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

[Chris]

I decided to test my new Celestron CN-6 6" f/5 Newtonian against my 6" f/8 Dob last night, it was first light for the f/5 so I was curious how it would stand up. The target was Jupiter, and well, in short the 6" f/8 Dob basically wiped the floor with the f/5! By this I mean the difference was immediately obvious:

@ 150x with my 8mm BST the f/8 image was sharp with fine detail and shading showing, I could see festoon action off the main belts, the image was engaging.

@ 125x with my 6mm SLV the f/5 wasn't quite as sharp, or at least I couldn't achieve a sharper focus very easily. Only the belts were visible at first, it took a bit more time to try and tease out anything else other than the main belts, and I couldn't see quite as much compared to the f/8 version. The obvious difference was the 125x verses 150x, so a 5mm SLV would have been handy! However, as the image wasn't as sharp as the f/8 at 150x, I'm not sure pumping up the mag on the f/5 version would have helped?

Both scopes were cooled and collimated.

I knew the f/8 would be a bit better as it's easier to get good focus with a longer scope, and they tend to have higher grade optics on average as slow optics are easier to figure. Also, slow scopes seem better in poorer seeing. It was average seeing for these parts last night. I wasn't prepared for how much better the f/8 was in practice...I've got to be honest despite the f/5 being a new purchase, and the temptation to big up the new scope.

Obviously this is based on a single session, although it was a long session. More sessions are needed for a more accurate appraisal, although my gut feeling is that a above will stand hence me putting pen to paper now..or fingers to keyboard I should say.

I star tested both scopes:

The f/8 had a pretty much identical star test both intra and extra focus, concrentric sharp uniform round rings.

The f/5 had wasn't bad, it had concentric round rings both intra and extra focus, but inside focus the outer and inner rings were noticeably brighter. There didn't seem to be much sign of astigmatism, pinching, or turned down edges on either mirrors. 

 I'm not the best at reading star tests, I don't think the f/5 mirror is bad at all, just that the f/8 mirror is very very good! 

The f/5 was setup on a motor driven EQ3 and this was great for keeping track with Jupiter and the eyepiece height and position was very comfy, whilst the f/8 is a Dobsonian so I needed to bend down and do nudge the scope to track. I much preferred the EQ3 setup, 

I've been so very impressed with this 6" f/8 scope since I got it at Christmas, I'm very tempted to get a 150PL so I can have the optics of the longer scope on a tracked mount.

Sorry for grammer spelling etc, I seized just a spare 5 minutes to put this together  

[nightfisher]

The longer focal length is going to be a winner in this situation, if nothing else you will be using a longer focal length ep to achieve magnification compared to the F5, you could always get a dovetail and tube rings to mount the dob tube on an EQ mount

[Alfian]

Thanks for an  interesting report. Doing the numbers, the 6" Dob makes such a lot of sense, its no great surprise it whipped the 6" f5.

[Stu]

Interesting report Chris. I too am surprised there was that much difference, but have to say I was impressed by the only 150P I have ever looked through, very good Jovian views.

[John]

Very interesting comparison

Apart from optical quality, the collimation "sweet spot" is much larger (and more forgiving) with the F/8 - 11mm vs just 2.8mm around the optical axis of the primary.

Does the F/8 have a smaller secondary as well ?

[Stu]

3 minutes ago, John said:

Apart from optical quality, the collimation "sweet spot" is much larger (and more forgiving) with the F/8 - 11mm vs just 2.8mm around the optical axis of the primary.

Interesting John, how is that calculated?

[Chris]

1 hour ago, nightfisher said:

The longer focal length is going to be a winner in this situation, if nothing else you will be using a longer focal length ep to achieve magnification compared to the F5, you could always get a dovetail and tube rings to mount the dob tube on an EQ mount

Hi Jules, I knew the f/8 would beat the f/5 but I'm very much surprised by how much! If I had to quantify it, I'd say the image was 50% better! As they are both 6" Newts, I'm a little surprised by the performance gap!

I've had it mind to try an EQ mount the Dob OTA, this is a good thought. I need to look/work out if the alt bearings on the tube would get in the way? in which case I might trade the Dob for the PL. I'm hoping the alt bearings won't be a problem as that option would be a bit cheaper.

[Chris]

14 minutes ago, Stu said:

Interesting report Chris. I too am surprised there was that much difference, but have to say I was impressed by the only 150P I have ever looked through, very good Jovian views.

Thanks Stu, Yes the last 150p I had was very good too, I'm now starting to wonder if there is something else other than the F/ratio difference?

I might need a bit of help deciphering the star test for the f/5.

[John]

3 minutes ago, Stu said:

Interesting John, how is that calculated?

I'm hopeless at maths Stu but this web page covers it:

https://www.astronomyclub.xyz/planetary-imaging/collimation.html

[Stu]

6 minutes ago, John said:

I'm hopeless at maths Stu but this web page covers it:

https://www.astronomyclub.xyz/planetary-imaging/collimation.html

 

Thanks!  

[Chris]

16 minutes ago, John said:

Very interesting comparison

Apart from optical quality, the collimation "sweet spot" is much larger (and more forgiving) with the F/8 - 11mm vs just 2.8mm around the optical axis of the primary.

Does the F/8 have a smaller secondary as well ?

Good point, John. I've not checked the collimation of the f/8 for 6 months. As for my new f/5, I collimated my laser collimator using a v-block, then collimated both the secondary and primary from there. I found that the 8mm BST placed the inner doughnut of the defocused star dead center, but with the 6mm SLV it did appear slightly off centre...I chose to trust the 8mm BST for collimating as I did take my time when collimating the laser and the scope. 

Maybe I should re-check this.

I think it's cool that I've not bothered with the collimation on the f/8 and it panned the f/5 which I took care over, it really does back up your numbers! 

I'll look into the secondary size, I'm not sure but it would be good to know  

[Chris]

1 hour ago, John said:

Does the F/8 have a smaller secondary as well ?

Just had a look and it is quite a bit smaller. The first pic is the f8,  and the second the f5. The secondary holders, aperture, and framing of the pic are the same or similar. 

[Astrobits]

It might well be that the mirrors and eyepieces  are all good. It's just that steeper light cone from the faster mirror that makes the difference. Many eyepieces work well with shallow ( slow mirror ) light cones but their performance deteriorates as the cone gets steeper ( faster mirrors ). 

 

Nigel

[Scooot]

If you'd like tracking for the f8 Dob you could stand it on an EQ Platform, I use mine when looking at planets, they also raise the eyepiece height slightly, which may or may not be helpful.  

http://www.equatorial-platforms-uk.co.uk/

[Chris]

7 minutes ago, Astrobits said:

It might well be that the mirrors and eyepieces  are all good. It's just that steeper light cone from the faster mirror that makes the difference. Many eyepieces work well with shallow ( slow mirror ) light cones but their performance deteriorates as the cone gets steeper ( faster mirrors ). 

 

Nigel

Hi Nigel, I used my 6mm Vixen SLV with the f/5, these EP's are orthoscopic over their 50 degrees  

I'm just going to conclude that slow Newts rule    I really have been amazed by this entry level £200 Dob over the last 6 months. I guess I just wanted to share my amazement really, as it toasts another scope I've put it up against. It has blown me away on a number of occasions for lunar and planetary so far, even for average seeing conditions.

I do plan on getting hold of a GP cam or ASI120mc for some near real time DSO imaging, and that's where the f/5 will find it's feet I'm sure.

I'm a proper confirmed long focus Newt convert now, and I'll have a bigger one once I've finished grinding it!

[Chris]

Just now, Scooot said:

If you'd like tracking for the f8 Dob you could stand it on an EQ Platform, I use mine when looking at planets, they also raise the eyepiece height slightly, which may or may not be helpful.  

http://www.equatorial-platforms-uk.co.uk/

Hi Richard, hmm? 500 quid is a bit rich for me I'm afraid (that's what all my scopes cost when added together), I'm probably going to cost up rings and a dovetail despite my particular EQ mount being a bit on the light side!   

[Scooot]

6 minutes ago, Lockie said:

Hi Richard, hmm? 500 quid is a bit rich for me I'm afraid (that's what all my scopes cost when added together), I'm probably going to cost up rings and a dovetail despite my particular EQ mount being a bit on the light side!   

Yes, I know they've gone up a bit, the watch house ones are hand made and put together I think so I suspect it's mostly labour costs. Telescop Service have a version as well but they're a lot dearer than they used to be. Thought I'd just show the option  

[Stu]

5 minutes ago, Lockie said:

Hi Richard, hmm? 500 quid is a bit rich for me I'm afraid (that's what all my scopes cost when added together), I'm probably going to cost up rings and a dovetail despite my particular EQ mount being a bit on the light side!   

Sumerian do one for 295 Euros

http://www.sumerianoptics.com/products-price/

[Moonshane]

I have a 6" f11 and other than wide field views there's really no comparison with a 6"f5 on most targets. 

That said I suspect the detail is generally there in the f5 you just have to work much harder for it.

[Chris]

Just now, Stu said:

Sumerian do one for 295 Euros

http://www.sumerianoptics.com/products-price/

Thanks for the link, Stu, that one does look keenly priced so I'd be sensible to consider it  

[DRT]

10 minutes ago, Moonshane said:

I have a 6" f11

So have I ?

Interesting comparison, Chris. I considered the 150PL for a long time but when the OO UK 150 F11 came up I couldn't resist

 

[Chris]

Just now, Scooot said:

Yes, I know they've gone up a bit, the watch house ones are hand made and put together I think so I suspect it's mostly labour costs. Telescop Service have a version as well but they're a lot dearer than they used to be. Thought I'd just show the option  

Appreciated, thanks for the thought  

[Chris]

Just now, DRT said:

So have I ?

Interesting comparison, Chris. I considered the 150PL for a long time but when the OO UK 150 F11 came up I couldn't resist

 

Well Derek, I'm not going to feel sorry for you there mate, I can only imagine the f/11 with its high grade mirror to be even better  

[DRT]

5 minutes ago, Lockie said:

Well Derek, I'm not going to feel sorry for you there mate, I can only imagine the f/11 with its high grade mirror to be even better  

If the Derbyshire clouds ever part for long enough I will let you know, Chris! ?

[John]

Interesting short review of the Skywatcher 150PL with some mentions of how it compares with an Orion Optics 150 F/11 (albeit a 1/6th wave PV one) here from Ade Ashford:

http://www.nightskies.net/scopetest/scopes/explorer/150PL.html