View Full Version here: : Too much Barlow?

28-05-2007, 11:20 AM

A while ago I bought a Televue 3x barlow for use with my Meade LPI and VC200L (8 inch) scope. I have not been able to use this for either visual (I'm not really surprised!) or imaging work though.

When fitted the LPI acts as a 6mm ep - thus native magnification is 300x, with a 3x balow in I am trying to work at a massive 900x (0.2" per pixel), I cannot get decent results though. With a 2x barlow I can get decent sharp frames at frequent enough intervals to be useful but seeing always seems to defeat me with the 3x in place...am I dreaming? I thought the extra image scale would help - in fact the opposite seems true with my images with the 2x looking sharper even scaled up...

Perhaps I have just never had really good seeing to work with...but, if true, that means those nights are so rare the 3x is not going to get much tube time...

28-05-2007, 01:10 PM
I almost always use a 3x Televue barlow with my 9.25 when imaging planets with the ToUcam.

What are you trying to image?

What's the focal length and f ratio of your scope?

What the pixel size (microns) of the chip in your LPI? Or is that a CMOS? That could be the problem.

28-05-2007, 01:34 PM
5umx5um and yes its CMOS.

I also have an Orion Starshoot (CCD based) would I be better off with that at the high magnifications?

28-05-2007, 01:55 PM
I'd be willing to bet the problem rests with the CMOS chip in the LPI.

They are also quite small pixels. The ToUcam has 50% larger photosites.

Try the same set-up (scope/barlow) but with a different camera.

Do you have access to a ToUcam?

That's about as much as I can offer.

28-05-2007, 02:31 PM
Also remember that you are multiplying bad seeing by 3 times as well. If the air is turbulent, the 3x barlow will make it 3 times as turbulent!

Barlowing requires fairly good seeing. It's not absolutely essential, but it certainly helps.

28-05-2007, 02:37 PM
I do all my planetary imaging with the 3x barlow, regardless of the seeing.

It's more of a critical factor for visual.

If the seeing doesn't hold up to a 3x barlow when imaging, I generally don't bother.

However, this again depends on what sort of image scale you are after?

It also depends on what quaility of image you are happy with.

Most planetary imagers work around the f30-f40 range to produce their images.

There are no hard and fast rules. It's all horses for courses.

You set the benchmark, you make the decision based on what you are hoping to achieve:)