Webcam AP and barlows

[ChrisThePuma]

Hi All

I am waiting for delivery of my new Philips webcam but i have a question about how the magnifications works...

I understand that the webcam goes in the focuser in place of the eyepiece (after modifying the webcam) but as the eyepiece usually supplies the magnification you need to use a barlow instead - firstly is my understanding of this correct?

Next - as barlows are only usually available in 2x,3x, 5x (thats all i have really seen) how will this magnify the image enough? Or does the focal length of the OTA come into play also?

For example - i am looking at taking pictures of planets where brightness is not usually a problem, what sort of mag could i expect to get fom a 2x or 3x barlow with just the webcam and no eyepiece? (seeing permitting)

thanks

chris

[JamesF]

When you're using an eyepiece then it's the focal length of the eyepiece that sets the final magnification, yes.

For imaging, just forget about magnification It's much more helpful to think about "plate scale", which is what area of the sky relates to a given area of the image plane or to a single pixel of the camera sensor.

Your telescope will create a focussed image of what is visible in the primary mirror at a position somewhere in the focuser tube. That is the image plane. When you're imaging with a webcam and no barlow, to get a focused image on the camera you have to place the camera sensor at the same position as the image plane. The image at the image plane will be somewhat larger than your camera sensor, so you'll see a small part of what's available. If you insert a barlow it spreads the light from the primary out and creates a new image plane that the camera sensor must be aligned with. A 2x barlow spreads the light out so the image at the image plane is twice as big, meaning that each camera pixel represents a quarter of the sky (half in each direction) that it did without the barlow. A 3x barlow spreads the image out to three times as big and thus one pixel represents an area of one ninth of the sky compared with no barlow.

The limiting factor here is that you can spread the light out too much for the camera sensor to be able to create a decent quality image. You can also spread it out beyond the limit of resolution of the telescope, so you're no longer adding detail. If you have a bright enough image, ideally you want to get as much detail as possible using the fewest number of pixels necessary.

If you have a larger sensor, for example in a DSLR, you can capture a larger part of the image at the image plane (though DSLRs are usually not ideal for planetary imaging for other reasons). If you have a sensor with smaller pixels you can capture more detail (subject to the amount of light available) than when using a camera with larger pixels in the same setup.

I wrote more details about this here:

http://www.tanstaafl.co.uk/2012/03/focal-ratio-capture-length-planetary-imaging/

James

[mart rich]

This very helpful, thanks.

Nice images on your site by the way.

Martin

[farunj]

Perfectly put James. That was informative!!

[JamesF]

Nice images on your site by the way.

Thank you. I just wish the weather would allow me out to do more

James

[michael.h.f.wilkinson]

I find that with my Philips webcam F/20-F/30 works best. A 2.5x or 3x barlow, PowerMate, or TeleXtender should work well with your F/8 scope.

[JamesF]

I find that with my Philips webcam F/20-F/30 works best. A 2.5x or 3x barlow, PowerMate, or TeleXtender should work well with your F/8 scope.

It really does depend on how much light you can get on the sensor though. This spring I was having no problems imaging Mars at around f/35, whereas I couldn't manage more than f/20 for Saturn. To some extent you have to try it and see what the conditions will allow.

James

[michael.h.f.wilkinson]

It really does depend on how much light you can get on the sensor though. This spring I was having no problems imaging Mars at around f/35, whereas I couldn't manage more than f/20 for Saturn. To some extent you have to try it and see what the conditions will allow.

James

True: F/20 for Saturn, F30 for Mars and Venus, F/25 for Jupiter is what works best for me (roughly)

[JamesF]

I think those are pretty good places to start, Michael. Last year I could manage perhaps as much as f/30 on Saturn, but this year I absolutely couldn't get beyond f/20 with the 127 Mak and even those images were a bit iffy if I'm absolutely honest.

James

[ChrisThePuma]

Hi All

Thanks for the comments and advice, this is starting to make sense...

Next question - can anyone recomend a good 2.5x or 3x barlow that wont break the bank? i am thinking less than £50?

thanks

chris

[JamesF]

I'm happy with my GSO/Revelation 2.5x barlow which is a smidge under £40 new I believe. I also have their 2x barlow which genuinely does appear to be 2x, but it's possible that's only available as part of their eyepiece kit. I've not seen it for sale anywhere on its own that I recall.

I used the 2.5x barlow with an additional extension tube for these images of Mars this spring:

James

[Stargazer33]

Hi Chris,

I got the Revelation Astro 2.5x barlow as a birthday present this year and although I'm a newb, I think it is a nice piece of kit. It has given me some nice increases on my eps. I haven't been able to get it working with my webcam yet as my mount is not driven and I can't tell if my target has drifted out of the field of view or if I am just so far out of focus after swapping from the ep that I can't see it!

With my scope having a focal length of f11.7, a 2.5x barlow may be increasing my focal length a bit too much for clear images of some of the planets. Some people on the forum have recommended no more than f20 for imaging some planets and a 2.5x barlow in my scope raises the f11.7 to f29.25. That is something to consider when choosing the x mag of your barlow.

If I ever get another clear night I will try again!!!

Bryan

[ChrisThePuma]

Hi

My scope is f8 so a 2.5 barlow will take it to f20 which should be fine.

i am waiting for my webcam to arrive and then i will bite the bullet and get the barlow along with some other bits and bobs (nose piece for the cam, filter to attach to it to protect the chip, etc etc)

cheers

chris