[Derry]

wondering if anyone shooting prime focus with a DSLR has tried to use any type of teleconverter between the camera body and the T mount attachment,, considering buying one for normal photo use and if it works to increase my TV85 scopes power, that would be double the benefit,,

thought someone may have tried this arrangement and would share their outcome,,

appreciate,,

[Sout Fork]

Originally Posted by Derry

wondering if anyone shooting prime focus with a DSLR has tried to use any type of teleconverter between the camera body and the T mount attachment,, considering buying one for normal photo use and if it works to increase my TV85 scopes power, that would be double the benefit,,

thought someone may have tried this arrangement and would share their outcome,,

appreciate,,

Because I don't use a normal telephoto lens I don't have a teleconverter. On the other hand because I do use astro scopes I do have and use a barlow for the same purpose on my astro scopes.

I would think, and I could be wrong, that barlowing your TV85 would give you better results than trying to kludge together a teleconverter to your Tele Vue. You could get a decent barlow for 50 bucks.

SF

[Derry]

have two TV barlows, 2.5X and 5X, only problem is they are 1.25" and need a 2" for the T mount,, will see what I can work out,,

most likely will buy the new 2x Oly converter,, will let ya know what it does besides the barlows,,

thanks,,

Originally Posted by Sout Fork

Because I don't use a normal telephoto lens I don't have a teleconverter. On the other hand because I do use astro scopes I do have and use a barlow for the same purpose on my astro scopes.

I would think, and I could be wrong, that barlowing your TV85 would give you better results than trying to kludge together a teleconverter to your Tele Vue. You could get a decent barlow for 50 bucks.

SF

[Sout Fork]

Originally Posted by Derry

have two TV barlows, 2.5X and 5X, only problem is they are 1.25" and need a 2" for the T mount,, will see what I can work out,,

No problem.
Get a 2" to 1.25" adapter WITH T threads - they cost 20 bucks here in the States.

Unscrew the barlow element from the barrel, install element in adaptor just as if it was an eyepiece and then screw in T ring to adapter and you should be good to go.

See pics

SF

[Paul Corfield]

I use a 2X and a 3X teleconverter prime focus and they work very well, nice and sharp across the whole photo. If you buy any then try and get the 7 element versions. A lot of the cheaper ones only have 4 glass elements and are inferior for sharpness. They are all cheap enough to experiment with on ebay though.

I've also tried 3 barlows so far and found them inferior to my teleconverters. They were all 1.25" versions and I found the photos were only sharp in the center and quite blurred at the edges no matter where I put the glass element in relation to the camera ccd. That may improve or disappear with a 2" barlow but I have none to try. I may try and get hold of a 2.5X Tele Vue Powermate next year as they are supposed to be way superior to even the apochromatic barlows. The Powermate is quite expensive though but I'll get one fairly soon. They have 4 glass elements and work slightly differently to barlows in that they restore the field rays back to their normal direction so you get pure magnification with no distortion.

At the moment I'm experimenting with eyepiece projection onto the camera ccd and that has given me my best photos so far out of any lens/eyepiece configurations I've tried to date. Just about to post some photos in my thread "Some digiscoping experiments with a dslr".

Paul.

[erniehatt]

Originally Posted by Paul Corfield

I use a 2X and a 3X teleconverter prime focus and they work very well, nice and sharp across the whole photo. If you buy any then try and get the 7 element versions. A lot of the cheaper ones only have 4 glass elements and are inferior for sharpness. They are all cheap enough to experiment with on ebay though.

I've also tried 3 barlows so far and found them inferior to my teleconverters. They were all 1.25" versions and I found the photos were only sharp in the center and quite blurred at the edges no matter where I put the glass element in relation to the camera ccd. That may improve or disappear with a 2" barlow but I have none to try. I may try and get hold of a 2.5X Tele Vue Powermate next year as they are supposed to be way superior to even the apochromatic barlows. The Powermate is quite expensive though but I'll get one fairly soon. They have 4 glass elements and work slightly differently to barlows in that they restore the field rays back to their normal direction so you get pure magnification with no distortion.

At the moment I'm experimenting with eyepiece projection onto the camera ccd and that has given me my best photos so far out of any lens/eyepiece configurations I've tried to date. Just about to post some photos in my thread "Some digiscoping experiments with a dslr".

Paul.

I have no problems using a 2" 2X barlow . Ernie

[Paul Corfield]

Originally Posted by erniehatt

I have no problems using a 2" 2X barlow . Ernie

Don't you use yours with a point and shoot camera though? With prime focus and a dslr the barlow is projecting the image onto the ccd without a camera lens being in the way. That means the barlow needs to be optically perfect as there's no other optics in the light path to correct it.

Paul.

[Sout Fork]

Originally Posted by Paul Corfield

Don't you use yours with a point and shoot camera though? With prime focus and a dslr the barlow is projecting the image onto the ccd without a camera lens being in the way. That means the barlow needs to be optically perfect as there's no other optics in the light path to correct it. Paul.

Paul,

As I understand it you may be confusing terms here. "Prime Focus" means to use the native focal length that is inherent to the scopes objective. So to use a negative focus device like a barlow in the optical path to increase the apparent focal length of a scope's objective's is not prime focus by definition. In other words a true prime focus setup has nothing between the front objectives of the scope and the focal plane of the camera but air. No barlows, eyepieces, camera adapters, focal reducers etc - just air.

In any case, all of the following were taken with scopes of various focal lengths using a DSLR and a $12, 1.25", two element, achromatic, GSO barlow as I show in the post. All are full frame except last one which is nearly full frame.

Judge for yourself.

SF

[Paul Corfield]

Originally Posted by Sout Fork

Paul,

As I understand it you may be confusing terms here. "Prime Focus" means to use the native focal length that is inherent to the scopes objective. So to use a negative focus device like a barlow in the optical path to increase the apparent focal length of a scope's objective's is not prime focus by definition. In other words a true prime focus setup has nothing between the front objectives of the scope and the focal plane of the camera but air. No barlows, eyepieces, camera adapters, focal reducers etc - just air.

In any case, all of the following were taken with scopes of various focal lengths using a DSLR and a $12, 1.25", two element, achromatic, GSO barlow as I show in the post. All are full frame except last one which is nearly full frame.

Judge for yourself.

SF

Yes, I was just making the point that there's no lens on the camera whereas Ernie is using his barlow with a digicam to give more eye relief. I suppose at the end of the day even a cameras prime lens has a piece of glass at the rear of the lens so it's not so different with a barlow.

Your photos look fine Sout Fork so I guess you are just lucky to get a barlow that doesn't distort. The most expensive one I tried was a Celestron 2X which I picked up on ebay. That was the worst, not only did it distort but the centre of the photo had a circular bright spot that would ruin the photo. The other two I tried were budget models. A simple test I try is to photograph something like a grid or a wire fence and it shows up the loss of image quality at the edges of the photo. I'll post an example later taken with a barlow and one with a teleconverter to show how much sharper the teleconverter is. With the barlow I think if I have it not so close to the ccd then the distorted part should fall outside of the ccd. It works in eyepiece projection so it should work the same with a barlow. I'll try that later too and post up the results.

Something else I'm thinking is that all of my barlows are short tube versions. For that reason the glass is a different shape to that used in the longer tube versions. Maybe that's where my distortion is coming from as it's probably easier to get a bad one. The GSO gets good reviews so I guess it's one of the better budget models.

Paul.

[Paul Corfield]

Here's some photographic examples why I prefer teleconverters over barlows. Also another good reason is that the camera can sit much closer to the scope and still achieve focus whereas with the camera/barlow needs to sit about another 1.5" further back.

For me the teleconverters are much better corrected and the images have more contrast straight off the camera. With the 3 barlows I've tried the images are more washed out as shown below.

These photos are the full images, uncropped, just resized to 800 pixels wide.

Paul.