Question for the imaging wizz's

[DBushell]

I'm currently using my SPC900 on the end of a 2x barlow, however, I was wondering if it would be possible to increase the size of the planetary image further but still be able to focus to get a decent sized image. Obviously I could go and spend hundreds on a better scope, but for now I want to get the most out of my existing scope.

If I were to use an extension tube, is there a formula that could be used to determine the maximum extension tube length for the SPC900 and my scope (650mm fl Astromaster 130eq).

I tried dismantling one of my barlows by unscrewing the lenses, however I was unable to focus the image sufficiently....

Any advice greatly received.

[Peter Drew]

Although I'm not an imaging "whizz", i do know that if you use an extension tube between the Barlow and the eyepiece and then refocus, you will get a larger image scale. The alternative would be to use a 3X Barlow, in each case the ability to refocus would determine the success.

[Tim]

But dont forget that the larger the image scale, the more difficult it is for the telescope to resolve that image satisfactorily. You might find that 3x is a step too far, unless the skies are very transparent and the seeing is excellent. It's worth a try though

[JamesF]

I often use a 2.5x barlow with a 40mm extension tube (my old Skywatcher kit barlow with the lenses removed) with my SPC900 in the 127 Mak. Works for me, and I get what I estimate to be about 3.3x overall.

James

[nytecam]

I'm currently using my SPC900 on the end of a 2x barlow, however, I was wondering if it would be possible to increase the size of the planetary image further but still be able to focus to get a decent sized image. Obviously I could go and spend hundreds on a better scope, but for now I want to get the most out of my existing scope.

If I were to use an extension tube, is there a formula that could be used to determine the maximum extension tube length for the SPC900 and my scope (650mm fl Astromaster 130eq). I tried dismantling one of my barlows by unscrewing the lenses, however I was unable to focus the image sufficiently....Any advice greatly received.

Increasing the distance between Barlow and sensor will increase the image scale AND reduce image brightness thus needing a longer exposure. The Barlow has a negative fl and if the separation of lens and sensor equal this value then the image is effectively projected to infinity [and is rather large!] Best not to disassemble a lens unless you make notes for its precise reeassembly

[opticalpath]

The formula for calculating the magnification you'll get at different spacing can be found in various places, e.g. here: http://www.astro-imaging.com/Tutorial/Calculations.html

It depends on knowing the negative focal length of your barlow lens, unfortunately, something that's not always easy to find out. Perhaps the simplest route is to just try some different spacers and take some images, then compare the size of the resulting images with the un-barlowed set up. That'll give you a rough estimate of the magnification factors you're getting at different spacings. Try it on the Moon, or on a distant terrestrial object, to make it a bit easier.

Adrian

[DrRobin]

Hi,

I agree with Adrian, I take pictures of a distant lit mast and measure the size between two lights in pixels to determine the magnification. Increasing the image size by 2x will lengthen the exposure by 4x. A good way to go is start with no Barlow, put your 2x Barlow in then add your extension, if you can't get focus you have gone too far. If your exposure gets too long you will see that there is no improvement in picture quality. Of course it depends on the seeing conditions, some nights you might not even be able to use your 2x Barlow.

Robin