Teleconverter or barlow for imaging with a DSLR

[Brinders]

Having just acquired a new DSLR (Pentax K20D), I was wondering what the best way of doubling the magnification was when it is connected to a telescope. Being the owner of a couple of barlows, I have used those, with variable results, on my old Samsung GX10 in the past. However, while researching which DSLR to buy, I came across an interesting piece at cameralabs.com, where the editor, Gordon Laing, describes using a teleconverter to double the image size. I was wondering what method members of this forum preferred and if anyone has experience of using a teleconverter in place of a barlow.

Out of curiosity, I have purchased a secondhand Soligor 2X teleconverter off ebay. As soon as it arrives, and I get the chance, I'll do a back to back with a 2x TAL barlow and the teleconverter. Still like to hear your views though, guys.

[jgs001]

I've tried both a 2xTC and a 2xBarlow. The 2xTC gives double, but a 2x barlow gives about 2.5x ish, and I think it's to do with the spacing between the camera and the barlow lens cell. If I use a 2xTC on the C80ED the moon about fills the frame. I used my 2x barlow tonight, and ended up having to go for two shots (about 2/3 of the moon each) and merge them as a panoramic.

[Brinders]

Cheers John.

[brendant]

Biggest problem with using Teleconverters of Barlows for imaging is the increased amount of glass in the imaging train introducing false colour and chromatic abberations and the reduction in light reaching the sensor increasing exposure times, OK for the Moon and brighter objects but not so good for anything else.

Either need to be ED or APO types to reduce CA and other artefact's, the least amount of glass in the system the better, have seen some great images with Canon Teleconverters being used on telescopes.

Brendan

[Brinders]

Biggest problem with using Teleconverters of Barlows for imaging is the increased amount of glass in the imaging train introducing false colour and chromatic abberations and the reduction in light reaching the sensor increasing exposure times, OK for the Moon and brighter objects but not so good for anything else.

Either need to be ED or APO types to reduce CA and other artefact's, the least amount of glass in the system the better, have seen some great images with Canon Teleconverters being used on telescopes.

Brendan

Hi Brendan, yes CA crossed my mind too. I have noticed a little with a barlow on my old DSLR. The thought also occurred to me that the more elements in the teleconverter the greater the chromatic aberration. Still for a modest outlay, it will be worth a try and my main interest is the moon, so may be useful if I try imaging in black and white.

[jgs001]

You can generally get away with the CA, if you get the focus spot on, the biggest problem I find with my 2xTC (as it's not for digital, but an ebay cheapy) is reflection effects from the sensor (it's quite a well known phenomenon with older camera optics, film didn't have the same reflective nature as sensors).

This was shot with the 2xTC (designed for film, not digital and cost £30 from ebay)

This is a 2 pane panoramic shot with my 2x barlow yesterday (this is an ED barlow)

HTH

[Brinders]

Thanks John. Both look very nice images, althoughI think there is some CA visible with the upper image using the 2x teleconverter - the ED barlow has the edge, but both look superb to me.

I've obtained my teleconverter from ebay also and will, I imagine, have been designed for film too. However, for £13.50 I thought it worth a punt, partucularly as I don't trust the single srew on my barlow to hold such a heavy camera as the K20D securely: I tought a teleconverter would be a more secure approach as the T adapter and nosepiece will be held by the brass compression ring in the draw tube.

Cheers,

Brinders

[jgs001]

Using the Barlow yesterday, I also used a diagonal... the 450d neck strap was firmly in place around my neck whilst I shot those just in case , I think the TC will give good results and as you say, for the money, it's worth it.