Alternate Barlow placement

[West End Wendy]

Does anyone place their barlow in front of a diagonal in order to get increased magnification from it?

The Antares 1.6 barlow, for instance, advertises that it becomes a 2.4 Barlow when used in this way.

This seems like an incredibly useful feature, but I don't hear it mentioned much. Is the reality not as straightforward as the theory?

Tim

[Chris H]

It will work as long as the barlow is short enough for you to still achieve focus. Give it a go.

[brantuk]

I've tried this and it did seem get get a bit more magnification but it wasn't a perfect night and the object was quite low so it was grainy. I was just messing with barlow combinations really.

The reason I wouldn't do it regularly is cos of the weight of a 2" diag and large ep hanging off it. The barlow screws are a bit difficult to tighten enough to hold everything confidently in the Antares 1.6 - I was afraid of stripping threads and overdoing it or being too loose and the lot dropping out.

I don't know the maths but if you can make it stable it does work somewhat.

[Damo636]

I have tried this with the Antares 1.6X but couldnt achieve focus with the f11, not enough inward travel as far as i can remember.

[Peter Drew]

It's perfectly possible to do this provided that you have enough in travel to acheive focus. Units have been constructed in the past employing a sliding tube Barlow giving a variable magnification, some of these are called zoom eyepieces! . The caveat with alternative Barlow placement is that some are ray traced to work best at their nominated amplification position ie in front of the eyepiece.

[rfdesigner]

Yes you can put a barlow in that position.

I tried ray tracing to place both barlows and plossels at the secondary point. In the end I decided that it was only possible to place a barlow lense at the secondary, I was aiming to reduce the secondary mirror size for something like an f5 instrument. To make it work well it would need to be a permanent fix as the focus position of the primary mirror would need to be somewhere inside the OTA, which would be fairly unusual. You also need a Barlow with a glass diameter that is at least as big as your secondary.. but ideally you don't want anything larger than your secondary mirror, so that really means using un mounted lenses and somehow fitting them without fattening them up.

Yes it can be done.. but to do it well is hard.

Derek