f/5 & f/4 reflectors, what's the difference?

[emadmoussa]

8 inch reflectors, one is f/4 and the other is f/5.Optically speaking, what are the advantages and disadvantges?

[John]

The F/4 will have a shorter focal length (obviously !) and therefore can show a wide field of view.

The collimation "sweet spot" of the F/4 is a lot smaller than an F/5 so accurate collimation and mechanical construation that maintains that is even more critical at F/4.

An F/4 shows quite a lot more coma than an F/5 - a coma corrector would be a "must have" I reckon.

An F/4 will be quite a bit tougher on wide angle eyepieces than an F/5.

An accurate F/4 mirror is that much harder to make than an accurate F/5 one.

Probably more but I can't think of it right now !

[Luke]

The main attractions of an F4 dob for me would be:

- lighter and more compact than the F5

- wider field of view

- if it's a big dob, fewer steps or none at all to climb :-o

It might make the difference for me between having a 10 inch F5 versus having a 12 inch F4, so depending on your needs it may allow you to go up a step in aperture if the F5 equivalent would be too heavy or long.

The F number itself does not bother me for viewing, it's more the impact on focal length/tube length.

[John]

The OP specified an 8" I think ?

At much larger apertures I agree that tube length becomes an issue to be taken into account.

[Luke]

Thanks, John, Sorry Emad, missed the 8 inch bit.

The OP specified an 8" I think ?

At much larger apertures I agree that tube length becomes an issue to be taken into account.

[ollypenrice]

For visual, at 8 inches, I'd go for the F5. Easier all round and not a huge loss in FOV. Smaller secondary so more contrast. Basically just what John said.

Olly

[emadmoussa]

The GSO f/4 reflector is 300 grams lighter and 23cm shorter than the f/5 version. For me this is a score...but like you say, more coma ( not sure if that bothes me much though)...and assuming like SW Quattro will require ultra precise collimation?

[John]

Much more coma and much more astigmatism in eyepieces, unless you spend big bucks on them. These aberrations increase really quickly below F/5.  

[Moonshane]

agree with all the comments above. is the weight reduction of 300 grams important? or the shorter tube? the weight would almost certainly be negated by a coma corrector. in all honesty you can use a f4 scope with no coma corrector -  with good quality eyepieces but the coma is still there even then.

[Naemeth]

The GSO f/4 reflector is 300 grams lighter and 23cm shorter than the f/5 version. For me this is a score...but like you say, more coma ( not sure if that bothes me much though)...and assuming like SW Quattro will require ultra precise collimation?

300g wouldn't make the slightest bit of difference for me. It's only an 8" scope, so it's not really that heavy - certainly much lighter than an NEQ-6! I'd go for the F/5, or even F/6 - save yourself a lot of money by not having to buy a coma corrector / very well corrected EPs.

[swamp thing]

The main attractions of an F4 dob for me would be: - wider field of view-

.

I feel I should add- For any given eyepiece, as the low power minimum is the same for both scopes. They'll just use different eyepiece combo's to get there.

Obviously if the f/4 is using a 50deg plossl at that power and the f/5 a 100deg Ethos then the f/5 will have a wider field of view. It kinda more depends on eyepieces used than f/ratio.

Although as Luke stated the f4 will give a wider FOV with any given same eyepiece used.

[Bodkin]

I feel I should add- For any given eyepiece, as the low power minimum is the same for both scopes. They'll just use different eyepiece combo's to get there.

Obviously if the f/4 is using a 50deg plossl at that power and the f/5 a 100deg Ethos then the f/5 will have a wider field of view. It kinda more depends on eyepieces used than f/ratio.

Although as Luke stated the f4 will give a wider FOV with any given same eyepiece used.

Absolutely right.  Unless you are interested in photography or need a lighter tube assembly there is no need to go to f/4.  The difficulty of manufacturing and then collimating a short focal ratio telescope's optics counts against it   The f/6 used to be the mainstay of most manufacturers reflector ranges for moderately sized telescopes.  My personal view is that their demise has not necessarily helped the average observer - expensive eyepieces and tricky collimation to get the best out of a telescope that is unlikely to be used for photography.

[emadmoussa]

300g wouldn't make the slightest bit of difference for me. It's only an 8" scope, so it's not really that heavy - certainly much lighter than an NEQ-6! I'd go for the F/5, or even F/6 - save yourself a lot of money by not having to buy a coma corrector / very well corrected EPs.

I think less weight and length will make a difference if you intend to mount the telescope on a giro, won't it?

[ollypenrice]

Absolutely right.  Unless you are interested in photography or need a lighter tube assembly there is no need to go to f/4.  The difficulty of manufacturing and then collimating a short focal ratio telescope's optics counts against it   The f/6 used to be the mainstay of most manufacturers reflector ranges for moderately sized telescopes.  My personal view is that their demise has not necessarily helped the average observer - expensive eyepieces and tricky collimation to get the best out of a telescope that is unlikely to be used for photography.

I agree. The desire to cater for the imaging markets has encouraged manufacturers to make over-fast Newts for visual observers. Once you get to the very big Dobs then the available FOV does begin to matter (to me) and so does the size, so I'm all for screamingly fast giant Dobs but what is wrong with F6 in an 8 inch? Nothing!

Olly

[emadmoussa]

I don't think I've ever had any Newtonians faster or slower than f/5 - then again, never had one bigger than 8" - SW 200P was the last one and if it weren't for the awkward eyepiece position (issue with all reflectors on equatorial mounts) I would've given it 5 out of 5.

[Bodkin]

I think less weight and length will make a difference if you intend to mount the telescope on a giro, won't it?

Yes and I would think the length will be particularly important.  I generally see these alt-az mounts as a great way for refractor and SCT users to get the benefits of convenience and stability that are normally the realm of the dobsonian user. Unless you already have a Giro, or want the flexibility to change assemblies, a nice OO dobsonian mount would probably be cheaper, lighter and more stable. 

[emadmoussa]

Yes and I would think the length will be particularly important.  I generally see these alt-az mounts as a great way for refractor and SCT users to get the benefits of convenience and stability that are normally the realm of the dobsonian user. Unless you already have a Giro, or want the flexibility to change assemblies, a nice OO dobsonian mount would probably be cheaper, lighter and more stable. 

I do have a giro, but like you say...Dobsonian bases are more convenient for reflectors. Anyway, I was just testing the water once I was hit by aperture fever again. At the moment I'm happy with simply set-up...On second thought -- and between you and me I'm always hit by the aperture fever, just a bit limited on space at the moment

I see you've got a 20"  Dob...do you use a step ladder with that?

[John]

I used to use a 10" F/4.8 on a Giro Mount. It worked well and the eyepiece was at a much more comfortable height for me than a conventional dob mount. I prefer to stand while observing. Your Giro could handle an 8" F/5 I'd have thought. You can pick those up for not much more than £100 on the used market.

[emadmoussa]

I used to use a 10" F/4.8 on a Giro Mount. It worked well and the eyepiece was at a much more comfortable height for me than a conventional dob mount. I prefer to stand while observing. Your Giro could handle an 8" F/5 I'd have thought. You can pick those up for not much more than £100 on the used market.

Yes, John, I was thinking about that. I need to organize my galaxy list  before going for an 8 inch reflector. Hopefully mid-winter. Otherwise, bigger aperture will require a Dobsonian.

[Macavity]

I think the problem is that, for Newtonian reflectors, the so-called "sweet spot" for collimation goes as the CUBE of the f-number. From memory (I have the "boring" [my oft-quoted] LINK somewhere!) we are talking: 1.4mm for an F4 Newt and 2.8mm for an F5 Newt.

Whatever the precise numbers, the difficulty (challenge!) of collimating an F4 Newt in a reproducible / "settable" way is 5^3 / 4^3 - A factor of TWO "harder". If you look at the (factory standard) construction method of budget F4 Newts, you immediately think - How the H*LL is this thing going to be (to remain) precise to less than 2mm! lol. Not least the spot size of (even good) laser calibrators is ~2mm across!

I think it "more luck than judgement" that the typical F5 Newt (Dob) works (maybe?) "off the peg"! I think, for the average user, tolerance of mis-collimation is more... tolerable than literature suggests! And F4 Newts are not *suddenly* "rubbish", but may require a bit of... coaxing. lol. For my application - VIDEO Astronomy, with smaller chip size, in a 6' wide observatory (5' 6" minus walls) the 8" GSO represents possibilities... To me, part of Astronomy is in "modding" scopes to get a bit better than "off the shelf".

The stuff I do gives but a TASTE of what is achievable (with greater physical / financial / skill resources)! But such is my aim. I leave the *stunning* (wide-field) imaging to others. Classical imagers are not "stuffy", they are dedicated people! On seeing my M27 (LRGB, 1hr total) Video rendition one was heard to exclaim: "Gosh, that's actually rather good!". We are all just "blind lovers", I suspect! And long may that continue...  

[Luke]

Well I know you aren't keen on SCT's, Emad, but I use an 8 inch Celestron Edge SCT as my mini light bucket on a giro, and it works very nicely. The views of the moon at low power have a touch of refractor about them to me in the Edge.

For an 8 inch dob, +1 for the slower scope. It will be easier on the optics and collimation, and the focal length won't be so long anyway at that aperture.

I think less weight and length will make a difference if you intend to mount the telescope on a giro, won't it?

[Luke]

Good points, Steve. I should have mentioned exit pupil. In my case, I would still be fine with my main widest eyepiece, a 21mm Ethos, in an F4 dob, so I would get a wider view than at F5.

But in some cases, others may not be happy using their lowest power eyepiece at F4 compared to F5, due to the higher exit pupil (my 35mm Panoptic - which I don't use in my fast scopes - gives a washed out view when the exit pupil is too high for me in fast dobs - a shame, because it is a fab eyepiece!).

So, because of exit pupil, you won't necessarily get a wider view - it may depend on your EP's and how exit pupil affects you.
 

I feel I should add- For any given eyepiece, as the low power minimum is the same for both scopes. They'll just use different eyepiece combo's to get there.
Obviously if the f/4 is using a 50deg plossl at that power and the f/5 a 100deg Ethos then the f/5 will have a wider field of view. It kinda more depends on eyepieces used than f/ratio.
Although as Luke stated the f4 will give a wider FOV with any given same eyepiece used.

[Moonshane]

I have said this a million times but there's no difference in collimatng an f6 or an f10 or an f4. basically, if you can collimate one newt you can do the others as it's the same process.if you are the sort of person who wants to collimate every now and again then get the slower scope. if you are happy to tweak it a very little each time then get the f4 with the caveat above re eyepiece height.

[emadmoussa]

I have said this a million times but there's no difference in collimatng an f6 or an f10 or an f4. basically, if you can collimate one newt you can do the others as it's the same process.

I actually expressed a similar view few months back, which was like an invitation for  a lot of members to unsheathe their swords on me...LOL

[Bodkin]

I have said this a million times but there's no difference in collimatng an f6 or an f10 or an f4. basically, if you can collimate one newt you can do the others as it's the same process.if you are the sort of person who wants to collimate every now and again then get the slower scope. if you are happy to tweak it a very little each time then get the f4 with the caveat above re eyepiece height.

You are right that the process is the same.  The difference is the relative tolerance of collimation errors.