telescope Focal Length ) * 206.265

arcsec/pixel = (5.6/4500)* 206.265 = 0.25 arcsec/pixel

A 0.25 arcsec/pixel sampling seems low, does that seem right to you ? I see different numbers regarding the sampling rate I should get (some say around 0.8 arcesec/pixel for planetary)

Of course my calculation (and understanding) may be way off !

I am being picky as I am sure there are many other factors (the 1st one being my cheap webcam) like seeing etc. But I am curious on the subject.

Cheers!

[Vox45]

Did a bit more ressearch

The sampling should be around the resolution power of my scope (to not oversample / undersample)

The resolution power formula is : Dawes limit devided by the Diameter

PR= 120/D

PR = 120/150 = 0.8 arcsec

According to this I should sample at 0.8 arcsec/pixel.

[JamesF]

My recollection is that Dawes and Rayleigh do not entirely agree on the limit of resolution.  The impression I have is that people generally use Rayleigh's formula for resolution in this case.

The other thing that comes into play is Nyquist's Sampiing Theorem which basically comes down to needing to sample at twice the rate needed to match the resolution.

And in fact because Rayleigh says resolution is dependent upon wavelength, what can be undersampling at one end of the spectrum could be oversampling at the other which is why "five to six times pixel size" can only ever be a rule of thumb.

I don't have any recollection of reading suggestions of 0.8 arcseconds per pixel for planetary imaging myself.  That seems wrong to me.  It would mean that at its largest Jupiter would only be about 60 pixels across.  I think generally we'd want rather more than that.

James

[Vox45]

I will start a new thread as this is a complicated issue and it could help other figuring this out

[Vox45]

New thread here

[Vox45]

I think it makes more sense to use separate components to allow you to better control the focal length at which you're imaging.  An IR filter may well fit into the end of the nosepiece if it is threaded, but other than that I'd personally want the flexibility to change barlow multipliers and to introduce extensions between the camera and barlow to achieve the optical focal ratio.

James

I followed your advice and bought a Bresser SA-Barlow 3x. I am doing some tests with it and may well go later with the extension you mentionned. Not sure how to calculate the extension needed though. I see that when I use a diagonal the image is bigger (but dimmer due to light absorbtion I guess) so I may as well put the barlow in the focuser and use an extension if I can achieve the same result.

Here is the result of last night (barlow 3x / diagonal) ... blurry even though I know the focus was good (bahtinov mask on Spica 2 hours before imaging) ... seeing was bad (3) so that may be the explanation. Also the sky is rather bluish ...

I'll keep at it

[JamesF]

That's a good effort.

Without the spec of the actual lenses it's probably impossible to calculate what extension you might need to achieve what effective focal length.  Probably the easiest way to do it is to try it and work backwards from the image size.

The image dims when you make it larger because you have the same amount of light spread over a larger number of pixels, so each pixel receives less light individually.

James

[Vox45]

That's a good effort.

Without the spec of the actual lenses it's probably impossible to calculate what extension you might need to achieve what effective focal length.  Probably the easiest way to do it is to try it and work backwards from the image size.

The image dims when you make it larger because you have the same amount of light spread over a larger number of pixels, so each pixel receives less light individually.

James

Ok thanks for the advice

[Vox45]

That's a good effort.

Without the spec of the actual lenses it's probably impossible to calculate what extension you might need to achieve what effective focal length.  Probably the easiest way to do it is to try it and work backwards from the image size.

The image dims when you make it larger because you have the same amount of light spread over a larger number of pixels, so each pixel receives less light individually.

James

I read that the Bresser SA-Barlow 3x is actually a TeleXtenders (or by their generic name: tele-centric barlows) so I did a bit of ressearch on this Tele-centric stuff...

From Wiki:

"Telecentric lenses, on the other hand, provide an orthographic projection, providing the same magnification at all distances. An object that is too close or too far from the lens may still be out of focus, but the resulting blurry image will be the same size as the correctly focused image would be.

Non-telecentric lenses exhibit varying magnification for objects at different distances from the lens. Most lenses are entocentric—objects further away have lower magnification. For pericentric lenses, objects further away have higher magnification"

From what I read, the extension would not change the 'magnification' (sorry James, I know I should not say magnification) in the case of this particular 'barlow' It seems that I am stuck with that image size, unless I do eyepiece projection I guess... 

[JamesF]

I would give it a go anyhow, to be honest.  It may not be quite as telecentric as they claim.

James

[Vox45]

I would give it a go anyhow, to be honest.  It may not be quite as telecentric as they claim.

James

There is a very good explanation from a forum user on Telecentric vs Barlow ... Just adding it to this thread for futur reference and some readers may find it useful