Rescued a Barlow from the trash...3.3x?

[szymon]

Hi there,

When I bought my telescope, the guy selling it to me included something that was marked up as a 3.3x barlow, but said that it "doesn't work".  Holding it up to the light, you couldn't see anything through it apart from a dull gray haze, so I accepted that assessment ;-)

Yesterday evening, as it was so cloudy anyway, I decided to take a look at why.  It turns out that it was just covered in sticky gunk.  Something sticky, which then got covered in dust, on which dirt had built up.  Using some muck-off (used to get rid of grease from my motorcycle!) followed by lens cleaning fluid and cloths, and a lot of patience, I cleaned it all out until it looked like a lens again.

I have to say, I was relatively impressed with the results!  When using it with my 25mm eyepiece, it gave significantly better views than the 12mm eyepiece I have (although that's probably a reflection on the quality of the 12mm eyepiece more than anything else).  Clear close-ups of the moon (when the clouds momentarily left it alone), and no real distortion or image quality issues.  25/3.3 is about 7.5, so on my 900mm scope that's 120x.  The moon is quite nice at 120x magnification :-)

In photography, using high-powered tele-extenders is usually a good way to dramatically compromise image quality.  If I saw something saying it's a 3.3x teleconverter, I'd dismiss it out of hand.  Can I assume the same is not true for astronomy?  I've bought a 2x barlow (Celestron Deluxe branded, mainly bought because it comes with a T-Ring on top) -- would it be reasonable to stack this with the 3.3x for imaging?

-simon

[JamesF]

It's important to understand why one might want to stack barlows for imaging and what benefits (if any) and downsides there might be.  It's not all just about "getting a bigger image".

For example, if the limit of resolution of your telescope is two arcseconds, and the limit of resolution of your camera at the native focal length of the telescope is one arcsecond, what do you gain by making the image six and a half times bigger?  You won't resolve any more detail as a result and given that the exposure will take almost 45 times as long to collect the same number of photons per pixel, you'd probably be far better off enlarging the image in processing.

In general you'd probably consider your target, telescope and camera together, then decide if a barlow is likely to be useful and what kind of multiplier might be a benefit.  I'd say that overall the high multiplication factors are more likely to find use in fast scopes when used for imaging solar system targets.  I'm using a C9.25 for my planetary imaging these days and don't tend to use a barlow with a multiplier more than 2x (in fact, smaller would be good if I could find a decent one) because its focal ratio is f/10.

James

[szymon]

Actually, I don't necessarily want to stack them -- I mainly bought the 2x barlow because it had a T-Ring on it and I didn't know that the scope I was going to buy would already have one on it!  Now however I have the option, using it "prime" or adding in the magnifying glasses.  I'll play with both options, I'm a radio amateur and we learn via experimentation :-)

The 2x barlow arrived in the post today, and using both (2x and 3.3x) gives a surprisingly crisp, clear, sharp daylight image with my 25mm eyepiece.  240x is meant to be "beyond" the capability of this telescope, I guess I'll find out when I try it on the skies at night (if we *ever* get clear skies).