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76mm reflector vs 70mm refractor

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#1 Lou3

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Posted 27 July 2018 - 03:29 PM

I was thinking about getting the Orion Space Probe 76mm reflector, but would a 70mm refractor be better because of the reflector's secondary?

 

Thanks!



#2 vtornado

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Posted 27 July 2018 - 04:19 PM

Hello,

 

By nature of refraction vs reflection, the 70mm refractor is going to deliver about 15% more light

to the eyepiece than the reflector.   That is because besides the obstruction caused by the secondary

A mirror will only reflect about 90% of the light that hits it.  You have two mirrors and the secondary shadow.   I can't find the size of the secondary on these scope but assuming it is 10% of the area of the primary mirror.  So about 72% of the light entering the scope will be delivered to the eyepiece.

A decent refractor lens that has anti reflective coatings will deliver about 95% of the collected light

to the eyepiece.  In this case the efficiency of the refractor beats the larger aperture of the reflector.

 

What is the focal ratio (f number) of the 70mm scope you intend to buy?

You should stay more than f/8 otherwise chromatic abberation comes into play.

This is the purple haze around and over objects.

 

A reflector does not have this problem, because the light does not pass through glass.

It is reflected off its surface.

 

From experience, an 80mm f/11 refractor will show you a bit more than a 70mm.

It is slightly larger and only slightly more expensive, just something for you to consider.


Edited by vtornado, 27 July 2018 - 04:23 PM.

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#3 CollinofAlabama

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Posted 27 July 2018 - 04:30 PM

Another vote for the 70mm refractor.  Reflectors start to come into their own at about 130mm, and even then, it's pretty even between a 130mm and a 4" refractor.  On the planets, I'd take the 4" refractor, whereas the 130mm might perform a little better on smaller DSOs.  But the refractors will not have any coma, and assuming you're dealing with F/7 or above refractors, you'll get a much flatter field with a refractor.  Reflectors start making sense at 130mm, get better at 150mm, and become the only logical choice at and above 200mm, unless the refractor already has an observatory for itself.


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#4 donniesoprano

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Posted 27 July 2018 - 04:31 PM

Depends on use.  The reflector will have just a slight edge over the refractor in aperture, but it won't be something you'll notice.  Assuming both of these are the Alt/Az versions.

 

Others will chime in, but I'll keep it high level:

 

Refractor

Advantages:

Easy to use & aim for a beginner

Day or night uses with little/zero additional investment (for solar projection if desired, or a spotting scope for terrestrial views)

Can be moved to another mount in the future if desired

Should not require collimation

Disadvantages:

Generally higher cost (though at 70mm it's probably similiar to the reflector)

False color on brighter objects at pretty much any f/ratio in the price range of the reflector you mentioned

Can put you in more difficult viewing positions as you reach apogee than the reflector

 

The opposite is true for the reflector.

 

Both have introduced people to the hobby and both are inexpensive entry points.  Often, it just comes down to personal preference.

 

This is the reflector forum, so your replies may be biased.

 

However, I would suggest that an 80mm refractor be your first telescope.  It's a measurable bump over the 70mm and will serve you longer term if you decide to stick with it.  There's many very experienced people who still employ an 80mm refractor aside their much larger telescopes.

 

That's my opinion, anyway,

 

ds



#5 CollinofAlabama

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Posted 27 July 2018 - 04:42 PM

I would agree that an 80mm refractor is a better choice than either, but a 70mm F/10 refractor has an aperture to Focal Ratio of 3.6, so better than the Sidgwick limit of 3, and honestly, with very little chromatic aberration.  It will have some, but the Orion Space Probe 76mm has a spherical mirror, and is unlikely to deliver defraction limited views, even with its F/9.2 focal ratio.  A 70mm F/10 will deliver much better views of the planets, double stars, etc.  I'd say it's not even close, really.  Get the 70mm F/10.  Of course, if you can afford it, go with an 80mm F/11.25, but between the 70mm and a spherical reflector less than F/14, I'd go with the diffraction limited optic, in this case, the refractor.



#6 tony_spina

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Posted 27 July 2018 - 04:48 PM

How much are you looking to spend? And any size/weight limitations?  



#7 stargazer193857

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Posted 27 July 2018 - 07:20 PM

Reflectors are not good for low aperture. They have a central obstruction. Anything under 114mm should be a tefractor, and can have much wider views.

Also the 76mm has terrible spherical aberration.

#8 Redbetter

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Posted 27 July 2018 - 07:33 PM

In general, very small reflectors don't have much going for them compared to a refractor of similar/lesser diameter.    Assuming this is a beginner scope, the reflector will be somewhat more challenging to start, because of the need to collimate it (at least initially.)  Putting up with a central obstruction is not necessary in this size range, and doesn't even really confer a price advantage.  I don't know how well the mount included would work with this scope.

 

Small aperture inexpensive refractors have some problems as well.  Many now ship with erect image prisms that are less than ideal for telescopic viewing, making them a likely early upgrade item.  Short ratio achromats tend to have more problematic optical quality and will definitely have more chromatic aberration.  The longer ratio ones (e.g. 70mm f/10) will have a focal length similar to what you are considering, so the field of view will be about the same.  Included Barlows are typically junk.



#9 Jon Isaacs

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Posted 27 July 2018 - 10:53 PM

It will have some, but the Orion Space Probe 76mm has a spherical mirror, and is unlikely to deliver defraction limited views, even with its F/9.2 focal ratio

 

I happen to own a Orion Spaceprobe 3 and have owned at least one 70mm F/10 achromat.  

 

A 76mm x 700mm spherical mirror is 1/12 wave.  There is no need for a parabola, a spherical mirror is a nearly perfect parabola.  The refractor has 4 surfaces that must be precise spheres and unlike the reflector, the surfaces not only need precise spheres, they also need precise radii to work together.  

 

I would normally prefer a refractor over a similar sized reflector but in this case, this 76mm reflector has some real advantages both optically and mechanically.  One advantage to the Newtonian is that the eyepiece is at the upper end of the telescope, this means the tripod can be lower and more stable. 

 

But regardless of which one has the better optics, the real issues with scopes like these are the mounts and accessories.  In general, the mounts undersized (flimsy) and prone to wobble and vibration. They make finding objects, tracking and focusing more difficult.  It can be done but it does get in the way.  

 

Jon


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#10 Chesterguy1

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Posted 28 July 2018 - 04:04 PM

My mom got my niece a Celestron Powerseeker 114 mm reflector.  The mount was so poor that it was nigh impossible to even keep something as easy to locate as Saturn or Jupiter in the fov using the two plastic bodied EPs that came included with the scope. It’s almost always the really badly undersized and mechanically suspect EQ mount and accessories rather than the optics that make these introductory scopes a bad decision.  I would argue that it’s ill conceived business practice to sell such inferior products as it will certainly discourage a percentage of budding enthusiasts who might move on to something user friendly and useful.  Of course we have the good folks of CN to direct them if they find their way here. 

 

Luckily for my niece I was able to show her the planets in a smaller 90mm refractor mounted on a rock solid Alt/Az mount to prove that even a small aperture instrument can offer considerable enjoyment.

 

Gogiboy


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#11 CollinofAlabama

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Posted 28 July 2018 - 04:25 PM

I agree with Jon and Gogiboy.  The reality of all these sub $200 scopes is the utter lack of functionality of the mounts.  I'd say get something like the SkyWatcher 6" traditional dob, or don't bother.  If you can't muster $285, you probably should just save your pennies till you can.  Spending less usually means you'll have to spend much more for a good mount for whatever optic you purchase, and ultimately spending more than $285 for something less capable.



#12 Lou3

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Posted 28 July 2018 - 04:32 PM

How much are you looking to spend? And any size/weight limitations?  

I was hoping to keep it under $100.  I know what that represents in the world of telescopes, but this isn't meant to be a major-league telescope.  I want something small and light that's easy for my young son to carry.  I had an Observer 70 EQ, but sold it because the EQ mount didn't suit my usual grab-and-go observing style.  I'm working on Astronomical League observing programs, and I usually head out for quick observing sessions (usually 30-45 minutes) to observe a small list of objects planned earlier that day.  I've become a stickler for getting eight hours of sleep, so I need to keep set-up time to a minimum on summer nights.  It's not that I race through my list without taking the time to appreciate what I observe; it's because I don't want to rush through it that I don't want to spend much time setting up.

 

I was thinking about getting an Observer 70 AZ since I'm already familiar with that OTA, but the fork-and-rod AZ mount looks like it's a bit of a hassle.  What has your experience been?  By the way, is there a name for that kind of fork-and-rod mount?

 

I just found out about Celestron's new Inspire line.  If the tripod and mount are reasonably stable, one of those might be the answer.  I like its features, particularly its modular and collapsible design.

 

I happen to own a Orion Spaceprobe 3 and have owned at least one 70mm F/10 achromat.  

 

A 76mm x 700mm spherical mirror is 1/12 wave.  There is no need for a parabola, a spherical mirror is a nearly perfect parabola.  The refractor has 4 surfaces that must be precise spheres and unlike the reflector, the surfaces not only need precise spheres, they also need precise radii to work together.  

 

I would normally prefer a refractor over a similar sized reflector but in this case, this 76mm reflector has some real advantages both optically and mechanically.  One advantage to the Newtonian is that the eyepiece is at the upper end of the telescope, this means the tripod can be lower and more stable. 

 

But regardless of which one has the better optics, the real issues with scopes like these are the mounts and accessories.  In general, the mounts undersized (flimsy) and prone to wobble and vibration. They make finding objects, tracking and focusing more difficult.  It can be done but it does get in the way.  

 

Jon

I too like the idea of the eyepiece being higher up on the SpaceProbe.  Good point about the lower tripod being more stable.  But I keep going back to that secondary obstructing the light path on a reflector of that diameter.  Given equally stable mounting and a refractor with lenses within specs, which telescope is likely to show more details on planets and the moon (within the limits of this diameter, of course)?


Edited by Lou3, 28 July 2018 - 04:33 PM.


#13 JeffreyAK

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Posted 28 July 2018 - 06:23 PM

I was hoping to keep it under $100. 

 

I'd go used then, check Craigslist or Ebay.  Telescopes are a bit like exercise machines, especially on the low end, people buy them and never use them and then sell them very cheap.  Personally I've had a lot of fun with a grab-and-go 3" reflector on a simple alt-azimuth tripod mount, and at 3" f/9 or f/10 the spherical mirrors are pretty good and the views can be very nice and much better than a comparable-price refractor - if the telescope is collimated.  Eyepieces in this price range are usually mediocre, but that's a common issue with both refractors and reflectors, and maybe best addressed separately.  A used Orion SpaceProbe-type reflector and a decent used eyepiece could get you into the $100 range and leave you with something useful.



#14 macdonjh

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Posted 28 July 2018 - 07:04 PM

Lou3, sticking to your original question, and given that your intended use is grab and go with your son, I'd pick the refractor because it won't need collimation.  I understand Jon I.'s comments, but for keeping a kid interested, simple is most important.  Your son won't be impressed if he has to spend more time setting up than looking.  What he'll want to do is carry his scope outside, set it down and aim it at that yellow dot you tell him is Saturn.



#15 Lou3

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Posted 29 July 2018 - 12:46 AM

Thanks for the replies.

 

I agree that a 6" dob is generally the ideal beginner's scope, but it's too much for my son to carry.  I think a refractor or a small reflector would be more approachable for him.  I've also considered the SkyQuest 4.5XT.  We'd need to sit on a low stool, or put it on a table.  But as macdonjh says, collimation might discourage him.  That alone probably necessitates a refractor.  I'll look for something used and will let you know what I get.



#16 JeffreyAK

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Posted 29 July 2018 - 01:57 AM

You didn't mention how old your son is, or whether or not he's the type who might enjoy learning to collimate a telescope.  I did, myself, when I was 8 and learning on my first scope (a 3" reflector, similar to the SpaceProbe), but if he's much younger than that, he'd probably need a father/son teaching experience (that I never got, my dad was more the "go figure it out yourself" kind of person).  There's some value, if he's old enough and has a methodical approach, to learning that we often need to put a bit of effort into making something work well, but the reward is worth it.



#17 Binojunky

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Posted 29 July 2018 - 01:18 PM

Well theirs 70mm refractors and 70mm refractors, a cheaper achromat and an ED  high end doublet or triplet, anyway as others have stated, 70mm refractor,D.



#18 stargazer193857

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Posted 29 July 2018 - 07:53 PM

I did not realize it was f9.2. in that case spherical aberation will be readonable controlled. Lenses have two surfaces to spread the lead over.

The 76mm x 300mm will show you what shperical aberation looks like. I don't regret buying and selling it, since I got to see the aberation first hand.

#19 X3782

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Posted 30 July 2018 - 08:16 PM

Vixen A70LF at Amazon (finder has to be replaced though), 159 USD.

Portamount is sorta ok.




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