TheSkySearchers

I am interested in a discussion on the following:

The comparison of visual effectiveness of the following three 150mm refractor telescopes of f/5, f/6.5, and f/8.

Assuming equal quality of glass etc, how the views may differ in your experience.
I am also interested in the theoretical.

If you use one of these refractors, is there a sweet spot ?
What are you most viewing?
What might you avoid viewing?
And if such a judgement can be made, What sky conditions would be better for one or the other?

I will thank you in advance for any participation.

I have outlined a few parameters here.

150 f/8 1200mm
ExP 2 with 16mm EP at 75X
ExP 1 with 8mm EP at 150X
ExP .5 with 4mm EP at 300X



152 f/6.5 988mm
ExP 2 with 13mm EP at 76X
ExP 1 with 6.5mm EP at 152X
ExP .6 with 4mm EP at 247X



150 f/5 750mm
Exp 2 with 10mm EP at 75X
ExP 1 with 5mm EP at 150X
ExP .8 with 4mm EP at 187X

Clear skies!

When it is financially responsible for me to do, I will be getting a 6" f/5. I like that it is lighter than the other focal ratio scopes. More violet color, but I will be using it for black and white DSO only. The f/8 is interesting. Just add a 4" aperture mask and you got a f/12 for the planets, moon, and other bright objects.

Thank you, Refractordude.
I have recently acquired the f/6.5. It came available at a good trade. I suppose I could use a mask on it as well.
I like your idea of B&W DSO.
Would the masking affect the color fringing?

I have not had opportunity to experiment, but I am wondering if Exit Pupil affects the color fringing.

Yes it does, as the use of magnification decreases (ExP enlarges) the visible CA lessens to a degree.
It does not go away as the CA is spread across the view as the colour blur I mentioned to you earlier.

As you know I have Celestron version of 6" F5. It is amazingly lightweight and compact. It is ideal for low to mid power wide field DSO views. It is my to go instrument for open clusters and extended nebulae. I also enjoy splinting doubles at lower powers.

I would say that in 20-100x range CA is not an issue. In this range stars are colorful and tight.

Bumping above 100x, CA really kicks in. You could still do planets and Moon, but even with good amount of details resolved they look mushy. I have better instruments for that job, so I hardly use 150ST.

For DSO hunting useful range of EPs are from 36mm to 7mm. I know some users reported using EPs in 3.5-5mm range. But for me stars bloat below 7mm and I just don't enjoy resulting views. I think viewing conditions and quality of telescope sampling are definitely factors there.

In 20x-100x range the scope really shines. Sagittarius Milky Way, fitting M31 in 3 deg FOV, Veils complex with OIII - mind changing views!

You need to watch for the field curvature though. To me it is more annoying than CA. Ironically, 82 deg EPs praised for the field correction in DOBs exaggerate field curvature in 150ST, and 14mm and 20mm Pentax XWs bashed for the FC on forums correct FC of 150ST. So, much EP to scope rule always applies.

Baader Aspheric 72 deg 36mm and 31mm and Pentax XWs 7mm to 20mm are the EPs of choice for my 150ST.

Thank you Andrey for participating here with your good experience and recommendations.

So far my experience on my AR152 f/6.5 has been limited to only two good night outings and some daytime fiddling...definitely had purple fringe in daylight. I flocked the drawtube and it seemed to help getting into focus in the daytime. I am not sure now what was affected but it seemed to help.

I am eager to get out at night and play with it more.

Don Quixote wrote: ↑Mon Sep 30, 2019 7:17 pm Thank you Andrey for participating here with your good experience and recommendations.

So far my experience on my AR152 f/6.5 has been limited to only two good night outings and some daytime fiddling...definitely had purple fringe in daylight. I flocked the drawtube and it seemed to help getting into focus in the daytime. I am not sure now what was affected but it seemed to help.

I am eager to get out at night and play with it more.

It is apples to oranges to some degree, since our scopes have somewhat different geometry and came from different vendors. But, once I upgraded the focuser on mine to GSO linear bearing I don't have any issues focusing at night or day. Yes, the fringe is there during the day. It also seems a function of EP and diagonal. I did not do side by side comparison during the day, but since I upgraded diagonal from Orion dielectric to Baader Clicklock and switched from Meade UWAs to Pentax XWs I see less purple during the day.

I can only answer for the 6 inch f/5.
Take into account that I observe from a LP area. Most widefield views are done with a 24mm Panoptic at 31X ,Exp 4.8 and 2.17 degree FOV. I sometimes use a 28mm Edmund RKE or a 30mm Eudiascopic. I haven't brought him with me to a dark location yet, but I can Imagine I use a 2 inch 31mm Baader Hyperion there. (24X, Exp 6.2, 3 degree FOV) Those big Exit pupils are ideal for using UHC or OIII filters.

I use the Leica ASPH Zoom for 17,8 to 8,9mm for 42X to 84X. ( Exp 3.6 to 1.8 mm ) The last one is becoming interesting for DSO-use. Field of view 1.4 to 0.9 degrees. Most Messiers fit in.
Even more interesting it becomes when I put the Zeiss barlow in between, mag. factor some 2,33X. Magnifications become 98X to 196X . The less bright objects emerge. Works well on small DSO's too.

Under my LP skies I would say the sweet spot lies at approximately 150X or so. ( Exp 1mm)

A F/5 system is not a Moon or planetary telescope, although much occasional work can be done with the 110mm opening in the dustcover. ( makes it a f/6.8) Jupiter is still the one to avoid though. An extra home made diaphragm in the dustcover makes it an 80mm F9.4 if needed.

CA is apparent, although I suppress it more or less with a prism at the eyepiece-end instead of a diagonal mirror. At low magnifications it is visible on stars to magnitude 4 or so. At high magnifictions the ordinary glare masks it and CA becomes less noticable. Most DSO's, even bright clusters, do not contain much bright stars, so most of the time I don't even notice CA. Moreover, once used to it it doesn't stand out any more.

At F5 the system is in need of a focusser with a 1:10 reducer. At higher magnifications, that is. The standard focusser is not bad for a F/8 system or slower but simply too course for the very small focus-depth of a F/5.

Hello John.
Among other details you have shared, your comment on the facility of UHC and OIII filters with larger ExP is a connection I have not thought of. Of course it makes sense.

On my next opportunity I am planning a dual mounting of the 6" f/5 Newt I have recently acquired with my 6" f/6.5 frac .

I realize that to some extent I am comparing apples and oranges but I think it will be interesting.

Thank you for participating in this thread John.
Godspeed naar jou mijn vriend.

Don Quixote wrote: ↑Sun Nov 03, 2019 12:52 am Hello John.
Among other details you have shared, your comment on the facility of UHC and OIII filters with larger ExP is a connection I have not thought of. Of course it makes sense.

On my next opportunity I am planning a dual mounting of the 6" f/5 Newt I have recently acquired with my 6" f/6.5 frac .

I realize that to some extent I am comparing apples and oranges but I think it will be interesting.

Thank you for participating in this thread John.
Godspeed naar jou mijn vriend.

Go for it Mark, that should be quite interesting comparison.

Good luck with the comparison Mark. So far I have compared the views in my 5.1" Newt and the 6" frac. The frac does provide superior views, though of course the aperture is a bit larger. I have noticed slight color fringing on some objects in the frac but so far it is manageable. And the wide field views are outstanding.

Hello Mark,

regarding the 6" F/5, I would not add much more besides what John and others have already posted.

As with any fast refractor, what matters is the right choice of the eyepieces designs.

The spherical aberration of the larger aperture fast refractors, and especially of the 6" F/5 limits the resolution, unless you will take a glass zenith prism, and cut the UV/violet with the filters like UV/IR cut.

On the 6" F/5, it is actually not as much the classical CA, as the sperochromatism, which makes the color blurr. In the plain language it is still called the CA fringe.
On the slower 6" F/6.5, the spherochromatism drops, the CA increases, and the both are eventually nearly heads on.
On the even slower 6" F/8 the sperochromatism has been hardly reported, the numerical CA deviations increase even more, but the CA color blur fades far away from the Airy disc, and the visual best focus looks clean.

Best,
JG

Hi Folks,

I am in agreement with the posters above about the CA of the f5. I don't find it at all objectionable except when you start to look at high power: using my 3-6mm Nagler it becomes pretty noticeable at 125x, and at 250x things get pretty blurred. With a 7.5mm Tak LE eyepiece (100x) it is fine.
I had a play the other day with the Baader Fringe Killer filter, and it made Jupiter a lot sharper at 250x! The view became quite acceptable. (Mind you, I am not a fan of this filter for daytime views as it imparts a yellowish tinge.)

At lower powers (up to 100x) it provides great views: in fact I was lucky enough to have a look through a Skywatcher Esprit 150 the other day, and the f5 achro held its own really well for lower powers.

- Dean

Thank you JG for coming in on this.

With my f/6.5 I have good visual focus with very little color fringing. On the moon tonight at 78X with 12.5 Docter there can be a very thin line of blue or yellow on the limiter depending on how I point my vision. It is in the peripheral part of my vision when the color line blinks at me. When I point my eye at the color it seems to disappear. It is in no way a detriment to the view.
I intended to make a comparison with my 6 inch f/5 Newt just for the fun, but when I got out there I had somehow knocked my secondary out of kilter so no comparison could be made.
There will be another time.
I enjoyed experimenting with the 6 inch frac untill the frost began to coat the outside of the tube. I viewed IC1805 with the f/6.5 tonight also, but transparency was quite bad so the nebulosity was not as evident as last weeks view under much better sky with the f/5 Newt.

This is the first frosty night for me this year. I am going to need to brake out my pocket warmer.

I really appreciate everyone's participation in this thread. It is very interesting and helpful to hear of each experience.