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Old 28-08-2013, 06:54 PM
Hans Tucker (Hans)
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Imaging at long focal lengths

Is it a pointless exercise to try and image with an f/12 refractor?
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Old 28-08-2013, 08:40 PM
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alocky (Andrew lockwood)
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Depends what you're imaging!
This is through a 4" f15 refractor
http://www.astrobin.com/18577/
But - I assume you're asking about deep sky, in which case you will find the relatively small aperture will add to the degree of difficulty.
cheers,
Andrew.

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Is it a pointless exercise to try and image with an f/12 refractor?
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Old 28-08-2013, 08:40 PM
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I think it could be done. You would just have to take longer exposures and preferably use a more sensitive CCD to compensate.

Its more the aperture rather than the F ratio. What is the aperture?

Greg.
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Old 28-08-2013, 08:53 PM
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Ive done some imaging with a 150mm apochromat at f11.1. It worked out ok, but I was using a very sensitive mono nabg camera, so that helped.
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Old 28-08-2013, 08:56 PM
Hans Tucker (Hans)
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Sorry, I should have been more specific. Yes, I am questioning if you can image Deep Sky with a long focal length. I would think that for planetary I could use the f/12 to an advantage. The scope is a 5" f/12 APO refractor. I do have the AP 0.67X Reducer so I can go from f/12 to f/8.
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Old 28-08-2013, 09:28 PM
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I did deep sky with it.... Lets see ...... Oh here's one...



www.billionsandbillions.com/centaurus_a.html

I did try using an ap reducer (27tvph .75 and .67). But because the field was very flat in the first place and the ap reducers have a slight flattening effect, they induced field curvature. I have a vc200 reducer that I was going to try, but never got around to it.
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Old 28-08-2013, 09:49 PM
Hans Tucker (Hans)
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Originally Posted by bert View Post
I did deep sky with it.... Lets see ...... Oh here's one...



www.billionsandbillions.com/centaurus_a.html

I did try using an ap reducer (27tvph .75 and .67). But because the field was very flat in the first place and the ap reducers have a slight flattening effect, they induced field curvature. I have a vc200 reducer that I was going to try, but never got around to it.
Wow, that Centaurus A image is impressive and inspiring. The scope in question that was used to produce this image was a f/10.5 wasn't it.
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Old 28-08-2013, 11:29 PM
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Hi Hans,
Interesting to read that it is a 5" f/12 "APO". Most APO's seem to be significantly shorter focal length, say F7-F9. The combination of F12 and "APO" would be really good in terms of chromatic aberrations, despite the limitations in terms of imaging. Can you say what the scope is? An early AP Super Planetary? Three lens objective? John

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Originally Posted by Hans Tucker View Post
Sorry, I should have been more specific. Yes, I am questioning if you can image Deep Sky with a long focal length. I would think that for planetary I could use the f/12 to an advantage. The scope is a 5" f/12 APO refractor. I do have the AP 0.67X Reducer so I can go from f/12 to f/8.

Last edited by johnt; 28-08-2013 at 11:57 PM.
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Old 29-08-2013, 06:12 AM
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Quote:
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Wow, that Centaurus A image is impressive and inspiring. The scope in question that was used to produce this image was a f/10.5 wasn't it.
Nope, it was 150mm f11.1. 1700mm focal length.
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Old 29-08-2013, 08:12 AM
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multiweb (Marc)
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Quote:
Originally Posted by Hans Tucker View Post
Is it a pointless exercise to try and image with an f/12 refractor?
No you can do DSOs at that FL. I did this one and this one with a 90mm refractor at F/20. Hard on optics and guiding but all long FL work needs impeccable guiding regardless.
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Old 29-08-2013, 04:13 PM
Hans Tucker (Hans)
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Originally Posted by johnt View Post
Hi Hans,
Interesting to read that it is a 5" f/12 "APO". Most APO's seem to be significantly shorter focal length, say F7-F9. The combination of F12 and "APO" would be really good in terms of chromatic aberrations, despite the limitations in terms of imaging. Can you say what the scope is? An early AP Super Planetary? Three lens objective? John
It is an Apomax 130mm f/12, with a 3-element objective built by Fred Mrozek. I believe only around 23 were made. My scope is the attached image but here is a review from CN.

http://www.cloudynights.com/item.php?item_id=897
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Old 29-08-2013, 04:35 PM
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Google "focal ratio myth". You should get many hits including articles from Stan Moore (CCDStack originator) and Craig Stark (PHD originator). You may find their views are contrary to those commonly held. Yes you can DS image at f12.
Geoff
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Old 29-08-2013, 07:42 PM
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Here’s the faint Globular Cluster HP1 at F12 with an ST2000XM and CFW9.

Cheers

Dennis
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Old 29-08-2013, 11:46 PM
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Quote:
Originally Posted by Hans Tucker View Post
It is an Apomax 130mm f/12, with a 3-element objective built by Fred Mrozek. I believe only around 23 were made. My scope is the attached image but here is a review from CN.

http://www.cloudynights.com/item.php?item_id=897

Very interesting Hans, I did not know these scopes existed. Doing a search, I saw that a few sold recently on CloudyNights, and were descibed as "brand new" so I'm guessing that they are recently made.

I have recently bought a Mallincam Xtreme camera for video astronomy and intend using it only with a video screen (not with computer capture for still imaging). However, I believe that your question about the suitability of longer focal lengths is also relevant to video astronomy.
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Old 30-08-2013, 07:39 AM
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I recently experimented with a 2X barlow + my TEC140, so I imaged at f14. It wasn't so easy, but I think I got a pretty decent result for the effort (and I learned a lot!). Next time I will do better with guiding and will use a more sensitive camera!


http://www.pbase.com/prejto/m83

Peter
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Old 30-08-2013, 08:20 PM
Hans Tucker (Hans)
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Originally Posted by PRejto View Post
I recently experimented with a 2X barlow + my TEC140, so I imaged at f14. It wasn't so easy, but I think I got a pretty decent result for the effort (and I learned a lot!). Next time I will do better with guiding and will use a more sensitive camera!


http://www.pbase.com/prejto/m83

Peter
Like the others I have viewed your results are impressive Peter.

I have got to admit the results being posted in this thread of images taken at longer focal length has got me more eager to give it a go.
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Old 30-08-2013, 08:57 PM
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Quote:
Originally Posted by Hans Tucker View Post
The scope is a 5" f/12 APO refractor. I do have the AP 0.67X Reducer so I can go from f/12 to f/8.
I think a lot will depend upon what you're imaging, and the image scale (arc sec / pixel) you'll be imaging at... e.g. trying to capture a faint nebula in narrowband using a camera with 4 um pixels might be somewhat futile, but imaging bright objects with broadband filters or using a camera with large pixels (or binning) might work out great.

Another way to think about it: at f/8, you'd be at the same focal length as most RCs. In fact, without a 50% secondary obstruction it would have the equivalent light gathering of a 5.7" f/8 RC scope. Considering what some people are producing with a 6" f/9 RC scope, I'm sure you'll have even better results
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Old 30-08-2013, 09:17 PM
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Quote:
Originally Posted by Hans Tucker View Post
Like the others I have viewed your results are impressive Peter.

I have got to admit the results being posted in this thread of images taken at longer focal length has got me more eager to give it a go.
For sure give it a go! You will need to guide with an OAG or ONAG and a sensitive guide camera. And, a really good mount will probably make or break the exercise. I'm sure we are all curious to see your results!!

Peter
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