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How much does a secondary affect the view?

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#1 Thaellar

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Posted 11 April 2018 - 11:27 PM

Let's use a hypothetical 8" dob. If the secondary blocks 25% of the tube, does that mean it would equal a 6 inch refractor with same focal length/ratio and they would have similar views?

 



#2 ShaulaB

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Posted 11 April 2018 - 11:41 PM

Yes, that's what the guys say in a lot of the posts. However, to buy a 6" refractor with zero chromatic abberation, you are handing over some serious dinero. Reflectors of any size do not have a problem with chromatic abberation. When they do, it is the eyepiece that is at fault. My feeling is, keep your reflector well collimated, use good orthoscopic or high quality eyepieces, and you will get nice crisp images of planets and the Moon. No need to stress out your mastercard buying the massive 6" refractor, and the massive mount and tripod it needs to ride upon.


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#3 SeattleScott

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Posted 12 April 2018 - 01:41 AM

A 25% CO means 25% of the diameter, not the surface area. I have a 10” with 25% CO. The secondary mirror blocks 6% of the light. A buddy has an 8” SCT. Something like 34% CO, blocks about 12-14% of the light. Not my scope so I don’t remember the exact figures.

In general I tend to give refractors an extra inch. My 6” Mak is advertised as being comparable to a 5” Apo. A 7” Apo is considered approximately equal to an 8” Mak newt. So yes refractors are more efficient, between no light being blocked by the secondary, and lenses transmitting more light than mirrors. But it isn’t like a 6” refractor will deliver just as bright of views as an 8” reflector or SCT. It would be more equivalent to a 7” obstructed scope.

And yes, the refractor (unless it is a very expensive Apo) will have issues with chromatic abberation which will limit its usefulness. Not saying there is no place for a 6” achro. But I expect 8” Dobs and 8” SCT each outsell 6” achros by a 10-1 margin.

Scott
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#4 sg6

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Posted 12 April 2018 - 03:30 AM

Since the blockage is by area then a 25% secondary is down at 6% by area so that makes a significant change.

 

I would say aperture blocked is almost insignificant, if your secondary blocks 6% then take into account the loss at the 2 reflection surfaces, they are likely to be down at 85% transmission so each mirror throws away 15%, 2 mirrors means 0.852 = 0.72 or 72% gets to the eyepiece. So at the mirrors you have lost 22%, the blockage of the sceondary is getting irrelevant.

 

That nice 6 element eyepiece = 12 surfaces, even if 1% loss at each surface means 0.9912 = 88% transmission so another 12% has gone. shocked.gif shocked.gif

 

So transmission at surfaces is now 0.72x0.88 = 0.63 or 63%. bawling.gif bawling.gif

 

The presumption is that everythiung reflects and transmits at 100% perfection. Except that is wrong and wrong by a lot. So your 6% blockage by the secondary eventually becomes irrelevant. Also the mirror coating is for protection and not for transmission so that is going to be pretty poor.

 

The above is likely why a good doublet well ant-reflection coated and simple eyepieces TV Plossl and Ortho's have such a following. It is also why you pay a fair chunk extra for the high reflection coatings.


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#5 Jon Isaacs

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Posted 12 April 2018 - 05:19 AM

That nice 6 element eyepiece = 12 surfaces, even if 1% loss at each surface means 0.9912 = 88% transmission so another 12% has gone. shocked.gif shocked.gif

 

 

In general, a 6 element eyepiece may consist of pairs of achromats so there are only 6 air - glass surfaces.  And multicoatings can be better than 99% transmission.  Tests on Type 6 Nagler eyepieces with 7 elements show the transmission to be in the 95%-96% range.  

 

As far as the original question, the effect of the Newtonian secondary: There is no simple answer to this question.  There are three factors to consider, the light throughput, the resolution and the fine scale "planetary" contrast.

 

-Throughput:  As has been discussed, a 25% secondary covers 6.25% of the mirror's area so the light throughput is 94%.  One also has to consider the reflective losses of the two mirrors. These numbers vary depending on the type of coating and the coatings age, brand new high quality coatings might be 95%.  If one uses 90% which seems like a reasonable number for relatively new coatings, then each mirror transmits 90% of light, the two of the 81%.  Multiple the losses together, this means overall throughout is 76%.  Compared to a telescope without a secondary and perfect transmisson, this would be the equivalent of a 7.0 inch.  A refractor with a dielectric diagonal is quite close to perfect, with a standard diagonal, it would lose about 10%.

 

- Resolution:  The pure resolving power, the ability to resolve a close double star, is unaffected by the central obstruction.  

 

- Fine scale "planetary contrast":  This is where things get more complicated.  The short story is the secondary obstruction causes a loss of fine scale contrast when viewing the planets.  There's a lot of analysis and discussion of the effect of the CO but there are a few generalizations:  A CO under 20% has very little effect.  There is an old "clear aperture" rule of thumb:  Subtract the diameter of the secondary from the aperture and you have the aperture of the equivalent unobstructed (refractor).  This is where the 8 inch Newtonian = 6 in refractor comes from.

 

The long story is this:  A objective or mirror does not produce stars that are truly points of light, they produce a dot of light surrounded by a series of faint rings, this is called the Airy disk.  The angular diameter of the Airy disk is inversely proportional to the aperture.  The greater the aperture, the smaller the disk.  This is why larger telescopes have greater resolving power, they produces smaller points.  A reasonably analogy is the pixels on your computer monitor or smartphone screen. The smaller the pixels, the greater the resolution.  This is an image of the Airy disk:

 

AiryDisk_1.gif

 

http://www.rocketmim...lvingPower.html

 

On steady night, one can see the Airy disk structure at high magnifications.  It's easier with smaller telescopes since the disk is larger.  

 

The effect of the secondary mirror is transfer energy from the bright central disk into the diffraction rings.  If one thinks of an image as being produced by a series of Airy disks the way your computer screen produces an image, then it can be seen that a dimmer central disk with brighter rings would tend to smear the image of very fine details.  

 

Bottom line:  Comparing telescopes of different types is not easy.  Many factors come into play, the quality of the optics, the thermal stability of the telescope, the central obstruction, the aperture of the telescope.  Far and away, the most important factors are the quality of the optics and the aperture. And of course the cost.  

 

Last year I was privately discussing splitting some close doubles with a fellow Cloudy Nights member on the east coast.  He was using a 175mm apochromatic triplet refractor that cost $20,000.  I was using a 10 inch (250mm) Dobsonian that I bought used on Astromart 15 years ago for $240.  One double in particular I had split cleanly with my $240 scope which had eluded the expensive refractor.  This was due to the greater resolving power of the larger aperture and the more stable atmosphere (better seeing) of my location. 

 

Jon


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#6 Alex McConahay

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Posted 12 April 2018 - 08:31 AM

One other thing to consider.....a typical SCT or newt of 8 inch aperture is a rather pedestrian assemblage. It is good and useful, but would not be considered a premium instrument. It would not be built to tight tolerances but according to the market. It would be a Ford, Chevy, Toyota.

 

A six inch refractor would probably be a pretty fancy schmancy telescope. Nobody is going to try to get a bargain on that much glass. So, it would be built to very high standards of optics, coatings, and mechanical engineering. It would be a Lincoln, Cadillac, Lexus.

 

Not necessarily true, of course, but probably. 

 

I would not expect the mass produced SCT or newt to perform as well as the handcrafted refractor. 

 

Alex


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#7 SeattleScott

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Posted 12 April 2018 - 11:54 AM

One other thing to consider.....a typical SCT or newt of 8 inch aperture is a rather pedestrian assemblage. It is good and useful, but would not be considered a premium instrument. It would not be built to tight tolerances but according to the market. It would be a Ford, Chevy, Toyota.

A six inch refractor would probably be a pretty fancy schmancy telescope. Nobody is going to try to get a bargain on that much glass. So, it would be built to very high standards of optics, coatings, and mechanical engineering. It would be a Lincoln, Cadillac, Lexus.

Not necessarily true, of course, but probably.

I would not expect the mass produced SCT or newt to perform as well as the handcrafted refractor.

Alex


I would agree with this assuming the 6” refractor is an Apo refractor. There are budget 6” achromats out there, like the Celestron 6” F5.

Scott

#8 JCAZ

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Posted 12 April 2018 - 12:05 PM

It has a huge impact. Without it you can't see the image created by the primary.
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#9 Feidb

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Posted 12 April 2018 - 12:20 PM

I don't even think about it or care. It's part of the system. All I care about is that it's clean, has a good coating and is flat. That's about it.

 

If I want super fine details, I just use an aperture mask and "cheat" the optical system. Tutherwise, not much use worrying about it. If you have any kind of conventional reflector scope, you can't avoid one. Let me be clear, "conventional," not tri-shiefspiegler, if I spelled that right, or tilted whatever.



#10 Alex McConahay

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Posted 12 April 2018 - 12:42 PM

>>>>>> Without it you can't see the image created by the primary.

 

Sure you can, but the image is degraded by the diffraction spikes caused by your silly old big ears.

 

Alex


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#11 pkrum

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Posted 12 April 2018 - 01:58 PM

How about the effect of greater aperture. At some point, the airy disk becomes too
small to see. So central obstruction size is more important with small scopes than big
ones. The question is when is the aperture so big that central obstruction ceases to
matter (besides the small loss in light gathering). Thoughts anyone?

#12 howardcano

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Posted 12 April 2018 - 02:04 PM

How about the effect of greater aperture. At some point, the airy disk becomes too
small to see. So central obstruction size is more important with small scopes than big
ones. The question is when is the aperture so big that central obstruction ceases to
matter (besides the small loss in light gathering). Thoughts anyone?

As Jon already mentioned, less than 20% is generally imperceptible.



#13 dgoldb

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Posted 12 April 2018 - 02:22 PM

Consider that the image in your eyepiece likely has a ~30-50% brightness drop-off from the center to the edge (yet most people can barely perceive it).  Your secondary is a fraction of that (as other said, its like ~6%).  So, no, a 6 inch refractor is not like an 8 inch dob because of the secondary.  The secondary has a small effect.  



#14 pkrum

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Posted 12 April 2018 - 02:58 PM

How about the effect of greater aperture. At some point, the airy disk becomes too
small to see. So central obstruction size is more important with small scopes than big
ones. The question is when is the aperture so big that central obstruction ceases to
matter (besides the small loss in light gathering). Thoughts anyone?

This refers to low contrast planetary where obstruction effects matter most.

#15 Redbetter

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Posted 12 April 2018 - 03:11 PM

It becomes very challenging to compare different designs and different apertures to arrive at a conclusion about what are similar or equivalent views.  Jon covered it well above. 

 

Equivalent or similar are also in the eye of the beholder so aesthetics are a major subjective consideration.  Some people find secondary diffraction spikes highly objectionable.  Refractors have an advantage in that they can put the most light in the central disk or first diffraction ring.  This can make the image appear more crisp.  Smaller aperture under greater magnification will have a darker background (smaller exit pupil) compared to larger aperture at the same magnification.  Obstructions can soften the image somewhat, as can roughness or various aberrations.    The smaller aperture and arrangement of the objective tends to make refractors more forgiving of seeing and thermal equalization--up to a point. 

 

I have tried to do some analysis of different sizes comparing the encircled energy of different obstructions at the same angular distance compared to a refractor to see where things cross.  All else being equal the contrast for fine detail of equivalent quality of figure/color correction/etc. seems to be rather close to the adjustment for linear obstruction percentage.  This doesn't account for greater image brightness of the larger aperture, or the somewhat greater point source resolution of high contrast areas (e.g. resolving two very close airy disks.)  However, it does seem to match some of what I have noticed comparing planetary views through my refractors, SCT, and Dobs.  (Now if I just had a 6" ED/apo for comparison...)  

 

One of the more noticeable aspects of the numerical evaluation that seems to carry over to the eyepiece has been the "softness" of planetary images at higher powers  in my SCT.  Some of this could be roughness or issues with the figure, but the calcs indicate that a surprisingly large remainder of light is spread over a much wider area than for no or low obstruction.  For me the result is that even when it was in near perfect seeing it topped out at around 38x/inch, vs. 50x/inch with ED refractors.  I have not had the same level of seeing to explore the limits of the 10" Dob (~25% obstruction) but I have had some indication that it is topping out somewhere above 400x for my eye, roughly 42x/inch, maybe a bit more. 

 

As an approximate rule of thumb for planetary detail, subtracting the linear obstruction seems to be a reasonable approximation.  It is probably somewhat conservative for evaluating the performance of the obstructed scope with respect to planetary detail, but not dramatically so. 

 

Limiting stellar magnitudes (and DSO's and moons) are a different matter more closely correlated to total system light transmission and effective aperture. 



#16 BillP

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Posted 12 April 2018 - 04:39 PM

Let's use a hypothetical 8" dob. If the secondary blocks 25% of the tube, does that mean it would equal a 6 inch refractor with same focal length/ratio and they would have similar views?

 

Let's assume the primary and secondary of your hypothetical 8" Newt with a 25% CO has enhanced coating on the mirrors that get you 94% reflectivity.  On the refractor, let's assume it is a triplet and each air-glass interface (6 of them) have 99% transmission and the diagonal is a 99% reflective dielectric.

 

Based on the above assumptions a 7.57" refractor would have the same light gathering as the 8" Newt.  So not enough of a resolution difference between the two to matter.  Of course, you cannot buy a production refractor that is 7.57".  But of course you can get a 152mm one quite easily.  In that case, a Newtonian would have to be 6.37" to have the same light gathering as the 6.0" refractor.

 

The views will be different between the two and the character of the view will be different: 

  • The Newtonian, if a standard faster focal ratio, will show coma in the off-axis and on bright stars and planets will show spikes of light from the spider vanes holding the secondary mirror. 
  • The visibility of scatter from dust on the optics or imperfections in the coatings is higher when on mirrors as opposed to being on refractive glass, so a mildly dusty mirror will show more scatter around a star point than a mildly dusty refractive optic (this is according to both theory and easily seen for me). 
  • Finally, given that the heat being shed from the Newtonian's mirror travels up the tube and through the light path, whereas the heat from the glass objective of the refractor is at the other end and rises immediately out of the light path, the Newtonian will show an unsteady view longer than a same glass mass refractive objective. 
  • And since the mirror in the Newtonian is deep in the tube, shielded more from ambient air flow, it will also take longer to acclimate. 

With all that said however, when both instruments are fully acclimated with no thermals, and the optics are very clean and well collimated, the views can be quite close (except for the coma and spider vane spikes in the Newtonian). 

  • However, if you buy a Paracorr to correct the coma and replace the standard spider with a curved vane spider, the views will get closer.  But after all of that, realize that the 8" Newt is hgher resolution so the spurious disk of a star point is smaller. 
  • That being so means that you will need higher magnification to see those pretty little classic airy disks from star points, and getting to the higher magnification to view those will be more problematic as it requires a steadier atmosphere. 

So generally that "aesthetic" view will be a little harder to attain from the larger aperture Newtonian.  With every gain, you have a loss.


Edited by BillP, 12 April 2018 - 04:57 PM.


#17 Stephen Kennedy

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Posted 12 April 2018 - 04:52 PM

When I saw the title of this thread I thought it was going to be about how the quality of the secondary affects the image. The quality of the secondary actually has a considerable impact on the quality of the image in a reflector. In the case of a Newtonian where the only purpose of the secondary is to reflect the image out the side of the OTA to a position where it can be conveniently viewed, quality of the secondary is important. In his book on "How to Make a Telescope" Texereau bemoans the fact that many very carefully made well corrected primary mirrors requiring many hours of work never reach their potential because they are paired with a sub-standard secondary.

To produce good images the secondary on a Newtonian has to be truly flat and making a mirror flat is not as trivial as it may sound. The secondary has to be flat to a precision of 1/10th of a wavelength and the better ones are lambda 15 or even lambda 20. This is not an easy task to accomplish and test.
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#18 rowdy388

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Posted 12 April 2018 - 09:40 PM

That's what I thought the thread would be about as well. Good post.


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#19 Asbytec

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Posted 13 April 2018 - 02:21 AM

"Some of this could be roughness or issues with the figure, but the calcs indicate that a surprisingly large remainder of light is spread over a much wider area than for no or low obstruction. For me the result is that even when it was in near perfect seeing it topped out at around 38x/inch, vs. 50x/inch with ED refractors."

Red, that's intetesting. It sounds as if your saying the reduction in peak intensity is partly responsible for image brightness. I've often wondered the same.

Like you, I max out on Jove near 0.6mm exit pupil or 40x/inch. I believe refractors can push a little higher, by all accounts, closer to 0.3mm exit pupil or above 50x/inch.

Discussion like this are fascinating, its great to know how our equipment works. Bottom line is, though, an 8" Newt is a capable scope in it's own right and despite the presence of 6" APOs.
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#20 Asbytec

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Posted 13 April 2018 - 02:24 AM

"With every gain, you have a loss."

Don't get me started. :)

Makes me wonder how we get anywhere. That said, one of the largest gains to be had has nothing to do with our equipment. Rather improving our own abilities

Edited by Asbytec, 13 April 2018 - 02:27 AM.


#21 Jon Isaacs

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Posted 13 April 2018 - 02:56 AM

That's what I thought the thread would be about as well. Good post.

 

Read the first post.  This thread was originally in the beginners forum. 

 

Jon



#22 Jon Isaacs

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Posted 13 April 2018 - 03:05 AM

Let's assume the primary and secondary of your hypothetical 8" Newt with a 25% CO has enhanced coating on the mirrors that get you 94% reflectivity.  On the refractor, let's assume it is a triplet and each air-glass interface (6 of them) have 99% transmission and the diagonal is a 99% reflective dielectric.

 

 

I think your numbers are optimistic for the Newtonian,  pessimistic for the refractor . For example,  A-P specs their through put at a minimum of 97%. That's for a triplet , a doublet could be slightly better. 

 

On the other hand Newtonian coatings degrade with time,  lens coatings do not.  I think that 90% reflectivity is optimistic for a mirror that is not brand new. 

 

Jon



#23 Redbetter

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Posted 13 April 2018 - 03:37 AM

"Some of this could be roughness or issues with the figure, but the calcs indicate that a surprisingly large remainder of light is spread over a much wider area than for no or low obstruction. For me the result is that even when it was in near perfect seeing it topped out at around 38x/inch, vs. 50x/inch with ED refractors."

Red, that's intetesting. It sounds as if your saying the reduction in peak intensity is partly responsible for image brightness. I've often wondered the same.

Like you, I max out on Jove near 0.6mm exit pupil or 40x/inch. I believe refractors can push a little higher, by all accounts, closer to 0.3mm exit pupil or above 50x/inch.

Discussion like this are fascinating, its great to know how our equipment works. Bottom line is, though, an 8" Newt is a capable scope in it's own right and despite the presence of 6" APOs.

There was a discussion about this a few weeks ago (can't recall where.)  From what I recall several of us were talking about the ratio of light outside of X number of diffraction rings vs. the encircled energy within.  It starts becoming rather significant at SCT type obstructions.  The calcs I have done in the past are derivative from Telescope optics.net central obstruction values--I did some interpolations which are only going to be generally correct rather than truly quantitative for these complex functions since key ones fall precisely midway between their table values. 

 

By the end of the 3rd ring refractors and unobstructed scopes have about 95% of the energy within, and surprisingly short range to that.  At 25% C.O. it drops slightly to a little under 94% (0.938) encircled energy and by 35% C.O. it is down to about 92% (0.918).  While the numbers don't sound that different, the recovery from there on out is slow so the ratio outside that is a better representation of the degradation of low contrast features.  Very roughly the inside/outside ratio past the 3rd ring will be nearly 20:1 for a refractor, and not too far from that with obstructions less than 20%.  At 25% the ratio is about 15:1, and by 35% is about 11:1.   (Also, this doesn't account for the somewhat slower recovery of all the obstructed apertures in the inner rings.)  So I guess one way to think of it is the SCT having far more noise that will reduce lower contrast details when side by side with typical Newt, and more so vs. a refractor.  This to me is the essence of the somewhat airbrushed effect I see in all of the SCT's I have viewed through, including my own.  

 

At very low obstructions, such as 15%, the ratio above is a still impressive 18:1. 

 

P.S. I think you meant to type 0.5mm vs. 0.3mm above. 



#24 Asbytec

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Posted 13 April 2018 - 04:20 AM

Red, yes, I think your hitting on the problem for unequal doubles within that range due to loss of contrast at small scales. Same with planetary detail at small scales. Does that also affect surface brightness? All of that diffracted light is still contributing to the image. Dunno.

I meant 0.3mm according to some claims of reaching higher power in a refractor before the image dims sufficiently on the retina.

#25 daquad

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Posted 13 April 2018 - 12:01 PM

To follow up a bit on what Jon said in post #5, consider the Orion 8" f5.9 dob.  The central obstruction is 47 mm, so the light loss to the obstruction is (47/203)^2 = 5.4 %.  The mirror coatings are advertised to be 94% reflective.  So, ignoring loss due to the spider vanes, the net throughput efficiency is (1-.054)X0.94^2 = 0.836. 

 

The equivalent aperture is SQRT(0.836)X8" = 7.3".  The coatings probably won't remain at 94%, and at some point may deteriorate a few percent, so a conservative estimate would put the equivalent aperture of the Orion 8" f/5.9 dob equal to a 7" aperture with 100% transmission efficiency.

 

Using the conservative rule of thumb that Jon alluded to, the performance of the 8" dob on subtle planetary detail should be around (203 -47) = 156 mm, that is, about the same as a 6" refractor with good color correction.

 

Not a bad deal.

 

Dom Q.




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