spaces or barlows without lenses to image the Moon?

[pixueto]

Dear all,

I am hoping that some of the imaging wizzards in the forum will be able to help me with this: I am using a Philips SPC 880 with and without a barlow and a SW200P. I have some spacers that I got for spectroscopy. Could I use them for imaging the Moon with the webcam? What would be the effect of getting the CCD chip further away from the barlow? Is anyone already doing this?

Some people are using a barlow without lense to image the Moon. They say that in this way they get all the light that's coming into the scope but I find it a bit baffling this issue about the light being taken by a normal barlow. I understand the constraints for very faint DSO's but the Moon being so bright, is this really a problem? There are wonderful images in threads with people using eyepiece projection.

As well as a 2X ultima barlow, I have the original 2X barlow that came with the 200P SW Explorer. I never use it as the quality is really bad. Is there a way of removing the lense without damaging it? What kind of magnification do you reckon I could get if I use a barlow without lense in this way? And has anyone tried stacking two barlows in this way with a webcam? So many questions! Sorry!

Finally, thanks again for your help and your time helping me out with this. I really appreciate it.

Regards

[Cornelius Varley]

The magnification of a barlow can be increased by a factor of 1 by placing an extension tube between the barlow lens and the camera equivalent to the focal length of the barlow. eg if a 2x barlow lens had a focal length of 30mm, adding a 30mm extension tube between the barlow and the camera would give a magnification factor of 3x.

Using a barlow with the lens removed has no optical effect, it just moves the camera further away. The is sometimes needed with refractors that don't have sufficient outwards focuser travel for the camera to reach focus without the aid of an extension tube, a barlow with the lens removed is a suitable extension tube.

Peter

[JamesF]

The important thing to note being that you can use an extension between the camera and a barlow to increase the image scale, but the extension on its own won't do anything to the image scale -- as Peter says, it just changes the position of the camera relative to the image plane, giving greater back focus if it's required.

James

[pixueto]

Thank you very much for your help! it's been very informative! i have a good Celestron 2X ultima barlow and a no-so-good Tal 3X barlow. reading your explanation I gather that I would be better off by using the ultima barlow with an extension tube to get 3X rather than using the Tal 3X barlow on its own. Am I correct or do a miss something here?

Thanks

[JamesF]

I thought that Tal barlows had a good reputation. Certainly they get recommended here on a regular basis.

You'll find the Celestron Ultima to be nearer 2.3x I think. Certainly it provides greater magnification than my Revelation 2x, but not as much as my Revelation 2.5x.

James

[pixueto]

I am aware of the Tal barlows' reputuation, that's why a got one but invariably, the celestron ultima pruduces much more detailed pictures. I got the impression that the images are better with the ultima than with no balow at all. It's as if the good quality of the ultima optics can improve the overall image as opposed to not using a barlow. As an example, I attach 3 pictures of the same area taken the same day: 1st no barlow, 2nd celestron ultima 2X barlow and 3rd Tal 3X barlox.

Regards

[JamesF]

They're certainly good images.

That the 3x one isn't as sharp may be no inherent fault of the barlow. It could just be that the seeing wasn't good enough to support that kind of image scale at the time the image was taken. It could also be that focus is much harder to get right as you increase the image scale. It doesn't look as though you've lost any detail, but rather failed to gain any, and perhaps there's a bit less contrast?

It might be informative to try the Ultima with an extension to compare the results with the Tal. I wouldn't write it off just yet.

Incidentally, as you have the original SW barlow, you can unscrew the internal lens retaining ring from the end and drop the lens out. There's only one, but if you'd like to use it as a barlow again you might want to mark which way up the lens goes. Without the lens it makes a very useful extension.

James