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Old 21-03-2018, 08:54 AM
glend (Glen)
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Wide field coverage refractor?

I need some help with field of view coverage. I am looking for a small refractor that can give me good wide field (4 degrees Az x 3 degrees Alt) for a particular faint target i am chasing. I have tried using my 80mm APO reduced to 442mm fl but the field is too large. I am thinking that something around 320mm fl might work ok. Can any of you math wizards point me to a FOV calculator that an idiot like me can use?
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Old 21-03-2018, 09:19 AM
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Merlin66 (Ken)
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Glen,
Visual or with camera??

CCDCalc is great for showing camera FOV with various cameras/ scopes.

http://www.newastro.com/book_new/camera_app.html
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Old 21-03-2018, 09:29 AM
JA
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Hi Glen,

This is one I have used....

https://www.scantips.com/lights/fieldofview.html

1. Input Focal Length
2. Select Sensor Size Option that suits or input exact size if not listed- 1 to 9 with button on left . (Usually 1,2,3 or 4 used)
3. CLICK Compute 1-9
4. Observe FOV results in decimal Degrees on Right Hand Side Top. There are also some field width results possible if one inputs subject distance (I don't use it. Move relevant to terrestrial and could do the same knowing the FOV with some trig)

Regarding your requirement for a 4 x 3 degree FOV, and as per the attached notated ScantipsFOV excerpt (for your case), you could also use a Full Frame camera with (36mm x 24mm SensorAspect) with your existing 442mm Focal Length refractor/reducer and that would give you a 4.7 x 3.1 degree FOV

Or at the sort of focal length you're interested in use a 300mm lens

Best
JA
Attached Thumbnails
Click for full-size image (FOV 442mm Focal Length Calc.jpg)
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Last edited by JA; 21-03-2018 at 10:21 AM. Reason: added ScantipsFOV Excerpt
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Old 21-03-2018, 09:34 AM
StuTodd
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CCDcalc is here at http://astronomy.tools/calculators/ccd_suitability
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Old 21-03-2018, 09:56 AM
glend (Glen)
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It's for imaging, camera is the ASI1600MM-C, 3.8 pixel size; 4/3rd sensor (17.7mm x 13.4mm), resolution 4656 x 3520.

I could try my Nikon D5300 (APS-C) but i need deep cooling and long narrowband subs. The Nikon is not suitable.

Looking at the calculators, 320mm might be ok if i work the diagonal axis into the framing.

Last edited by glend; 21-03-2018 at 10:10 AM.
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Old 21-03-2018, 10:06 AM
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Hmmm
According to CCDCalc you'd need a focal length around 230mm
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Old 21-03-2018, 10:12 AM
glend (Glen)
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Hmm, maybe its time to tackle mosaics.

Last edited by glend; 21-03-2018 at 10:40 AM.
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Old 21-03-2018, 10:42 AM
Wavytone
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Shortest suitable refractors AFAIK are a 50mm f/6.6 and 81mm f/4.4 from TeleskopService.

Shorter than those means a camera lens and for m4/3 there are two:

- Olympus m4/3 300mm f/2.8, at almost $9k.

- Panasonic 200mm f2.8 at $4k.

Last edited by Wavytone; 21-03-2018 at 12:38 PM.
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Old 21-03-2018, 01:08 PM
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OzEclipse (Joe Cali)
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Hi Glen,

I get 253mm using attached Excel spread sheet

Mosaics might be more economical.

Joe
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File Type: zip Field of view calculator.zip (43.8 KB, 7 views)
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Old 21-03-2018, 02:07 PM
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Glen

You need to be a tester for the new Skywatcher Evostar72 by the sound of it.

Chris
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Old 21-03-2018, 02:31 PM
glend (Glen)
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Quote:
Originally Posted by ChrisV View Post
Glen

You need to be a tester for the new Skywatcher Evostar72 by the sound of it.

Chris
Chris, at 420mm fl it is too long and would need reducing. However, the Astrotech ED60 (aka the TS60), both f6 (360mm), FPL-53 and Lanthanum, looks pretty good.

https://www.teleskop-express.de/shop...-RED-Line.html

The Astrotech model is listed cheaper in USD to AUD conversion,

https://www.astronomics.com/astro-te...et_p20522.aspx

The TS 2" Photoline Reducer would fit them both to go down to 284mm fl.

A lower cost solution (and more color corrected) than a pure camera lens i would think.
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Old 21-03-2018, 02:34 PM
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Glen, they both look the same (or vice verse!) as the WO Z61 I bought last year. When I ordered mine from WO direct it arrived 2nd day. TS seems pricey with the current $=€

360mm is still a way from your goal. I’ve been experimenting with my WO 0.8x and have got it focused against the garden hedge...weather has so far prevented me from seeing if it’s focused under the stars.

Even then, I’d expect it’s a little too long still...
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Old 21-03-2018, 04:46 PM
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billdan (Bill)
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The Orion Mini guide scope or a Skywatcher finder scope is 50mm f3.2 or 160mm focal length, a 2X barlow will extend that to 320mm focal length. Or a 1.5x will get you to 240mm focal length.

Last edited by billdan; 21-03-2018 at 04:59 PM.
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Old 21-03-2018, 05:27 PM
Wilso (Darren)
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Borg 55 fluorite 250mm f4.5 need I say more
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Old 21-03-2018, 05:44 PM
glend (Glen)
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Borg 55 fluorite 250mm f4.5 need I say more
For sure the Borg 55 is at the top of the food chain at that focal length but too expensive for me.
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Old 22-03-2018, 12:38 PM
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How about the new Skywatcher EvoGuide 50ED?

It has a focal length of 242mm.

Not sure how it will go for imaging but it looks promising.
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Old 22-03-2018, 12:44 PM
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If you don’t mind going a bit wider you could go for the Rokinon 135mm F/2?
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Old 22-03-2018, 08:31 PM
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I use a MZ135 lens for widefield with my ASI1600, it works very well. I stop it down to f/4 though, to avoid the worst of the edge aberrations.
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Old 23-03-2018, 06:28 AM
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I'd be looking at older, manual focus camera lenses like the Tamron 200mm f/2.8 Adaptall or if you are feeling affluent, the Canon FD 200mm f/1.8 L lens.
I have the Tamron and it has a little CA at the edges of APSC when wide open but you'll be be well inside that with the ASI1600.

Steve
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Old 23-03-2018, 06:54 AM
glend (Glen)
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Quote:
Originally Posted by Icearcher View Post
How about the new Skywatcher EvoGuide 50ED?

It has a focal length of 242mm.

Thanks, Not sure how it will go for imaging but it looks promising.
Yes i have been following the CN thread on this 50ED but so far setting it up for imaging involves adapting it.

Quote:
Originally Posted by Atmos View Post
If you donít mind going a bit wider you could go for the Rokinon 135mm F/2?
Thanks, but I am trying to steer clear of camera lenses, feeling that scopes provide better optical quality.

Quote:
Originally Posted by Amaranthus View Post
I use a MZ135 lens for widefield with my ASI1600, it works very well. I stop it down to f/4 though, to avoid the worst of the edge aberrations.
Thanks but as i said above.

Quote:
Originally Posted by SteveInNZ View Post
I'd be looking at older, manual focus camera lenses like the Tamron 200mm f/2.8 Adaptall or if you are feeling affluent, the Canon FD 200mm f/1.8 L lens.
I have the Tamron and it has a little CA at the edges of APSC when wide open but you'll be be well inside that with the ASI1600.

Steve
Thanks Steve, and all other camera lense respondents, i will investigate scope solutions, including upgrading my setup for mosaic automation through SGP, which i already have. This will preserve resolution.
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