What is your favourite f ratio ?

[Bivanus]

First scope I've bought was a Bresser Classic 70/350 so an f/5 Also an achromat and not among the best of them , I named it Violet, guess why 😄

The second scope hit me fast and hard due to a TS sale , slightly used guaranteed as new, 102 f/7 apo doublet. Still wondering how that happen, so does my wife...

After breaking my nerves with the 102 on a Twilight I and my back with it on the EQ6-R  I've also got an SW 72ED that is f/5.8  that is a tad lighter on both back and nerves. 

All of them are used for both visual and EAA (still working towards AP...) with the Bresser used as finder/guiding , the TS mostly visual and the SW mostly EAA, but still I like the 102 f/7 better , my medium term goal is to learn how to use it at it's full potential with the EQ6-R and NINA.

What you've got , how are you using it and what is your favourite? ( and how do you deal with related nerves/back issues 🙂) 

[JeremyS]

Someone on here seems to favour f/15, @F15Rules 🤔

[Louis D]

For most Newtonians, I favor f/5 to f/6 to keep the tube more compact without growing the secondary too much by using a low profile focuser.

Edited August 8, 2023 by Louis D

[DirkSteele]

Takahashi FS-60 f/5.9

APM LZOS 105/650 f/6.2

APM LZOS 115/805 f/7

APM LZOS 180/1260 f/7

Takahashi FC-76 f/7.5

APM LZOS 130/1200 f/9.2

Takahashi FS-60Q f/10

Celestron C11 f/10

Takahashi FC-76Q f/12.6

Picking a favourite is hard but in refactor land, the longer focal lengths tend to perform better but the trade off is they get long and harder to mount. Something in and around the middle would shake out as my favourite.

[Piero]

For me, it depends on the telescope and its focal length.

- 8" dob, I like f6

- 12" dob, I like f5-6

- 16" dob, I like around f4.

These f-ratios allow one to observe without a ladder.

Regarding refractors, I like sub f7.5 EDs, but these can be pricey.

Edited August 8, 2023 by Piero

[Bivanus]

@Piero: Touche on the ladder 🪜 It's funny that usually I mention the collimation issues as why I shy away from Newts / Dobs totally forgetting the size & not always friendly EP positioning 😁 Please don't get me wrong, have nothing against the reflectors and only respect for those patient enough to use them 🤗

[Louis D]

1 hour ago, Bivanus said:

@Piero: Touche on the ladder 🪜 It's funny that usually I mention the collimation issues as why I shy away from Newts / Dobs totally forgetting the size & not always friendly EP positioning 😁 Please don't get me wrong, have nothing against the reflectors and only respect for those patient enough to use them 🤗

Think how unfriendly it would be to mount a 16" APO refractor if you could get one made for you at any price.  Imagine hauling that out to a dark sky site and setting it up by yourself.  I'll take dealing with an f/4 or faster 16" or larger Dob's viewing position any day over that.

[vlaiv]

Surely F/8 (I don't own a single scope with this F/ratio).

It allows you to get max exit pupil (7mm with 56mm 2" plossl eyepiece), while still being easiest on the eyepieces (slower the scope - better image in the eyepiece except for the most expensive eyepieces).

[Elp]

2

[Bivanus]

@Louis D: I am thinking at least once a week...and drool...20" of greatness....

https://www.professional-telescopes.com/Product-Line/Apochromates/LZOS-51cm-Giant-Apo

Something like this but as an 8 wheeler

https://www.eoportal.org/other-space-activities/mobile-observatory#mobile-observatory-nissans-navara-dark-sky-concept-vehicle

Edited August 8, 2023 by Bivanus
Added link

[Elp]

You could do a Clement, and build your own 70"er.

[inFINNity Deck]

All suggested scopes are way too fast, my favourite scope is f/50 (or rather f/49.67) at 1490mm focal length (30mm aperture): https://www.dehilster.info/astronomy/gtt60.php

I use it on a daily basis for counting sunspots, the results of which are used for scientific research: https://www.aavso.org/solar-observing-project

Nicolàs

[Bivanus]

@Elp: I will not lie , before getting into it , I was reading a lot about all kind of scopes , and one of my favorites stories was of this 42" Dob from Germany built by Erhard Hänssgen https://www.cruxis.com/scope/scope1070.htm  i mean , look at that tinny garage ...or at least tinny compared to the star cannon in front of it !

Edited August 9, 2023 by Bivanus
bad grammar

[Elp]

44 minutes ago, Bivanus said:

@Elp: I will not lie , before getting into it , I was reading a lot about all kind of scopes , and one of my favorites stories was of this 42" Dob from Germany built by Erhard Hänssgen https://www.cruxis.com/scope/scope1070.htm  i mean , look at that tinny garage ...or at least tinny compared to the star cannon in front of it !

Is he going to attempt fitting it into the car? Or tow? 350Kg does not conform to the definition of portable. It's impressive anyway.

Edited August 9, 2023 by Elp

[dan_adi]

16 hours ago, Bivanus said:

@Elp: I will not lie , before getting into it , I was reading a lot about all kind of scopes , and one of my favorites stories was of this 42" Dob from Germany built by Erhard Hänssgen https://www.cruxis.com/scope/scope1070.htm  i mean , look at that tinny garage ...or at least tinny compared to the star cannon in front of it !

Great scope! Also great to see a fellow amateur astronomer from the same region

[bosun21]

17 hours ago, Bivanus said:

@Elp: I will not lie , before getting into it , I was reading a lot about all kind of scopes , and one of my favorites stories was of this 42" Dob from Germany built by Erhard Hänssgen https://www.cruxis.com/scope/scope1070.htm  i mean , look at that tinny garage ...or at least tinny compared to the star cannon in front of it !

Where's the mirror? I imagine that the mirror will be no lightweight either.

[Louis D]

42 minutes ago, bosun21 said:

Where's the mirror? I imagine that the mirror will be no lightweight either.

It's probably under that tan piece of wood in the bottom of the mirror box.  Most big Dobs designs have a board to protect the mirror from falling objects.  I know my Tectron 15" does.

[IB20]

Currently have an f12.75, 2 x f10s, f7 and an f5.9. Have to say I like the super long and slow fracs, just cope much better with my local atmospheric conditions and garden configuration. 

[lunator]

My 3 scopes are F5, F6 and F9

They all have their place but for being the best all rounder I have found anything better than my F6 newt.

Cheers

Ian

[F15Rules]

Q. What is your favourite f ratio ?

A. I'm not telling. Just don't look at my SGL handle..🙃

Dave

Edited August 10, 2023 by F15Rules

[Elp]

2 minutes ago, F15Rules said:

Q. What is your favourite f ratio ?

A. I'm not telling. Just don't look at my SGL handle..🙃

Dave

Thought you really really liked the fighter jet

[Gonariu]

On 8/8/2023 at 4:31 PM, Bivanus said:

•  

@Louis D: I am thinking at least once a week...and drool...20" of greatness....

https://www.professional-telescopes.com/Product-Line/Apochromates/LZOS-51cm-Giant-Apo

 

Guuuulllpppp, I want it!!!!!! 😍

Edited August 10, 2023 by Gonariu

[Swoop1]

I have-

f5 SW 150P Newtonian

f10 Celestron C6 SCT

f6 RVO Horizon 72ED refractor.

The 72ED is easiest to cary, transport and mount but, my favourite is the one that works best on the night.

By that I mean the one that I decide to use when I'm doing whatever takes my fancy. For deep sky, that is currently the 72ED. For planetary/ lunar, that will be the SCT, for outreach, the Newt.

[AstralFields]

I love my f/6 of my 8" DOB. Seems like the perfect focal ratio without physically becoming too big.

I tested a 10" DOB with an f/4.7 and that was really not a great experience (it could use a coma corrector). Too bad they don't make the f/6 10" DOBs that much, otherwise I would totally think about it.

With this being said, I think f/5 will be doable when I get a 12".. will probably need the CC though.

[John]

Currently all my scopes are refractors ranging from F/6 to F/9.2. 

As long as the optics are close to apochromatic I guess F/8 would be pretty optimal (I am a visual astronomer only) but I don't actually currently own a refractor of that exact focal ratio.

When I've had newtonians I have found F/5.3 - F/6 a good focal ratio range for visual use.