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Old 11-04-2021, 07:34 AM
Ribodab
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Long focal ratios may not be so bad?

When doing visual astronomy, lower power means brighter image, wider field, but less detail in image.

Magnification somewhat seems to bring out more detail and noticed it in the likes of the Orion Nebula.

I know in astrophotography, its always shorter focal length the better approach, because of being faster photography easier tracking, wider field, but the tradeoff maybe less detail and resolving power?

Would a longer focal length scope even though its slow photographically and harder to get good tracking allow more image detail?
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Old 11-04-2021, 09:38 AM
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gregbradley
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Quote:
Originally Posted by Ribodab View Post
When doing visual astronomy, lower power means brighter image, wider field, but less detail in image.

Magnification somewhat seems to bring out more detail and noticed it in the likes of the Orion Nebula.

I know in astrophotography, its always shorter focal length the better approach, because of being faster photography easier tracking, wider field, but the tradeoff maybe less detail and resolving power?

Would a longer focal length scope even though its slow photographically and harder to get good tracking allow more image detail?
The view with longer focal length would have a darker background which would help.

Longer focal length scopes are more affected by the seeing conditions than widefield.

As far as would it show more detail than a faster focal ratio scope? No I don't think so, its more determined by aperture and seeing. Also pixel size and sensitivity of the sensor being used.

The greatest detail I have achieved was with a CDK17 F6.8 (so not really a slow F ratio) with good seeing. It outperformed a 12 inch F3.8 scope but not massively but noticeably.

Planetary imagers use very long focal lengths and lucky imaging where thousands of video type files are captured and then sorted by sharpness due to seeing fluctuations.

Greg.
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Old 11-04-2021, 09:43 AM
glend (Glen)
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This is potentially a big subject, but keeping it simple: You do get greater magnification at long focal length, bearing in mind that with imaging there is no eye piece to boost it, you are stuck with the scopes native focal length. Longer focal length also means narrower field of view, and the trend for some years has certainly been for wide field, fast, imaging ( where time starved imagers can at least produce something). It is common for reducer corrector to be used with long focal length, which gives you some improvement in speed but usually these devices are limited to f ratio reductions of (0.6x at best, many at around 0.8x). Long focal length imaging usually means much more data acquisition time, which also means good tracking, guiding, - basically very experienced people who know their stuff. My advice (on the assumption you are just starting out imaging) would be to start out with fast short focal length wide field imaging of larger target or field objects. Take your time to learn the craft, skills, processing, etc before diving into longer focal length.
Of course your equipment needs to match your targets, and importantly the camera pixel size/image scale in relation to your scope, etc is crucial. There are plenty of online tutorials to read. This is a simple answer to your question.
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Old 11-04-2021, 12:22 PM
Ribodab
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For me its about EAA not astrophotography so much easier and forgiving at any focal length as your basically live viewing and stretching, histogram etc on the fly and not stacking on pc. While no match for astrophotography its quicker and easier and less time consuming.

If I want a snapshot just hit the save png as single image from the stacked image its built up so far.

Better than visual but not as good as astrophotography but inbetween both.

Its possible to do good EAA even on alt alz mount, but of course an EQ mount does refine it somewhat.

Remember years ago astrophotography was done on long refractors, or SCT F10 with great results, guess with less patience today, using short focus has become the trend, but image scale is smaller, there are advantages and disadvantages to both, at f2 your image scale is small, and rvtn though will be extremely fast, less image scale, so less fine details.

Way back then they had no reducers, or using fast scopes like we do now. Using a long focal ratio is a skill many wont have.

Same as back then hand guiding or manual guiding. I dought any astrophotography today would have the skill.

We are all spoiled by computers doing most of the work and automating things.
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Old 11-04-2021, 04:00 PM
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multiweb (Marc)
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As long as you have the necessary aperture increasing FL will benefit your image scale. To a point.
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