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120mm f/5 Refractor or 150mm f/5 Reflector for DSOs?

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#1 hawkinsky

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Posted 17 September 2018 - 02:44 AM

Hi!

I am thinking about buying a scope for DSO observing, now close up to two options:

 

Orion StarBlast 6 Astro Reflector Telescope, and Orion 120mm f/5.0 Refractor,

 

Which one of them will be a better choice? Have read some posts here still not sure. Any suggestion will be appreciated!



#2 Sky Muse

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Posted 17 September 2018 - 03:38 AM

I have the Orion StarBlast 6...

 

6 f5v2.jpg

 

Under darker skies, it's great for observing deep-sky objects.  Also, given a Newtonian's total apochromaticism, it's good for brighter objects, too.

 

The Orion 120mm f/5 achromat would also be very good for deep-sky observing, but not so much for brighter objects due to the excessive false-colour produced by short achromats when viewing same.  Also, there would be no need to collimate the telescope; as there would be with a Newtonian, initially, and on occasion thereafter.

 

In either event, most DSOs are rather small, so plan on getting at least a 2x barlow, and perhaps even a 3x barlow.  For example, I once saw the Trapezium of Orion that made my jaw drop, and with my 6" f/5 Newtonian.  I had used a 12mm 60° eyepiece with a 2.8x barlow, and for a simulated 4.3mm(174x), at the time.

 

As you can see within my image, above, I quickly abandoned the original Dobson-type mount, and for a tripod-type alt-azimuth.

 

Would you be observing under urban, suburban, or rural skies?  



#3 deepwoods1

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Posted 17 September 2018 - 04:37 AM

My 120 f/5 pushed to 120X is a little soft. I don’t think I’ve seen anyone stating that their pulling apo like magnification out of it. That being said, it’s not made for higher mags. It excels at lower power. I never found myself pushing magnication in a fast instrument. I did find it cooled quicker than a 6” f/5, so it was kept and the reflector was sold. Have you considered a 8” Dob? The greater light gathering really helps with DSO’s! Fairly light and portable for what it does. Usually has a good mirror that can take some power. 



#4 otocycle

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Posted 17 September 2018 - 06:14 AM

I would choose the 120mm f/5 refractor because of the 2" focuser.



#5 Jeff Struve

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Posted 17 September 2018 - 06:20 AM

I would recomend holding on any purchases until you visit a club... even if you have to drive a few hours each way... go to a number of  meetings, go to a number of events... decide on the type of astronomy that you are most interested in... check out what the view is like in the scopes... what it takes to set them up and take them down, transport them store them... club members may even have a great scope to be leant out or used scopes for purchase at the same or lower price than a new lower quality scope. 

 

My 2 cents


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#6 Jeff Morgan

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Posted 17 September 2018 - 09:25 AM

I owned a Jaegers 6" f/5 refractor. On DSO's it was formidable.

 

But the weakness was magnification. It was great - sensational - using a 35 Panoptic at 21x. But when I put in the 22 Panoptic at 34x - the color and other aberrations were very noticeable, and it only got worse from there.

 

Could have been that particular objective - could be the breed.

 

The reflector would be a more versatile performer.



#7 jcj380

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Posted 17 September 2018 - 01:30 PM

I have an ST120 on an alt-az that I use exclusively for DSOs.   My favorite EP is a 15mm Orion Expanse that gives about 1.5 degrees FOV and 40X.  However, I'm in heavy LP, so I usually go up a notch to a 12mm Plossl or a 10mm SLV to give a little more contrast and / or bring objects out of the soup.  When I can take the 120 to a dark site though, I get in-your-face views.

 

I have a 6mm Expanse, but I have to say it seems a little mushy.  Not sure how much is scope and how much is EP since I don't have another 6mm to compare.  Might buy an ES just to experiment.

 

I don't see much CA on bright stars.

 

(I also have an old 40mm Kellner but using that is almost like looking through my finder.)



#8 havasman

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Posted 17 September 2018 - 04:11 PM

Hello hawkinsky and welcome to the forums!

 

I have had a very similar 5" f5 tabletop reflector and found it to be very useful, particularly for widefield observations of DSO's. The view of M31/32/110 all in the field is still a favorite.

 

The problems with entry level refractors usually include a poor mount/tripod combo that make them hard to use and, as above, limited capability for higher magnification.

 

Neither of these scopes really needs a large eyepiece kit. The value of a 2" focuser over a 1.25" focuser is lost on these scopes. The higher weight of a 2" ep will just exacerbate the shaking of the lightweight mount carrying the refractor.

 

If it was my $$, I'd get the little Dob and consider adding only 2 eyepieces: an Explore Scientific 68o 24mm and either a Meade Series 5000 825.5mm or an Explore Scientific 82o 4.7mm. (Those can relatively often be found in the classifieds here and at AstroMart for significant savings.) Then find you some dark skies and that gear will give you years of high class observing.



#9 hawkinsky

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Posted 17 September 2018 - 11:35 PM

I have the Orion StarBlast 6...

 

attachicon.gif 6 f5v2.jpg

 

Under darker skies, it's great for observing deep-sky objects.  Also, given a Newtonian's total apochromaticism, it's good for brighter objects, too.

 

The Orion 120mm f/5 achromat would also be very good for deep-sky observing, but not so much for brighter objects due to the excessive false-colour produced by short achromats when viewing same.  Also, there would be no need to collimate the telescope; as there would be with a Newtonian, initially, and on occasion thereafter.

 

In either event, most DSOs are rather small, so plan on getting at least a 2x barlow, and perhaps even a 3x barlow.  For example, I once saw the Trapezium of Orion that made my jaw drop, and with my 6" f/5 Newtonian.  I had used a 12mm 60° eyepiece with a 2.8x barlow, and for a simulated 4.3mm(174x), at the time.

 

As you can see within my image, above, I quickly abandoned the original Dobson-type mount, and for a tripod-type alt-azimuth.

 

Would you be observing under urban, suburban, or rural skies?  

Thank you so much for your information, Alan. I have a Meade 80mm short tube, which works fine and super portable, but not powerful enough for DSOs. I do lean on the 6'' reflector for the bigger aperture. but I never used a reflector before, really want to give it a try. Do you think how long is the mirror's coating going to last under reasonable maintenance? 


Edited by hawkinsky, 17 September 2018 - 11:37 PM.


#10 hawkinsky

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Posted 17 September 2018 - 11:49 PM

My 120 f/5 pushed to 120X is a little soft. I don’t think I’ve seen anyone stating that their pulling apo like magnification out of it. That being said, it’s not made for higher mags. It excels at lower power. I never found myself pushing magnication in a fast instrument. I did find it cooled quicker than a 6” f/5, so it was kept and the reflector was sold. Have you considered a 8” Dob? The greater light gathering really helps with DSO’s! Fairly light and portable for what it does. Usually has a good mirror that can take some power. 

I have a Meade 80ST, also bought for the low power, I really enjoyed this short tube, just want to push up a little for better DSO observing. 8'' would be ideal, but out of my reach from my budget and for portability. I live in Seattle urban area, I usually spend every summer weekend camping in mountains for dark sky, portable is one of my big concern.



#11 otocycle

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Posted 18 September 2018 - 12:26 AM

There will not be a significant difference in DSO brightness between the two if those are the primary objects.   The Starblast Newtonian does offer a less colorful and slightly higher resolution view for lunar and planetary.   The 120mm f/5 refractor will have better contrast/less scatter in my experience (Celestron 150mm f/5 XLT).

 

See this CN thread....

 

https://www.cloudyni...or-worth-doing/


Edited by otocycle, 18 September 2018 - 02:31 AM.

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#12 hawkinsky

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Posted 18 September 2018 - 12:26 AM

Hello hawkinsky and welcome to the forums!

 

I have had a very similar 5" f5 tabletop reflector and found it to be very useful, particularly for widefield observations of DSO's. The view of M31/32/110 all in the field is still a favorite.

 

The problems with entry level refractors usually include a poor mount/tripod combo that make them hard to use and, as above, limited capability for higher magnification.

 

Neither of these scopes really needs a large eyepiece kit. The value of a 2" focuser over a 1.25" focuser is lost on these scopes. The higher weight of a 2" ep will just exacerbate the shaking of the lightweight mount carrying the refractor.

 

If it was my $$, I'd get the little Dob and consider adding only 2 eyepieces: an Explore Scientific 68o 24mm and either a Meade Series 5000 825.5mm or an Explore Scientific 82o 4.7mm. (Those can relatively often be found in the classifieds here and at AstroMart for significant savings.) Then find you some dark skies and that gear will give you years of high class observing.

Thank you for those great information, sir. I bought my first telescope from a gentleman on Craigslist about a year ago, he gave me lots of free information to kick start my astronomy life, introduced CloudyNights to me. I have been an avid visitor to this forum thereafter. This is truly a fantastic place for learning and asking help. Portability is fairly important to me, which makes the 6'' Orion Dobs very attractive. 


Edited by hawkinsky, 18 September 2018 - 12:29 AM.

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#13 BrooksObs

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Posted 18 September 2018 - 01:02 PM

I have the Orion 120mm f/5 refractor and an older Coulter 150mm f/5 Newtonian. The refractor does a better job for my purposes, which are mainly observing brighter comets (particularly naked-eye ones) and variable stars. However, the Newtonian isn't all that different in performance except for its more limited FOV because of the 1.25" eyepiece holder. I would add that, with the refractor mounted on an old Unitron alt-az mount, it is more convenient to use for quick looks and particularly if space to position the scope is tight, or the scope needs to be moved around at intervals.

 

Either instrument is specialized and neither is designed to be used at high magnification. You will be buying either one specifically for low power viewing, so don't think that they are applicable to every observing purpose, or you will be very disappointed. 

 

BrooksObs


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#14 Phillip Creed

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Posted 20 September 2018 - 07:22 AM

The 120-ST will have the advantage in terms of FOV.  The Starblast 6 will have the advantage for any magnification beyond double digits.   Almost anything worth looking at in the sky is smaller than the 2.1°-FOV of the Starblast 6 with its 1.25"-focuser. That being said, if you need WIDE fields of view, the 120-ST can do up to a staggering 4.4°.

I have owned both telescopes, with the Starblast 6 being the most recent.  I had mine modded with a 2"-focuser, and can now get 3.2°-FOV with a 31T5.  If I wanted to, I could eek out 3.5°.  Either is overkill, but it's nice having the RFT capability in a 6" scope that's fully apochromatic and still dirt cheap.

 

The 120-ST is a one-trick pony (low-power DSO work), but it's popular because it does that one trick VERY well.  If you're a widefield junkie and you don't want to mod the scope, the 120-ST is the better bet.  If the chromatic aberration on the 120-ST bothers you but you still want to preserve a lot of the widefield capability, consider a Starblast 6 and a used dual-speed 2"-GSO Newtonian focuser.

One thing's certain--you LITERALLY don't have much to lose with either choice, especially if buying used.

Clear Skies,

Phil



#15 Sky Muse

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Posted 20 September 2018 - 11:06 AM

Thank you so much for your information, Alan. I have a Meade 80mm short tube, which works fine and super portable, but not powerful enough for DSOs. I do lean on the 6'' reflector for the bigger aperture. but I never used a reflector before, really want to give it a try. Do you think how long is the mirror's coating going to last under reasonable maintenance? 

I have an 80mm f/6 achromat, the tube only 80mm longer.  Your Meade is an 80mm f/5, the tube only 80mm shorter.  I'm well aware of the limitations of both.

 

The 6" f/5 Newtonian of the Orion "StarBlast 6" is Synta's base model.  The plastic focusser is dreadful, in both construction and performance; but the adjustable cell which holds the primary-mirror is first-rate.

 

This one, however, comes with a 2" focusser, which can be useful for the low-to-medium powers in future...

 

https://www.highpoin...1hoCHXsQAvD_BwE

 

It's a house-brand, but it's manufactured by GSO; a very good thing, that.  You'd get a much better telescope in getting that one over the Orion kit, and this mount will support it...

 

https://www.telescop...LRoCXIwQAvD_BwE

 

...and at or near the price of the Orion "StarBlast 6".  This mount would also support it, and comes with slow-motion controls...

 

https://agenaastro.c...ltaz-mount.html

 

Both mounts would also support the Orion "120ST" achromat, for that matter.  I'm somewhat curious as to why you did not enquire as to why I had abandoned the particle-board Dobson-mount of my "StarBlast 6".  Didn't you suspect that something must've been wrong with it? 

 

It's up in the attic now.  I got tired of wrangling around with it outside.  It's compact, but not all that small, and heavy.  I got a bit of a strain on one side just before I chucked it upstairs.  Now that it's on a tripod, it's like a whole other telescope, and much easier to use, hence my suggestion of the GSO 6" f/5 and the Orion or GSO alt-azimuth.  Now, there's really only one advantage with the "StarBlast 6" over that: it can be upgraded to a computerised push-to in future, if desired...

 

https://www.telescop.../46/p/99636.uts

 

Or, you can get it already equipped with same... https://www.telescop...yCategoryId=340

 

It's not a go-to, however, as it doesn't include motors.

 

Incidentally, a 6" f/5 Newtonian is the closest you can get to an all-rounder, with magnifications ranging from a low binocular-like 20x, and up to 200x and beyond with the aid of 2x and 3x barlows.  That would give you the ability to observe the gamut; everything in the sky, satisfactorily, and with no false-colour.  I've had my own trained on Jupiter, at high power, and with a variable-polariser; gorgeous it was, the festoons and whorls sharp as a tack during a few moments of steady seeing.  Saturn wowed and amazed, and with its moon, Titan, trailing alongside. I agree that a 6" f/5 is specialised, somewhat, however it is certainly most capable of high-power observations; max out its aperture even, and up to 300x, and beyond, else you'll never know what you might've missed otherwise.  There is one thing, however: the higher in power you go, the harder the telescope must work to keep up, and that requires a near-perfect or perfect collimation(alignment) of the optical-system.

 

Modern mirror coatings can last ten years, fifteen years, even twenty, depending, then you send them off to be stripped and re-coated, but only if the figurement of the primary-mirror had proved exemplary, or even above-average, during those years.  Salt, however, is a mirror's enemy, as it is for many things.


Edited by Sky Muse, 20 September 2018 - 11:06 AM.


#16 Mr. Bill

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Posted 22 September 2018 - 01:36 PM

I own a good sample of the ST 120mm.....great up to about 50x; these are not lunar or planetary scopes; for those you want an apochromat. Great on the wide field Milky Way views with my 26mm Nagler.

 

The weak link is the focuser; I replaced with a Moonlite with the short focus tube designed for fast refractors. This also adds much needed weight aft to balance objective cell weight.

 

I also own a Celestron 150mm f/5 XLT OTA that is my favorite rich field refractor and if you can get one I recommend it over the ST120mm; awesome light gathering for low contrast extended objects like MW bright/dark nebulae.

 

 

Attached Thumbnails

  • ST 120 + 475.JPG
  • Celestron 6 inch f-5.JPG

Edited by Mr. Bill, 22 September 2018 - 01:42 PM.

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