Effect of using camera lens instead of barlow?

[mattias]

Hi everyone!

I was thinking if it is possible to use a zoom lens mounted on the camera and connected directly to the telescope with a step down ring and T2 adapter as a replacement for a barlow? (as attached pic)

I mean, the optics within the lens are much more sophisticated than a barlow, at least if you compare price versus optical quality.

I've got a Canon EOS 600D and one of my lenses are the EF-S 60MM 2.8 Macro. It's extremely sharp and according to my calculations it would have a magnification factor of 1.92. (60x1.6/50) I suppose this would be able to compare against a x2 barlow?

Another lens of mine is the EF-S 15-85, and att highest zoom that would give me a magnification factor of 2.72.

One problem could of course be the focus distance but I think that in the case of the 60 mm it is not a big problem due to the very short lens.

All the best

Mattias

[mattias]

Attaching file...

[Astrobits]

All camera lenses are positive lenses so one would not act in the same way as a barlow. As a positive lens it will act as a transfer lens, when outside of the primary focus, with the magnification depending on the placement of the lens with respect to the focal plane of the telescope and the sensor of the camera. That is, the camera lens is acting in the same way as eyepiece projection which can give much larger magnifications than just using a barlow.

Nigel

[E621Keith]

Hi everyone!

I was thinking if it is possible to use a zoom lens mounted on the camera and connected directly to the telescope with a step down ring and T2 adapter as a replacement for a barlow? (as attached pic)

I mean, the optics within the lens are much more sophisticated than a barlow, at least if you compare price versus optical quality.

I've got a Canon EOS 600D and one of my lenses are the EF-S 60MM 2.8 Macro. It's extremely sharp and according to my calculations it would have a magnification factor of 1.92. (60x1.6/50) I suppose this would be able to compare against a x2 barlow?

Another lens of mine is the EF-S 15-85, and att highest zoom that would give me a magnification factor of 2.72.

One problem could of course be the focus distance but I think that in the case of the 60 mm it is not a big problem due to the very short lens.

All the best

Mattias

Are you planning to image with this or attach an eyepiece to the back of the camera lens for viewing. Your post isn't very clear and you posted in the eyepiece rather than the camera forum.

Camera lenses in general have inferior optics to telescope, even most L telephoto lens are not defraction limited whereas most telescopes are. Each lens elements introduce additional error, and most camera lens suffers from coma, CA, and SA wide open. After all, camera lenses are not designed for shooting a field of point sources, which is even more challenging than shooting test chats.

http://www.astropix.com/HTML/I_ASTROP/EQ_TESTS/C300MM.HTM

As Nigel pointed out, if you use camera lens, it will behaves more like eyepiece projection than a barlow. A tele extender like the Canon EF 1.4X III or EF 2.0x III are more similar to a barlow.

[The Sailor]

Even if this worked my worry would be the strain at the camera lens junction to the telescope, the filter thread is not that wide and although the Canon is lightish there must still be some weight on that piece of thread especially during a slew.

Even if someone was to say it was okay I would still have doubts.

Jim

[mattias]

Are you planning to image with this or attach an eyepiece to the back of the camera lens for viewing. Your post isn't very clear and you posted in the eyepiece rather than the camera forum.

Camera lenses in general have inferior optics to telescope, even most L telephoto lens are not defraction limited whereas most telescopes are. Each lens elements introduce additional error, and most camera lens suffers from coma, CA, and SA wide open. After all, camera lenses are not designed for shooting a field of point sources, which is even more challenging than shooting test chats.

http://www.astropix....ESTS/C300MM.HTM

As Nigel pointed out, if you use camera lens, it will behaves more like eyepiece projection than a barlow. A tele extender like the Canon EF 1.4X III or EF 2.0x III are more similar to a barlow.

The reason I posted here was that I intend to use it as a replacement for a barlow (at least I imaged that it could be used for it).

My intentions with this setup are to take photos/film of planets only. I know that the step down ring will leave a coma around the image but that only affects the outer parts of the image and the planets would definitely end up in a small part of the center so that is not a problem in itself.

Regarding weight this is something to evaluate. If I believe that the camera or lens/threads would break, I will find myself a good barlow instead :-)

[mattias]

So, if the lens acts like an ocular, how does the focal length of the lens corresponding to the focal length of the ocular?

Would you for example gain higher magnification by choosing a wide lens instead of a zoom lens?

[Astrobits]

What you will be effectively doing is to photograph the focal plane of the telescope ( where the primary image is ) as if it is a macro subject. To get a 1:1 ratio the lens-subject distance will equal the lens-image distance. If the lens is 50mm f/l then the subject and image distances from the lens will be in the region of 100mm each. If you use a 100mm f/l lens then the distances will be about 200mm each. To get the image twice the size then the image distance will increase and the subject distance will decrease according to the optical formula 1/u + 1/v = 1/f where u is the subject distance, v is the image distance and f is the focal length of the lens. Compound lens systems can be considered as a single lens for this calculation but you will have problems deciding where a single lens of equivalent focal length would be placed for the measurements. To get any meaningful increase in magnification with a normal camera lens is going to be difficult as the extensions needed are going to be ridiculous. And don't forget that when going beyond 1:1 with a camera it is better to reverse the lens otherwise the image quality deteriorates.

Far better is to use a camera adaptor that takes an astronomical eyepiece.

Nigel

[The Sailor]

Far better is to use a camera adaptor that takes an astronomical eyepiece.

Nigel

Nigel, do you mean this item:

http://compare.ebay.co.uk/like/160684490095?var=lv&ltyp=AllFixedPriceItemTypes&var=sbar&adtype=pla&crdt=0

Jim

[mattias]

What you will be effectively doing is to photograph the focal plane of the telescope ( where the primary image is ) as if it is a macro subject. To get a 1:1 ratio the lens-subject distance will equal the lens-image distance. If the lens is 50mm f/l then the subject and image distances from the lens will be in the region of 100mm each. If you use a 100mm f/l lens then the distances will be about 200mm each. To get the image twice the size then the image distance will increase and the subject distance will decrease according to the optical formula 1/u + 1/v = 1/f where u is the subject distance, v is the image distance and f is the focal length of the lens. Compound lens systems can be considered as a single lens for this calculation but you will have problems deciding where a single lens of equivalent focal length would be placed for the measurements. To get any meaningful increase in magnification with a normal camera lens is going to be difficult as the extensions needed are going to be ridiculous. And don't forget that when going beyond 1:1 with a camera it is better to reverse the lens otherwise the image quality deteriorates.

Far better is to use a camera adaptor that takes an astronomical eyepiece.

Nigel

I think I understand.

In this case, it's a macro lens with the ratio 1:1 at 20 cm. For the object to appear the same size on the chip as at the focal plane on the telescope the lens end needs to be as far as 20 cm from the focal plane on the telescope?

If so, I would not gain anything at all putting the lens between the telescope and the camera. The 1:1 situation would appear anyway when you put the camera chip in the prime focus of the telescope.

Am I right?

[Astrobits]

Jim

That looks like some sort of torture device

I have the simple type that looks like an eyepiece barrel but can contain a basic eyepiece within. 1 1/4" one end and T2 thread at the other. Like this:

http://www.telescopehouse.com/acatalog/Meade_Basic_Camera_Adapter_1_25__.html

Mattias

You are correct.

Nigel

[The Sailor]

I have a few of those items in your link Nigel, thought maybe you were on about the one in my link which I also have though have not yet used it

Jim