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

[Moonshane]

LOL. I should add my comments relate to visual use as you need to be even more critical with imaging of course but that I know nothing about.

[Moonshane]

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

very true but given that the only usual adjustment is the primary, it's really not hard to  ensure it's correctly aligned. 

[foundaplanet]

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.

Based on the center of a circle always being the center of a circle..

[Naemeth]

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

It will make a tiny bit of difference, your giro should easily be able to handle an 8" F/5. Alternatively a dobsonian mount would be convenient, although the eyepiece would not be at standing height.

[emadmoussa]

It will make a tiny bit of difference, your giro should easily be able to handle an 8" F/5. Alternatively a dobsonian mount would be convenient, although the eyepiece would not be at standing height.

I'm too lazy to stand all night...

[Naemeth]

I'm too lazy to stand all night...

We observe differently, clearly. I hate sitting down.

[emadmoussa]

I think it's probably governed by the type of instruments you use. I mostly use refractors, so sitting down is the most comfortable considering the eyepiece can get low when pointing at the zenith. Having said that, I remember with the 200P on NEQ6 I often observed standing up.

[Naemeth]

I think it's probably governed by the type of instruments you use. I mostly use refractors, so sitting down is the most comfortable considering the eyepiece can get low when pointing at the zenith. Having said that, I remember with the 200P on NEQ6 I often observed standing up.

Perhaps, but it depends on how long the refractor is (and how high the tripod goes!) - my 66mm F/6 is at perfect height for standing, but the ED80 on the AZ4 isn't. You could always put the newt on a box if you wanted to stand - I should think 8" F/5 is comfortable for sitting too .

[emadmoussa]

Not to mention it's warmer to sit down...you can just huddle up!!

[Naemeth]

Not to mention it's warmer to sit down...you can just huddle up!!

You can jog on the spot standing up .

[emadmoussa]

You can jog on the spot standing up .

LOL, so much of keeping the vibrations at minimum

[Naemeth]

LOL, so much of keeping the vibrations at minimum

Away from the scope of course...

Have you decided on an F/4 or F/5 then?

[emadmoussa]

The last thing I'm willing to keep doing is collimation, so probably an f/5 should do the job better for me. However, this is only for the future a couple of month from now perhaps - I need to see whether or not I'm moving and this will be a major deciding factor which might lead to a 12" dob if I am.

[laser_jock99]

I did a rough calculation (someone correct me if this wrong!) but the difference between F4 & F5 is 40 minutes.

i.e. 1 hours worth of exposure at F4 requires 1 hour 40 mins at F5 to get the same data (or indeed 30 mins at F2.8). Worth considering for the imagers....

[AlexB67]

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

I do recall looking at so numbers for this, how precise you need to be, so for a big Dob for example you need to be every so accurate on the tilt of the secondary to get the most out of it, fractions of a degree can make the difference.  That demonstrates to me that good collimation tools and techniques are a must for F/4 ( as a rough guide )  or faster  BIG dobs .  Often the f ratio dependence is discussed ( other powers for astigmatism and other aberrations ), but as you say the increased aperture means the collimation tolerance/limit changes rapidly also, this is perhaps not often stated, but worth a mention. 

[Naemeth]

I did a rough calculation (someone correct me if this wrong!) but the difference between F4 & F5 is 40 minutes.

i.e. 1 hours worth of exposure at F4 requires 1 hour 40 mins at F5 to get the same data (or indeed 30 mins at F2.8). Worth considering for the imagers....

Indeed, it makes a huge difference for imaging - but not for visual.

The way I see it is that faster focal ratios are chosen in visual to shorten the size of the OTA, especially important if you're considering 16"+, but at 8" it doesn't really make much difference .

[John]

On the size of the collimation "sweet spot", ie: deviation from the optical axis, to allow your scope to perform reasonably well, ie: to a diffraction limited level, here are the "sweet spot" diameters:

F/4 = 1.4mm

F/4.5 = 2.0mm

F/5 = 2.8mm

F/6= 4.8mm

F/8= 11mm

F/10= 22mm

No wonder Shanes F/11 6" newt does so consistently well - he has over 2cm to play with

I'm sure his faster scopes are spot on as well though 

Makes you wonder how accurately the centre spots on primary mirrors are placed doesn't it ?. They don't have to be far off to cause havoc with faster scopes.

[AlexB67]

On the size of the collimation "sweet spot" (ie: deviation from the optical axis), to allow your scope to perform reasonably well, ie: to a diffraction limited level, here are the "sweet spot" diameters:

F/4 = 1.4mm

F/4.5 = 2.0mm

F/5 = 2.8mm

F/6= 4.8mm

F/8= 11mm

F/10= 22mm

No wonder Shanes F/11 6" newt does so consistently well - he has over 2cm to play with

I'm sure his faster scopes are spot on as well though 

Makes you wonder how accurately the centre spots on primary mirrors are placed doesn't it ?. They don't have to be far off to cause havoc with faster scopes.

There was a rather interesting article written on this topic , it has a table in it and some graphs, it elaborates further on the implications on all of this

http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/forum/AA_error_article.pdf

Something I wanted to build into my calculator  and it can compute these things ( and therefore came across it in searching for the info )

Also worth  noting that there is an angular version as well as the linear version of collimation tolerance. Not a lot you can afford to go wrong in a big fast Dob if you want to optimise collimation. Note  in that article, the dependence on aperture and how that affects things also

[Singlin]

I did a rough calculation (someone correct me if this wrong!) but the difference between F4 & F5 is 40 minutes.

i.e. 1 hours worth of exposure at F4 requires 1 hour 40 mins at F5 to get the same data (or indeed 30 mins at F2.8). Worth considering for the imagers....

Is their a table with the differences in minutes from f8 to f4?

[Singlin]

I am losing my English (30 years here in Italy)

What I am looking for is a table with the differences in minutes of all the f numbers from 8 to 4.

[Luke]

You compare the square of the F ratios - F8 is four times slower than F4: (8*8) / (4*4)

So my quick maths of the equivalent of 1 min at F4:

F4: 1 min

F5: 1 min 34s

F6: 2 mins 15s

F7: 3 mins 4s

F8: 4 mins

I am losing my English (30 years here in Italy)

What I am looking for is a table with the differences in minutes of all the f numbers from 8 to 4.

[Singlin]

So therefore Lazer jocks calcultion is wrong?

[Rustysplit]

Emad, I had an 10" Quattro for imaging before I was saved and found the joys of visual again. It does, or at least mine did, have great optics and was capable of good visual use up to a point. Unless you are willing to pay for premium ep's and invest in a decent coma corrector, the coma visible really spoils the image. As an example, I bought a Meade 5000 UWA 14mm to try in it. Whilst the centre of the fov was superb, this very quickly degraded into soft mush for the outer 1/3rd. At the time this was a relatively expensive ep. I ended up getting a Televue Radian to improve matters.

For visual, for me, my previous 8" 200p was far superior. After prolonged use the coma at f4 may really start to bug you. As others have said, f4 for imaging or large aperture dobs where height is a concern, at 8" and visual the cons far out weigh any benefit imho.

[laser_jock99]

So therefore Lazer jocks calcultion is wrong?

LOL- that will teach me for trying to work out 25/16 quickly in my head! Six minutes out......