Skywatcher 120 mm Startravel versus 150mm F5 Newtonian telescopes..

[Grump Martian]

I have a Startravel 120mm F5 refractor. This has given me some wonderful widefield views. But it is nowhere near an all round performer. OK I know that there is really no such thing as an all round telescope. The views of the double cluster and some nebulas have been great. But it does give chromatic aberration on some objects. Bright planets for example. The colours of Alberieo the double star are not correct. So the view is not as striking.

But I am starting to think about the possible advantages of a 150 mm F5 Newtonian reflector telescope. Especially one with a 1/10 th wave figured primary mirror. An extra 30mm of apature, better quality optics, still the same focal length F5 as the Startravel. So nice wide field,low power views. But with no chromatic aberration. The view of the bright planets and double stars would be improved. Brighter aswell with the extra apature. But hang on, there is the central obstruction of the secondary mirror holder.

Some people would be put off the Newtonian because of the collimation of mirrors. But a six inch Newtonian would be easy to collimate and would'nt deviate too much.

Which of these telescopes would give the better views I wonder? Should I change from the Startravel to the Newtonian?

Edited September 15 by Grump Martian
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[Grump Martian]

Apologies. I have seen a similar post . This is specific to a particular refractor. But if mods feel that it infringes.

[dweller25]

I think the 150mm Newt is a really good idea - more light gathering and APOchromatic performance for a low price 👍

Edited September 15 by dweller25

[vlaiv]

6" F/5 will beat 120ST in almost anything except very wide field.

With newtonian you will have both coma and vignetting on 2" eyepieces with large field stop (it would need very large secondary to fully illuminate 46mm field stop - most imaging newtonians have larger secondary than visual scopes and struggle to illuminate APS-C size which is 28mm diagonal).

There is also issue with mounting such scope - you don't want EQ mount and some AltAz that happily hold ST120 (AZ4 for example) won't be able to hold 6" Newt. Weight won't be the problem - OTA diameter and tube will for observing near zenith. It would be happy on something like SkyTee2 for example - of small dob mount of course.

I was also in the market for "all around" scope in 4-6" class and decided to go with this:

https://www.firstlightoptics.com/stellamira-telescopes/stellamira-110mm-ed-f6-refractor-telescope.html

Don't know if it fits your budget, but it is cracking little scope. It has far far less CA than ST120 - in fact, if you compare it to ST120 then it can be said it is color free. It will show very wide field views with suitable EP (I'm using 40mm ED 69 degrees EP for that).

In fact - our team used this scope on mini Messier marathon this summer on local Astro camp and people who used it and judges were very impressed with performance of the scope. In comparison to Evostar 102 (4" F/10 achromat) - it shows no color in focus (very distinct red/green CA while out of focus though) on planets and the moon and it has "higher fidelity" - so to speak. It shows all the features as excellent Evostar 102 but Evostar shows those features "in bold" / "greasy looking" while StellaMira shows them quite delicate.

[John]

I'd like to see a detailed comparison on visual DSO's between these 2 scopes. The paper advantage of the newt is 30mm of additional aperture plus it's free of false colour but the secondary and it's supports plus the lower light transmission of the reflecting system tend to scrub away at that aperture advantage I reckon. Then there is coma in the newt if you want wide views which are sharp right across.

Refractors are noted for their contrast due to the unobstructed optical system and that benefits DSO's as well as solar system targets. The 120mm F/5 achromat is not a great solar system performer though due to it's CA.

The newt might be the better all rounder but I seem to recall that you have a 6 inch SCT as well ?, so it's possible that the 120mm F/5 refractor might make a better companion for that scope, if it's allowed to play to it's strengths.

If I was looking to replace the SCT and have something of an all rounder / one scope solution I might think along the lines of a 150mm F/6 rather than the F/5 versions.

 

Edited September 15 by John

[Backyardscope]

Come and see me about my OmniXLT150 Newt Grumpy.

[Louis D]

I love my GSO 6" f/5 Newtonian with GSO CC.  It does pretty much everything very well for a modest cost.  It is also not very heavy or bulky.  My DSV2B mount has no issues with it.

I don't have a 120ST, but I do have a 152mm f/5.9 achromat, and I have yet to find anything it does better than the Newtonian.  The CA and SA are just too much to overcome in the frac.  Even low power star field sweeping isn't very satisfying because the star colors are messed up and they're not pinpoint.  Add in the fact it weighs roughly double the Newt and cost 3 times as much even when bought used, and it's a no-brainer to go for the Newt.  Once set, I haven't detected any collimation shift in the Newt, either.

[Ratlet]

I went through other way with a 5" newt to a 4" refractor.  I went for an apo though.

I think, so long as you can cope with the diffraction spikes (they don't bother me) you'll enjoy the newt.

The only caution I would offer is that if you are using an EQ mount be prepared for the eyepiece to end up in some very awkward places and having to rotate the scope.  I tried it once and got so frustrated I never tried again.  It's part of the reason I went for a refractor.

[NGC 1502]

I own a 120 f5 Achro and a Hinds 6” f6 Newtonian.  Ok the 6” is not the f5 you mention but it’s reasonably close.

The 120 f5 excels at very low power wide field. If you want to view objects like Kemble’s Cascade or large nebulae, dark nebulae, sweep the Milky Way etc…..that’s where it shines.

In all other observing the 6” Newtonian gives far better views, from Solar System to deep-sky. Plus of course it’s an Apo with zero false colour. With a low power wide field eyepiece I can get a well over 2 degree field of view without the exit pupil being too large. Coma? acceptable. Any loss of contrast from the central obstruction and spider vanes is more than compensated by the lack of false colour and extra light grasp.  Collimation? Zero problem when you learn how.

Manual star hopping is easier with a Newtonian because star charts can be turned upside down to match the Newtonian view. You cannot do that effectively with a mirror reversed view assuming you’re using a star diagonal on a refractor.

A 6” Newtonian is grab and go, what’s not to like😊