6" f/5 vs 6" f/8 Newt - Planetary Shoot out.

[Stu]

Shane, I can't remember if I told you but I bought the scope you now have from Ade Ashford. It's not the one referred to in this review but I think he did review it elsewhere.

[Chris]

"The smaller central obstruction (25mm vs. 36mm) and Hilux coatings of the OD150L gave it the edge for planetary work, but the Explorer-150PL held up against its more expensive competitor staggeringly well. "

I'm happy with that!  

[Astrobits]

Here for interest are two pages of spot diagrams for some common eyepieces for F/10 and F/5 telescopes taken from the book:

Telescope Optics  by Harrie Rutten and Martin van Venrooij

The one labelled Abbe is the orthoscopic design that he ( Ernst Abbe ) developed. You can see the deterioration of all the eyepieces when going from an F/10 to an F/5 telescope.

Nigel

 

[Chris]

26 minutes ago, Astrobits said:

Here for interest are two pages of spot diagrams for some common eyepieces for F/10 and F/5 telescopes taken from the book:

Telescope Optics  by Harrie Rutten and Martin van Venrooij

The one labelled Abbe is the orthoscopic design that he ( Ernst Abbe ) developed. You can see the deterioration of all the eyepieces when going from an F/10 to an F/5 telescope.

Nigel

Wow! at f/5 the Plossl looks better than the Ortho on axis! I must admit I didn't realise it would make such a difference purely on axis between f/5 an f/10, the on axis spot diagrams are very much tighter at f/10. 

So, can we conclude that slow optics with cheapish eyepieces give better results than fast optics with expensive eyepieces?

I'm liking slower scopes even more, cheers Nigel    

[John]

52 minutes ago, Lockie said:

So, can we conclude that slow optics with cheapish eyepieces give better results than fast optics with expensive eyepieces?

 

Not really. To produce an eyepiece that will work well right across the field of view with a fast scope costs more but it certainly can be done. It is fair to say that slower scopes place less demands on low cost eyepieces.

I've found that good abbe orthos work as well in my F/5.3 dobsonian as good plossls, possibly even slightly better in the central field area.

 

[Chris]

Just now, John said:

It is fair to say that slower scopes place less demands on low cost eyepieces.

 

Yes of course   I'm just a bit surprised by the spot diagrams illustrated above. It shows a Plossl at f/10 soundly beating an Abbe Ortho at f/5 on axis, and them there Abbe Ortho's can be relatively expensive. Then again it also shows when perfectly on axis the Plossl at f/5 beats the Ortho at f/5! so I'm not sure what's going on anymore? 

[Ruud]

Heads up, beginners: a 6 inch f/8 Dob is about the best telescope you can get!

[Chris]

Just now, Ruud said:

Heads up, beginners: a 6 inch f/8 Dob is about the best telescope you can get!

For visual on a budget I absolutely agree! a 6" high grade optic plus mount for £200, it's crazy good. I know these are often thought of as entry level scopes, but the truth is they could provide a lifetime of planetary and lunar enjoyment, and aren't half bad with the brighter DSO's also  

[YKSE]

20 minutes ago, Lockie said:

Yes of course   I'm just a bit surprised by the spot diagrams illustrated above. It shows a Plossl at f/10 soundly beating an Abbe Ortho at f/5 on axis, and them there Abbe Ortho's can be relatively expensive. Then again it also shows when perfectly on axis the Plossl at f/5 beats the Ortho at f/5! so I'm not sure what's going on anymore? 

The confusion arises when there's difference between the plossl and plossls

The plossl showed in the ray trace is the original plossl, i.e. with unsymmetrical doublet, where plossls in nowaday's market, are all symmetrical doublets (TV plossls included), the diagram in this page show how sysmmetrical doublets (achromat pair, i.e.) compare with true plossl or abbe orthos:

http://www.telescope-optics.net/eyepiece_aberration_2.htm

TV plossls should have better edge correction than anything showed in the diagram though, because of Al's patent work.

[Chris]

6 minutes ago, YKSE said:

The confusion arises when there's difference between the plossl and plossls

The plossl showed in the ray trace is the original plossl, i.e. with unsymmetrical doublet, where plossls in nowaday's market, are all symmetrical doublets (TV plossls included), the diagram in this page show how sysmmetrical doublets (achromat pair, i.e.) compare with true plossl or abbe orthos:

http://www.telescope-optics.net/eyepiece_aberration_2.htm

TV plossls should have better edge correction than anything showed in the diagram though, because of Al's patent work.

Few! thanks! That's more inline with what I previously thought/assumed to be the case  

p.s. the spot diagram for the Monocentric looks very interesting! 

[John]

I had a TMB Supermonocentric 5mm for a while. probably the best high power eyepiece I've ever owned / used.

The downsides were just 30 degrees of apparent field of view, very small eye lens and rather short eye relief. One for the purist I reckon

They cost around £350-£400 if you can find one now

Having tried dozens of different eyepiece brands and designs over the years I've found that the vast majority of them work very well on axis even in fast scopes. It's as the field moves away from the axis of the optical system that the abberrations make themselves known and this happens more with faster scopes.

 

 

[Alan White]

Lockie I was confused for a minute with your name change.

Thanks for the comparison, very interesting indeed.

I had a 150p for a while and rather liked it, but swapped out for a refractor only.

As you sit in both camps, how does the 150pl work as your grab and go for you?

 

[JG777]

I would be interested to also hear about any differences in your testing of wide field and DSO objects as well as I guess the longer focal length will always perform better on planets and this stands for other types of telescope as well. I liked the idea of an F8 tube but planets being fairly low down on my list of favourites with globs and wide field at the top and the fact that a shorter tube is so much easier for grab n go it made the F5 a more compelling choice.

[michael.h.f.wilkinson]

Very interesting discussion. I used to have a 6" F/8 Newtonian with very good mirrors. Killer planetary scope, and no slouch on DSOs either (except wide-field). They are sometimes referred to as "APO-killers"

 

[Stu]

2 hours ago, michael.h.f.wilkinson said:

Very interesting discussion. I used to have a 6" F/8 Newtonian with very good mirrors. Killer planetary scope, and no slouch on DSOs either (except wide-field). They are sometimes referred to as "APO-killers"

 

Always fancied an 8" f8 OO scope, pretty impractical but on an EQ platform could be a cracker on planets 

[Peter Drew]

1 hour ago, Stu said:

Always fancied an 8" f8 OO scope, pretty impractical but on an EQ platform could be a cracker on planets 

I have a self built scope containing 8" F8 1/20 wave optics by OO, the planetary views are indeed exceptional. Needed a Fullerscopes MkIV mount to carry it!. 

[Chris]

1 hour ago, Alan White said:

Locked, I was confused for a minute with your name change.

Thanks for the comparison, very interesting indeed.

I had a 150p for a while and rather liked it, but swapped out for a refractor only.

As you sit in both camps, how does the 150pl work as your grab and go for you?

 

Hi Alan, the 150p Skyliner Dob works very well as a grab n go, I just pick the entire thing up, plonk it down, remove dust cover, done   

[YKSE]

21 hours ago, Stu said:

Interesting John, how is that calculated?

It looks like the sweet spot means coma free zone, there're a number of slightly different ways defining it, one is

0.022mm x f-ratio^3

It gives the coma free zone diameter 2.8mm for f5 and 11.3mm for f8.

[Chris]

41 minutes ago, JG777 said:

I would be interested to also hear about any differences in your testing of wide field and DSO objects as well as I guess the longer focal length will always perform better on planets and this stands for other types of telescope as well. I liked the idea of an F8 tube but planets being fairly low down on my list of favourites with globs and wide field at the top and the fact that a shorter tube is so much easier for grab n go it made the F5 a more compelling choice.

Hi, I did try and detect M81/82 at the time with both scopes, but the sky was just too bright. I'm pretty certain it would be excellent for globs and PN's at least. I don't have the best skies for DSO's hence I tend to lean towards planetary and Lunar unless a camera is involved  

[Chris]

Just now, Peter Drew said:

I have a self built scope containing 8" F8 1/20 wave optics by OO, the planetary views are indeed exceptional. Needed a Fullerscopes MkIV mount to carry it!. 

  Wow, you sure do have some incredible scopes, Peter, do you have favorite/ or favorites? 

[Peter Drew]

56 minutes ago, Lockie said:

  Wow, you sure do have some incredible scopes, Peter, do you have favorite/ or favorites? 

I do have a 12" F8.5 Newtonian as well. I do like all my telescopes for various reasons but these days my 6" F10 PST mod is my most used.