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How does a focal reducer work?

Zeffer

By Zeffer
in Discussions - Cameras

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Zeffer

Zeffer

Posted
Posted

Go easy on me - my degree is in English! I can't get my head round how adding a focal reducer works.

Can anyone help? Is there some loss somewhere else? And loads of other questions I don't even know I should be asking?!

Thanks

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GazOC

GazOC

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Zef,

Both Atik and Antares do an imaging only 0.6 focal reducer (same model, different badge) for around £20. It work OK, using it I can get the whole of the moon (theres a song in there somewhere) on the Toucam chip using a 100mm f5.

Gaz

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The Warthog

The Warthog

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Actually, the question was about focal reducers, which in their simplest terms are a Barlow that reduces the effective focal length, whereas a regular Barlow increases the focal length. The reducer has convex lens elements, the Barlow, concave ones.

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Kaptain Klevtsov

Kaptain Klevtsov

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With a DSLR you get so much vignetting with the Antares 1.25" FR that the useable bit covers a smaller field of view than you get without the focal reducer. You do get shorter exposure times so if you don't mind losing resolution you can just use the centre of the sensor and get more subs done in the time between the clouds.

The 2" version doesn't vignette nearly as much but I can't get the thing to focus on conventional 'scopes and the inward travel is so great with the Skelescope that I lose half the light cone by moving the secondary so much closer to the primary. (Anybody know of a really short 2" nosepiece? Mine is kind of stepped inside so I can't just saw the end off like the 1.25" one.)

Another thing to watch out for is focus travel. With a DSLR I can't get enough inwards travel on my 'scopes to get the FR to work, different matter with the webcams as they have the sensor closer to the FR lens thereby reducing the effect. With my webcam I have cut down a nosepiece to allow the FR to get closer to the chip and make it work.

HTH

Captain Chaos

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CELESCOPE

CELESCOPE

Posted
Posted

you will need a f6.3 reducer anyway Zeff on the GPS , thats for starters , if i was u if you are using the dslr i would get the t ring for it using it through the 6.3 or either piggy back it on the celestron , it has an adapter for that already , another perk i left on .

Rog .

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MartinB

MartinB

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you will need a f6.3 reducer anyway Zeff on the GPS , thats for starters , if i was u if you are using the dslr i would get the t ring for it using it through the 6.3 or either piggy back it on the celestron ,

Rog .

Yep, you bump your head on it everytime you look through the eyepiece :wink:

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Zeffer

Zeffer

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I can see I have a lot to learn. Thanks Rog, I think my first imaging will be by piggybacking. How long can I image with the GPS without a wedge?

I have had a look now and there are 3.3 focal reducers as well. Which would be my better purchase?

Oh, and I will get that T-mount, there's one on astro buy and sell for £10 - is this the right one?

http://www.astrobuysell.com/uk/propview.php?view=8406

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G1ZmO

G1ZmO

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On the subject,

Can you get a F6.3 focal reducer that fits a standard 1.25" push-fit focusser?

All the ones I've seen seem to be some sort of screw fitting e.g. Celestron

I bought an ATIK focal reducer but it doesnt go anywhere near getting the moon onto my ToUcam ccd.

Paul

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Zeffer

Zeffer

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Good point - I was assuming that if I wanted the focal length, say for planets, then the focal reducer would help and use the fastar for DSOs?

I will be looking into a modified webcam. But does anyone know how bad the vignetting would be with the f6.3?

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GazOC

GazOC

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The last thing you want for planets is a focal reducer, you want to be trying to increase the focal length with a Barlow lens when imaging planets for increased size and detail.

Planets are generally bright enough that you don't have to worry about reducing the focal length so enough light lands on the webcam chip.

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Zeffer

Zeffer

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Righto - I am starting to get this, honest.

I hadn't clicked that the focal reducer changes more than just the f-number. (Daft I guess).

Why would I need one then with the fastar? Is it for an intermediate focal length / speed when imaging?

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Ambermile

Ambermile

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You NS is f/10. The Fastar makes it f/1.9 I think. For some that's a bit radical so you could get a 6.3 or 3.3 reducer, respectively making your scope f/6.3 or f/3.3. The 6.3 will be usable for visual as well but the 3.3 is also a bit too radical a stretch for visual.

Arthur

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