Skywatcher ST 150 versus 150mm F 5 Newtonian

[Guest]

Just remembered the Skywatcher ST 150. It has a focal ratio of F5. This ratio is the same as a 150mm F5 Newtonian. Although the Newtonian does have a central obstruction. In principle the ST would give some colour, but an unobstructed view. Whereas looking through the Newtonian you would not expect colour aberration. The tube weight for the ST 150 is 6.8 kgs. The tube weight for an aluminium tube 150 mm Newtonian is 4.5 kgs. Probably easier to handle than the ST weight wise.

But which of these two differing telescopes would give a brighter clearer and more pleasing view I wonder?

Edited June 3, 2020 by Guest

[Alan White]

That very same question has crossed my mind as well.
So look forward to the replies.

[SimM]

The ST 150 is a bit of a bargain. There is also a 150 SW at 10x the price. For the latter, FLO have an offer to save £50 if you buy the reducer at the same time. For that price you might expect them to include it!

You are trading different things. The Newtonian has no glass, so avoids any chromatic aberration, that the more expensive refractor seeks to avoid. The ST 150 avoids collimating issues that Newtonian are known for. All Newtonian scopes (which includes the ones with a parabolic mirror) suffer from coma, so crow’s feet at the outer edges, which can be corrected. A Newtonian can have spiral canes which introduces four points around bright stars, which can be eliminated with curved vanes.

There are practical issues with the Newtonian e.g. have to rotate the scope in the cradles to get a good viewing angle and you may want a short mount or a “booster” seat to be comfortable. Some Newtonian are big for their size e.g. some bigger ones are in smaller or shorter tubes and be less susceptible to wind etc. A camera may not reach focus on a Newtonian, otherwise the EP would be too far out for normal use. A refractor may require a pier to get sufficient clearance on a Eq. mount.
 

Overall the best image is the one you prefer, with lots of other things to take into account.

Simon

[John]

5 hours ago, Grumpy Martian said:

.... the ST would give some colour....

A great deal of false colour I think.

Personally I would much rather have one of the 152mm F/5.9's such as the one below for deep sky observing:

http://ensoptical.co.uk/ts-optics-152mm-f5.9

 

 

[CraigT82]

5 hours ago, SimM said:

The ST 150 avoids collimating issues that Newtonian are known for. All Newtonian scopes (which includes the ones with a parabolic mirror) suffer from coma, so crow’s feet at the outer edges, which can be corrected.

This is true, but refractors are not immune to collimation issues, and as you do get coma with newts you get field curvature with fracs (well, and pretty much all scopes really, to a certain extent)

Edited June 3, 2020 by CraigT82

[John]

24 minutes ago, CraigT82 said:

This is true, but refractors are not immune to collimation issues, and as you do get coma with newts you get field curvature with fracs (well, and pretty much all scopes really, to a certain extent)

Quite right and with the 150 having no collimation provision, if it is out (and they frequently are) it is harder to do anything about it.

 

[mikeDnight]

If its a good specimen I'd prefer the ST150. They can be spectacular rich field/deep sky scopes. Some can suffer from spherical aberration so quality can vary, but if you get a good one, it would be the perfect RFT, much better than the reflector. Colour isn't really an issue as it only shows up on the brightest stars, but on lunar it will definitely be obvious. The ST is really a specialist scope where as the reflector could be viewed as a more good general purpose instrument. Using curved spider veins on the reflector doesn't remove the spider diffraction, it only alters its pattern, so the effect on contrast is still there. Take a look at the Double Cluster in Perseus through each, and id pretty much guarantee you'll walk away with the ST. 

[rl]

In my experience the short focus refractors can suffer from excessive spherical aberration as well, which is often not noticed over the CA. I've had 2 examples of the ST120 which differed markedly in this respect. They are wonderful for their intended job; widefield views on deep sky. Having said that, the field curvature can be a bit of an issue. It's obvious in some of the bigger TV eyepieces that have a guaranteed flat image plane. 

The Newtonian might be the better all-rounder if it's got really good mirrors if you have the enthusiasm to fettle it. 

Which would I choose? I've got both a ST120 and a 6" f/6 Newtonian with very good mirrors (which is a bit longer than your question asks). The Newt is the better overall scope by some margin because it will make a decent fist of everything,,  but it's the ST120 that generally gets chucked in the back of the car simply because I know I won't have to faff about with the collimation on site.

[FenlandPaul]

I had an ST150 for a while, Martin (one that’s done the rounds on SGL actually!).  I did have a bit of a soft spot for it.  On open clusters and the star fields of the Milky Way, it was quite hard to beat for all round relaxed viewing pleasure.  I seem to remember good colours on things that weren’t too bright - never had a lovelier view of Uranus’ colour.

Need a decent mount though.  I had it for a while on an AZ4 and it was very much on (or beyond) it’s limit.  

[John]

45 minutes ago, rl said:

In my experience the short focus refractors can suffer from excessive spherical aberration as well, which is often not noticed over the CA....

So do many of the longer ones from these sources