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Relative advantage of 4.5 newt vs 80mm refractor vs 90mm mak

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#1 boyd

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Posted 21 March 2020 - 08:52 AM

4.5 f11 newt
80mm f5 refractor
90mm f11 mak

Other than the wide field of he refractor, what is the relative difference in ds viewing. I understand the focal length issues. I’m really talking light gathering. Newt has central obstruction, as does mak. Aren’t all three essentially the same?
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#2 junomike

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Posted 21 March 2020 - 08:59 AM

The Newt even factoring in the Secondary works out to about 110mm which IME is a noticeable increase in light gathering  on DSO's.

I'd use the Newt for smaller DSO's and keep the 80mm F5 to compliment it for RFT use.


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#3 DLuders

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Posted 21 March 2020 - 09:00 AM

The 4.5" f/11 Newtonian Reflector telescope has a primary mirror diameter of 114.3 mm.  Even with the secondary mirror obstructing SOME of the incoming light, its light-gathering capability should still be more than the (unobstructed) 80mm f/5 Refractor telescope, and the (obstructed) 90mm f/11 Maksutov-Cassegrain Telelescope (MCT).

 

You could do some math and figure out the surface area of each telescope's light-gathering surface (taking the Reflector's and the Mak's central obstruction into account).  

Area = (pi) x (diameter/2)= (3.14) x (d/2) x (d/2)   graduate.sml.gif 


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#4 wrnchhead

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Posted 21 March 2020 - 09:27 AM

I own or have owned all three of these telescopes. Yes the newt gets the most light but they are all fairly dim on DSO‘s. Stars are nicer in the 80mm.
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#5 tleroy1

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Posted 21 March 2020 - 09:36 AM

Hi boyd,

 

Have you ever considered a large Dobsoniain for DSO's? Is it not an option for you? Too big and bulky?

 

Kind regards,

 

Ted



#6 sg6

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Posted 21 March 2020 - 09:59 AM

I have often said that instead of asking the good points and advantages, ask and find the bad points.

 

Everyone will tell you the good points: The 114 is bigger., hey that is simple, just read the numbers.

The 114 is a spherical mirror - not so good.

The 114 will have a set of diffraction spikes - usually not so good.

The 114 secondary assembly also causes "softer" less distinct views - not so good.

 

So can you live with the not so good points?

 

At times astronomy and cars are similar:

Would you like a nice powerful fast car?

 

Would you like the noise,

Would you like the fuel cost,

Would you like the harsh sports ride,

Would you like the high insurance.

 

Likely the answer to 1 is yes, but 2, 3, 4, 5 are likely no.

 

The only scopes that come close to doing everything people want have the slight disadvantage of costing a fortune and need to be permanently housed in a purpose built observatory.


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#7 mikeDnight

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Posted 21 March 2020 - 11:25 AM

If the 4.5" F11 has a well figured primary mirror with a truly flat secondary, and the coatings are in good condition and well colimated, it would be my number one choice out of the three for DSO's. With the exception of spider diffraction, the 4.5" Newtonian will give as bright a view as a good 4" refractor, and a 4" refractor is a great deep sky scope. Back in the early 90's a friends 4.5" F8 Newtonian gave a superb view, on a transparent night, of M81 and its supernova. I was using a 5" F6.3 triplet refractor, but in the triplet the view was dim by comparison. The little Newt' was a real eye opener!

 

Years later, I was observing Saturn using my astro clubs  Vixen FL1 102 apo, when I was invited to look through a 4.5" F11 Newtonian. I reluctantly walked over to the little Newt', but I'll tell you with hand on heart, the detailed view of Saturn in the little reflector left the apo standing. Usually reflectors just don't do it for me no matter what aperture and I much prefer the refractor view, but on both the above occasions it was a 4.5" Newtonian reflector that wowed me. Stunned me in fact! Whatever the scope, it's the optical quality that is the true king. 

 

It was the little 4.5" Newt' below that blew my beautiful old brass triplet (figured by the late Horace Dall) out of the water on M81 and its supernova. I never looked through that triplet again after that.

IMG_5432.jpg


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#8 mikeDnight

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Posted 21 March 2020 - 11:31 AM

At F11 a 4.5" Newtonian wouldn't need a paraboloidal primary and should give text book perfect images, so I wouldn't worry if the primary is spherical. At F8 it would become a problem.


Edited by mikeDnight, 21 March 2020 - 11:33 AM.

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#9 boyd

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Posted 21 March 2020 - 01:01 PM

I have owned four different 4.5 scopes. Two f/11 straight newts, an f/6 Celestron goto horrid thing, and a bird-jones short tube.

Of these the bj short tube was the easiest to use. Never Gave a moments problem, gave me more than the moon and Jupiter which were really all I ever saw in my old old sears refractor.

If patient the f/11 was a fine scope. But now that I’m back in refractor world, I’m not sure any of the 4.5s were any better. I may be biased, as the 80mm refractor seems to pull in what my abilities and the seeing allow.

Just not sure.

I have an old c90, but it is temperamental to focus with a narrow field. Still the 80mm seems it’s equal.
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#10 Jon Isaacs

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Posted 21 March 2020 - 02:45 PM

Everyone will tell you the good points: The 114 is bigger., hey that is simple, just read the numbers.

The 114 is a spherical mirror - not so good.

The 114 will have a set of diffraction spikes - usually not so good.

The 114 secondary assembly also causes "softer" less distinct views - not so good

 

 

- A spherical 4.5 inch F/8 is 1/5th wave.  A spherical 4.5 inch F/11 is 1/13 th wave.  I know of no easily available standard 4.5 inch F/11's, they're all F/8s or faster.  But 1/5 wave is actually very good and better than what the Mak and ST-80 will be.

 

- It will have diffraction spikes, no biggie.

 

- Secondary obstruction causes a loss of fine scale contrast due to diffraction effects.  Worse though is the diffraction effect of the smaller apertures.  And the 90mm Mak has a bigger secondary and the 80mm F/5 refractor has serious chromatic aberration.. 

 

In terms of through put, a 114mm Newtonian with a 30% CO and 90% reflective coatings has the through-put of a 93mm refractor with perfect transmission.  It has the resolution of a 114mm anythinig and it would be the better planetary scope, assuming equal quality.

 

The question though is which 114mm Newtonian are we discussing. Some are very good, mostly the older Japanese models like the Celestron C-4.5, made in Japan by Vixen.  Newer 114mm F/8s are generally entry level scopes and lacking in quality.

 

3747294-Celestron C 4point5.jpg
 
Jon

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#11 boyd

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Posted 21 March 2020 - 06:33 PM

There is a local selling a Celestron 114 for 50 bucks. I was thinking about swooping in and picking it up, but I just don’t think it’s enough of an upgrade to bother. I think I’m going to hold onto my money in hopes that sometime next year I’ll have enough for the Celestron 150 XLT. That or a big honking refractor 👌

#12 Diego

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Posted 21 March 2020 - 07:44 PM

I have a 4.5" f8 Celestron version and I find the optics to be quite good. The only disadvantage I find is the necessary cool down to avoid tube currents. In summer I can take it outside with no problem, but during winter tube currents are really bad so it won't work in my situation as a grab and go. I saw the GRS for the first time in the little reflector.

 

Edit: regarding your question on DSO, I was able to resolve Omega Centauri and the image was very similar in brightness to my 6" reflector, yet the 4.5" is way lighter and easier to handle and mount. At a very dark site I can imagine it's a very powerful scope.


Edited by Diego, 21 March 2020 - 07:48 PM.

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#13 chris charen

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Posted 21 March 2020 - 07:56 PM

Over the decades I had all 3 of the various scopes you have mentioned. Ultimately it is not 'just' about light gathering, as you have to 'use' a scope so other factors come in play like portability, mounting, the sky conditions, etc. The old adage remains true, 'your best scope is the one you use the most'. Yes, for pure light gathering a good well collimated long tube 114 Newt. is great. Avoid the more mass produced 'E Bay' type Newts as they are clearly inferior and try to go for a high quality one,  or an earlier Japanese version. Even the entry level versions which come from the 'name' producers, [Celestron / Meade] etc. I would tend to avoid. I have had some excellent earlier Japanese made Vixens and SYW Newts, which were well built and optically very good, [Avoid ++ at all costs the short tube versions with the in build 2x barlow lens, which is just an attempt to reduce all the bad optical aberrations they produce]. A 'good' FMC S/T 80 mm refactor is portable and good for lower powers, however above 80x +  you get increasing CA .The ones I have owned, [Orion / Skywatcher etc], I have refitted a good 2x speed focus unit. Also good good for terrestrial use. I have had several 90 mm Maks over the years, my main concern is that I just find them 'dim' [IMHO] of course you also get the inherent narrower FOV with Maks. I recently had a great Meade ETX 90, great optics and sharp, by ultimately at higher mags, ie 100x it I found it just became 'dim'. I recently I acquired a Synta 102 mm Mak. [Skywatcher version] and at 65 years old it has become my main 'Grap and Go' scope, It gathers 28 % more light then its 90 mm counterpart. I can go to 160x + and the image remains usable and relatively sharp. It can be used terrestrial and be mounted on a decent camera tripod. They are not dramatically more expensive or larger then a 90 Mak. Sorry, as I know this is not on your list but include in your options if you can and you budget lets you,

 

Chris


Edited by chris charen, 22 March 2020 - 05:37 AM.

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#14 PXR-5

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Posted 21 March 2020 - 08:25 PM

The refractor will have CA, but if you are not viewing planets, you will not notice it.

I use my 80mm f5 for wide field sky scanning.

I use the 90mm Mak on planets and the Moon.

They complement each other nicely :)
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#15 boyd

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Posted 22 March 2020 - 05:16 AM

I am finding the 80mm really cannot deal with ambient light from my neighborhood. M1 was impossible last night. Nothing in Leo at near zenith. I’m thinking my original plan was flawed. I need something less effected by light pollution, so I was considering a 4.5 on the cheap for home viewing and let the 80mm serve in the field.
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#16 Jon Isaacs

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Posted 22 March 2020 - 07:22 AM

There is a local selling a Celestron 114 for 50 bucks. I was thinking about swooping in and picking it up, but I just don’t think it’s enough of an upgrade to bother. I think I’m going to hold onto my money in hopes that sometime next year I’ll have enough for the Celestron 150 XLT. That or a big honking refractor

 

It does depend on the telescope itself.  A good 114mm F/8 on a solid mount would be a worthwhile upgrade.  But I see a Celestron Powerseeker 114mm on the Deltona Craigslist and while the optics might be OK, the mount is unacceptable.. That scope is not an upgrade in my opinion.

 

Jon


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#17 lylver

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Posted 22 March 2020 - 07:51 AM

The 4.5f11 designed as it was in the past is to work with 24.5mm Zeiss/Vixen eyepieces. With a 22mm 70% illuminated aperture, fully illuminated on Moon diameter.

The secondary is to be 40mm giving 35% obstruction, 88% illumination versus non obstructed.

 

Design strehl is very high compared to parabolic mirror, the loss is under 2% Strehl.

This is the best design of the three proposed if well collimated : and it is low sensitive to decollimation.

Coma is low, no problem with moon diameter, few loss of sharpness.

 

35% obstruction makes it sensitive to turbulence but its diameter is under 6" so despite a loss of contrast it is a very nice telescope, a keeper with cheap eyepieces.

 

Spherical design has been pushed up to 6" f/8 with the TAL-2 scope, tank robust same secondary size (40mm 27% obstruction).

In France, with Texereau, it was usual to build 6"f10 in the past with 27mm aperture diameter flat (small thickness) cabestan focuser for planetary observation.

-----------------

Mak 90 : smaller diameter

80f/5 : cannot reach planetary required sharpness, but can be a rich field scope and decent on moon (with a filter). Only high quality & longer f/D 80mm can challenge this newt on moon/planetary.

 

For magnification, my Scopetech 80/1000 with his high strehl is better than the perfectly collimated Vixen VMC110L I had. The VMC was better for nebulae/rich field.

80-1000.jpg


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#18 Steve Cox

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Posted 22 March 2020 - 08:20 AM

Posted in wrong forum and thread.  Deleted.


Edited by Steve Cox, 22 March 2020 - 08:45 AM.

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#19 Jon Isaacs

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Posted 22 March 2020 - 08:25 AM

Never had a 4.5" Newt, but have had the 90mm through 127mm Maks, along with 80mm and 102mm ED refractors, and can say that your findings pretty much mirror mine - the 80mm ED walks all over the 90mm and 102mm Maks, and the 102mm refractor walks all over the 127mm Mak.  I would also imagine that if you included a 102mm ED or triplet into the mix, it would easily beat the 4.5" newt.

 

In this thread, we are not discussing ED/apo refractors, the discussion is about an ST-80, a 90mm Mak of unknown description and a long focal length 4.5 inch Newtonian.  

 

The budget seems to be about $50..

 

Jon


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#20 lylver

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Posted 22 March 2020 - 08:31 AM

In this thread, we are not discussing ED/apo refractors, the discussion is about an ST-80, a 90mm Mak of unknown description and a long focal length 4.5 inch Newtonian.  

 

The budget seems to be about $50..

 

Jon

It is a very low budget, second hand only.


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#21 Steve Cox

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Posted 22 March 2020 - 08:44 AM

In this thread, we are not discussing ED/apo refractors, the discussion is about an ST-80, a 90mm Mak of unknown description and a long focal length 4.5 inch Newtonian.  

 

The budget seems to be about $50..

 

Jon

My apologies Jon, and to the OP.  There is a very similar thread over in the Equipment forum and the first time I read CN this morning I was on my phone, and guess I thought I was in the Refractor forum.  I'll delete my post.


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#22 Ford Prefect

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Posted 22 March 2020 - 10:34 AM

4.5 f11 newt
80mm f5 refractor
90mm f11 mak

Other than the wide field of he refractor, what is the relative difference in ds viewing. I understand the focal length issues. I’m really talking light gathering. Newt has central obstruction, as does mak. Aren’t all three essentially the same?

Some years ago I was at a dark site with a friend of mine, he had a 114/500 newtonian (f/4.4 so larger obstruction than a f/11) and I had my old 90mm Mak.

Even if the Mak impressed me on many objects (M31, M27, M57), the 114/500 showed much more light gathering on many target, and gave a magnificent view of the Veil with a 20mm eyepiece. The same night, I observed the veil through both an 80mm ED refractor and a 80mm f/5 refractor of other people, and the newtonian was better. I did not manage to see the Veil with the Mak, it was much dimmer than with the 80/500 and could point only a portion of the nebula at a time.


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#23 mikeDnight

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Posted 22 March 2020 - 12:38 PM

The 4.5" F11 I mentioned in my previous post as "leaving the Vixen FL102 standing" while looking at Saturn, was an old Japanese model. It had a cast front cell that incorporated a cast two vein spider that was quite thick. The scope had just been realuminized and brought back to its new condition cosmetically. I imagine most of us here know just how amazing the Vixen fluorite's are, and I like many others here love refractors with a passion;  but for whatever reason on that night, the 4.5" Newtonian outperformed the apo'. It was certainly not a toy in my view, but a very capable scope. I have no idea who the manufacturer was, and for all i know it could even have been a Vixen or a Tak'.Truthfulness has to prevail, and small reflectors shouldn't be so easily dismissed as being the poor cousin, as its not always the case. 


Edited by mikeDnight, 22 March 2020 - 05:15 PM.

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#24 Jond105

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Posted 22 March 2020 - 01:22 PM

Owning an ST80 in the past, I’m not a fan of these, it’s a fun scope. Super easy. I’ll say this though, as much as I’m not a fan of it, I’d prefer it over any of the other posted. A 90mm mak(I’ve recently become a huge fan of maks), is too little with a central obstruction, for me personally, and a 114 meet isn’t worth the spiking just Incase I want to push the power. I don’t think your plan is flawed. Even in my Bortle 9 sky, an 80mm f/5 would be the better choice if I could only take one and don’t think they are the same with the types of views you get. 


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#25 Steve Allison

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Posted 22 March 2020 - 06:05 PM

The 4.25" Edmund f/10 reflector I used to own gave superb images, even on the moon and planets. This was with its original rectangular secondary mirror, which was not ideal.


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