Practical differences between f5 and f6 scopes

[JulianFR]

Hi... I'm afraid I have another very basic question. Can you explain to me what practical differences I could expect between an f5 and an f6 Newtonian/Dobsoniam telescope. I'm only thinking about visual observing, not imaging.

For instance, does it affect eye piece choice (most of mine are BST/Starguiders); is pollination accuracy more or less important; anything else?

As always, many thanks. Julian

[rowan46]

f5 scopes are not as long and its about this focal ratio where they start to get fussy about eyepieces. Here is where the edges break down considerably on poorer wide eyepieces. your bsts will be fine on an f5 . yes they will be softer at the edges and coma will be more evident but they will not suddenly become bad eyepieces its just that some eyepieces will be better ( they are also more expensive) and yes f5 is where it starts to get fussier about collimation, but its no harder to collimate an f5 than an f6 scope

[rwilkey]

Hi Julian, for eyepiece choice, the f/6 is a better and kinder focal length, though the BST StarGuiders will work well with both.  I also think f/6 is better on visual observing as well.

[ChrisLX200]

In theory the secondary could be a bit smaller in the f/6 scope than the f/5 which will be less of an obstruction to the lightpath, but the difference won't be great here. An ideal planetary scope is an f/8 Newt which offers lovely high contrast  views. Collimation is finer to adjust, smaller tweaks needed to get it exactly right.

ChrisH

[JulianFR]

Thanks all. Does that mean an f5 scope is inherently worse, or at least not as good as, a slower (?) scope?

Eye piece-wise is there a particular type that is best, or is it cost=quality?

[ChrisLX200]

No, these short focal length scopes excel at wide-field / bright FOV for imaging. That is the main reason for buying one.

ChrisH

[JulianFR]

Excellent. I appreciate everyone's comments. Julian

[AlexB67]

Julian, I just put something together, a list of resources, you may find it useful as it touches on that topic.

http://stargazerslounge.com/topic/207739-coma-astigmatism-and-newts-some-useful-notes-perhaps/?p=2208912

[JulianFR]

Thanks Alex... I'm going to study the links. Looks like a coma corrector might be in order.

[bomberbaz]

http://www.firstlightoptics.com/coma-correctors/baader-mpcc.html

I have the same scope as you Julian and although I mainly use very good ehepieces with it, I sometimes use sw stock ones. Imo the difference is noticeable but not huge.

I did use a coma corrector before I bought the delos and xw, it was the baader one in the link.

paracorrs are highly regarded but very expensive.

erm guess I am saying you may not need one but if you do, my money would be on the baader.

steve

[YKSE]

@JulianFR From your sig, I don't think you need a para corr. Coma will be most prominent in low mag or wide field EPs, and your 24mm MV should not show much coma in a f5. When I had the f5 explorer 130p, there no coma in TS 32mm plössl, nor in the TS HR 58deg eps (15mm, 9mm and 5mm). Para corr will not correct astigamatism which lies in the EPs.

[JulianFR]

Thank you Yong and Steve... My but there's a lot to understand in this great hobby/discipline, isn't there!

So, it looks like f5s can be demanding of EPs and collimation and are susceptible to coma... But they are good for deep space viewing (which is what I was hoping) and the EPs I have should be ok (ie the BSTs and the Maxvision. I don't need a coma corrector, but if I do get one it might help... And I should go for the Baader.

Hope my summary is accurate. This has been really helpful, thanks.

[YKSE]

Here's link you may find it helpful in identifying different kind of abberations

http://www.umich.edu/~lowbrows/reflections/2007/dscobel.27.html

[itmo]

Differences between f5 and f6 of similar (reflector) aperture are:

-f6 is easier to collimate

-f5 has wider field of view

-f6 has a longer focal length , hence bigger magnification with the same eyepieces 

-they both have the same maximum magnification

-f6 is physically longer and so it is usually more bulky

-f6 mirror is easier to make and hence might be of better quality

-f6 has smaller aberrations and is easier for older eyepiece designs. 

So a 12"/300mm F5 would be 1.5 meters long (60") , F6 would be 72"/1800mm