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Old 21-09-2013, 06:06 AM
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nebulosity. (Jo)
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A New Lens For Wide Fields

Gíday everyone,

Iíve been thinking and have decided that as my home built mount is not able to have autoguiding, and I hate manual guiding, Iím going to stick to doing wide field stuff until I get around to building a new mount.

Now, currently Iíve been using a Panasonic GX1 and a Nikon 85mm lens, which, as the GX1 has a 17x13mm sensor, has given me a nice 170mm focal length.

The GX1 is a nice little camera and has given me some great results but is limited by the fact that it can only do 128 second exposures so recently Iíve been working on cooling a Canon 350d to use as my wide field camera.

But I got a bit of a problem with the 350d, as it has a larger sensor than the GX1, with the 85mm lens Iím only going to be getting a 136mm focal length, and added to the fact that itís only 8mp vs 16 mp for the GX1 I donít think 136mm is going to give me the resolution Iím looking for, what do you reckon?

So, Iím looking for a lens around 100 to 200mm and the only criteria is that it has to be super sharp at f4, have minimal CA, and have a second hand price of under $500.

Any ideas?

The 85mm lens would probably work nice but I was hoping to get a bit more close and personal, I reckon the max unguided exposures I could get on my mount at 200mm would be around 5 to 8 minuets, it that good enough?

If you know of a cracker lens thats over my budget you might as well mention it as well, I like to goggle.

Thanks a lot.

Cheers
Jo

PS Nikon lenses are not out of the question as I have a Nikon to Canon adapter.
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  #2  
Old 21-09-2013, 06:33 AM
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rcheshire (Rowland)
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Hi Jo. I use a Canon 200mm f2.8L USM II, prime lens. Reported to be very sharp. But there are other factors that determine image sharpness in AP. With an APS-C sensor, it looks like 320mm. I find this provides a field not so wide that deep sky objects loose too much detail.

The lens needs stopping down. The best resolution is obtained at f5.6 - a completely flat field at f6.3. This is nice at iso400 under light polluted skies with exposure/integration times of 3 - 3.5 minutes with a 1000D.

I think it meets your criteria at f4, but you will get slightly eggy stars in the corners. Price second hand, might make it?

Iris diffraction spikes are very prominent on larger stars. Alternatively you can use an apeture ring on the front of the lens. Set at f2,.8, or tape over the contacts on the back of the lens.

You can of course image wide open or at any other aperture, but the edges will need cropping.
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Old 21-09-2013, 06:43 AM
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Nikon 180mm F2.8 ED is a classic. You can get adapters for Nikon to Canon easily on ebay.

I use one occasionally.

Greg.
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Old 21-09-2013, 10:05 AM
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Octane (Humayun)
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+1 for the EF 200mm f/2.8L II USM.

A beautiful lens for the price.

H
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Old 21-09-2013, 10:29 AM
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Thanks heaps for the coments guys, I'll check them out.

Jo
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Old 21-09-2013, 11:39 AM
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+ 1 for the 180 ED AF , I've just bought one to use with my Fujifilm X-E1 Mono ,
The Canon 200 is a good lens but how do you control the aperture on a non canon body ?
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Old 21-09-2013, 01:09 PM
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You'll need to have a pretty good worm drive to do unguided exposures with a 200mm lens.

6.4 micron pixels @200mm is 6 arc sec/pixel

What drive do you have on your home made mount? You can get away with much bigger drive errors with shorter lenses.

Joe
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Old 21-09-2013, 02:15 PM
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alocky (Andrew lockwood)
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Add another for the nikon 180 f2.8 D series. I've used it on a polarie with acceptable results, so it fits your needs. Available second hand!
Cheers,
Andrew.
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Old 21-09-2013, 04:46 PM
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Here's an example from the Nikon 180 F2.8 ED at F4:

http://www.pbase.com/gregbradley/image/152449036/large

Round stars to the corner of a FLI Proline 16803. Very few lenses can do that. There is a tad amount of tilt here as the adapter needs to be unscrewed a little bit from the filter wheel otherwise it catches the filters and stops them from turning. But its minor.

I did notice minor chromatic aberration in some other images I used it for but again its minor compared to most lenses. It often goes for less than US$450 from places like keh.com.

There are a few versions of the lens. The crinkled paint finish version goes for more. I have had both and they both are very good and I think the differences are more cosmetic and to do with autofocus which is not needed.

Plus it has an aperture ring which not all lenses these days have which is also sometimes a pain.
On Canon APSc with 1.6 crop factor that is 288mm. On a Fuji it is 270mm (Fuji APSc is 1.5 crop).

Even with Polarie you would be limited to a couple of minutes tops without a proper mount or autoguiding. I don't think Polarie will handle that long a focal length for several minutes.

Greg.
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Old 22-09-2013, 05:01 PM
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Quote:
Originally Posted by OzEclipse View Post
You'll need to have a pretty good worm drive to do unguided exposures with a 200mm lens.

6.4 micron pixels @200mm is 6 arc sec/pixel

What drive do you have on your home made mount? You can get away with much bigger drive errors with shorter lenses.

Joe
I have roller drives on my mount so no PE.

Quote:
Originally Posted by gregbradley View Post
Here's an example from the Nikon 180 F2.8 ED at F4:

http://www.pbase.com/gregbradley/image/152449036/large

Round stars to the corner of a FLI Proline 16803. Very few lenses can do that. There is a tad amount of tilt here as the adapter needs to be unscrewed a little bit from the filter wheel otherwise it catches the filters and stops them from turning. But its minor.

I did notice minor chromatic aberration in some other images I used it for but again its minor compared to most lenses. It often goes for less than US$450 from places like keh.com.

There are a few versions of the lens. The crinkled paint finish version goes for more. I have had both and they both are very good and I think the differences are more cosmetic and to do with autofocus which is not needed.

Plus it has an aperture ring which not all lenses these days have which is also sometimes a pain.
On Canon APSc with 1.6 crop factor that is 288mm. On a Fuji it is 270mm (Fuji APSc is 1.5 crop).

Even with Polarie you would be limited to a couple of minutes tops without a proper mount or autoguiding. I don't think Polarie will handle that long a focal length for several minutes.

Greg.
you've sure convinced me Greg I'm gonna get one

I did some maths and worked out that with the 350d FOV is 7.072 by 4.71 degrees, 7.37 arcsec/px.

It looks like it will suit my needs very well, so thanks everyone for your comments and I hope to be able to show you some results once I get my camera going.

Cheers
Jo
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Old 23-09-2013, 12:11 PM
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Quote:
Originally Posted by astroboy View Post
+ 1 for the 180 ED AF , I've just bought one to use with my Fujifilm X-E1 Mono ,
The Canon 200 is a good lens but how do you control the aperture on a non canon body ?
Fit an aperture ring. I use step up/down rings back to front.
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Old 23-09-2013, 01:19 PM
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Regulus (Trevor)
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For what it's worth Jo, don't go past looking at the old M42 mount lenses.
Since you are doing it all manual the electronic connections are moot. I have been looking at eBay for old 50 and 100mm Macro lenses and notice very good ones selling under $300.
An adapter from the same source is under $10.

Trevor
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