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Old 21-11-2018, 06:16 PM
Jethro777 (Jethro)
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Is a focal ratio on a 80mm APO refractor of f/6.9 too slow?

I intend to use it for Observation and down the track Astrophotography.
Specs are:

It's a APO 80 Triplet
80mm(3.15")
f550
6.9
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Old 21-11-2018, 06:44 PM
astro744
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For visual observing it is just about perfect. For imaging you will likely want/need to add a focal reducer/field flattener with will typically be 0.8x so your effective focal ratio will then be f5.5, 440mm. Something like http://www.televue.com/engine/TV3b_p...id=71&Tab=_TRF
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Old 21-11-2018, 10:12 PM
Jethro777 (Jethro)
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That is so helpful!
Does a field flattener improve or decrease the quality of an observation?
Should the changed focal ratio be considered then when selecting/buying eyepieces?
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Old 21-11-2018, 10:34 PM
Wavytone
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The flattener is for photography - not particularly useful for visual on such a small scope.

The point is that the focal plane of refractors is curved, like a sphere but the sensor in a camera is flat so it can’t get it all in focus simultaneously. The flattener corrects this curvature.

For visual however the eye will tolerate this and most eyepieces aren’t designed with a flat field in mind anyway so a flattener doesn’t have much point for visual users.
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Old 22-11-2018, 04:14 AM
astro744
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Do you already have this elescope? What are your main observing interests? What are your main photographic interests? What mount do you gave or intend on getting?

You will find there is no one best telescope and for each different focal length/ratio there is no one best set of eyepieces although if you just buy quality one at a time even you can use good eyepieces with any telescope. It will come down to what eye relief and apparent field you prefer and you won't know this until you look through a few different eyepieces.

An 80mm APO us a great grab and go for quick look at planets and the brighter DSOs but for most DSOs you will want greater aperture. My first 'scope 33 years ago was a 6" f5.5 on an EQ mount which I still have and it has provided me with lots of great planetary and DSO views. If I was starting again now I would either get a 6" f8 DOB or more likely an 8" f6 Dob for brighter views of galaxies. I have other telescopes and often observe with my TV-60 for quick views of planets in the evening dusk. Mostly though I use my 10.1" f6.4 Dob which was one I built but had my mirror professionally refigured.

I just cannot seem to get interested in astrophotography although I did dabble with it on film with my 6" f5.5 many years ago. You need a very heavy duty mount with good tracking for astrophotography but can get satisfactory results with basic equipment. I'll let others chime in on the astrophotography path.

Whatever you choose, enjoy!
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Old 22-11-2018, 08:12 AM
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ChrisV (Chris)
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+1 on the advice below, especially adding a flattener/reducer at some stage. An 80mm Apo is a great start for AP. Which one is it? Always worth trying it as it is.
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Old 22-11-2018, 08:27 AM
Jethro777 (Jethro)
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I am on a severe budget, but I know I want APO
I intend to get this one. I have not bought it yet, but want it for observing first, then astro.

http://www.ksonoptics.com:8081/Product/PrdDtls/164/
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Old 22-11-2018, 08:45 AM
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pluto (Hugh)
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Not sure if you saw this just pop up for sale but seems similar to what you're after:
http://www.iceinspace.com.au/forum/s...d.php?t=171623

Looks like a really nice scope, if I hadn't just emptied the coffers moving house I'd buy it myself!
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Old 23-11-2018, 11:01 PM
Wavytone
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Agreed for an 80mm that’s as good as it gets unless you’re preparedness to pay a great deal more for something exotic.
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Old 24-11-2018, 01:10 AM
glend (Glen)
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Quote:
Originally Posted by pluto View Post
Not sure if you saw this just pop up for sale but seems similar to what you're after:
http://www.iceinspace.com.au/forum/s...d.php?t=171623

Looks like a really nice scope, if I hadn't just emptied the coffers moving house I'd buy it myself!
Yes it is a nice scope, that was mine and it has been sold now. The OP needs to work out how important certain things are for his purposes. The 80mm size is a good place to start astrophotography as it gives you good field width (reduced) for most of the larger southern hemisphere nebula. It is a size I use a fair bit for that reason.
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Old 24-11-2018, 03:56 PM
Jethro777 (Jethro)
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What I would like to do is use it for visual observation first, and then later, when I have the mount for it, to go AP.

I'm determined to save a little more, and then my options will grow further. It's a good time to ask questions.
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