23 replies to this topic

[agmoonsolns]

How much larger does a good Maksutov-Newtonian need to be before it's on par with a good apochromatic refractor for planetary observing? I am curious about at what point the two are close to being equivalent? For example, is a 150mm Intes MN about the same as a Takahashi FC-125 at showing planetary details? What have been your experiences when it comes to comparing the two types of telescopes?

 

I am asking because I had very interesting results when doing such a comparison and I want to see if others have made similar observations. After letting this thread run for a while without biasing it with my own experiences, I'll post my results.

[Tyson M]

 

Interested in hearing the responses.   

 

I have an Intes 150 mak newt.  So far so good, but I have never had a tak, especially in that size range. Ive had a SW 120 apo though, just not at the same time. 

 

Seeing and transparency have been awful in the few times I have taken it out so far so I cannot offer anything to say on lunar or planetary.  Lunar views look promising.  I get a bit of vignetting on stars in the outer field because of the feathertouch focuser, but only on the edges of 70 degree EP's with 10mm EP's or smaller.

 

Collimation with laser is easy and it holds it fairly well, only needing the smallest of tweaks. 

 

It has replaced a really nice 6" achro and no regrets.  I really need to make a smaller custom dob base for it though. 

[Astro-Master]

I just bought the ES 6" F4.8 Mak-Newt, but can't compare it to my 105 APO yet because, guess what its cloudy and raining.  Sorry for all the bad weather in California, I guess I'm to blame.

 

However, my Intes 180mm Mak-Cass will put the APO to shame.

[Eddgie]

If it is an Intes Micro MN Deluxe model, it will be better than the Tak.  On par with a 6" Apo.  Amazing scopes.   It is the Intes Micro quality that makes this true as much as the small obstruction.   I have had the 6" and 5" and both had essentially perfect optics.  6" Put up views about the same as my 6" Astro Physics non-ED triplet.  Hard to tell the difference.

[dr.who]

In general with the central obstruction and the corresponding light loss as well as the loss of light from the reflectivity throughput of the mirrors a good reflector is usually comparable to a 1”/25mm smaller well made APO (true APO with three lenses that are well matched and figured not a two lens Achro or “ED APO”) refractor.

So using that your 6”/150mm Mak when properly cooled and collimated will perform as well as a 5”/127mm APO. However this assumes the same magnification in both scopes.

Both the Mak, especially a IM Mak, and a good quality APO will give you very nice views.

[Traveler]

A good Newton 6 inch F8 will give a 5inch APO a considerable hard time on planets as well.

[Erik Bakker]

Please keep in mind that thermally, these scopes behave very differently. So depending on location and observing conditions, differences between a top quality 6” MN and FC125 can be small to huge. Generally, the FC125 will perform well more often. When performing at it’s best, a good MN is wonderful.

[spencerj]

If it is an Intes Micro MN Deluxe model, it will be better than the Tak.  On par with a 6" Apo.  Amazing scopes.   It is the Intes Micro quality that makes this true as much as the small obstruction.   I have had the 6" and 5" and both had essentially perfect optics.  6" Put up views about the same as my 6" Astro Physics non-ED triplet.  Hard to tell the difference.

Agreed.  It was the Intes Micro optical quality that made those Mak Newts special.  The design itself is not super special.  Like all telescope designs, it has strengths and weaknesses.  When these scopes came out, the Apos they competed against were much more expensive (and there was a lot less variety).  At that time, they were a fantastic bargain for observers.  The landscape of available competition is much different today, but the Intes Micro Mak Newts still hod up well.     

[Jeff Morgan]

Please keep in mind that thermally, these scopes behave very differently. So depending on location and observing conditions, differences between a top quality 6” MN and FC125 can be small to huge. Generally, the FC125 will perform well more often. When performing at it’s best, a good MN is wonderful.

 

Over the summer I was using a Ceravolo MakNewt, and my local high desert climate features some rather sharp temperature drops after sunset. With some planning ahead (and the use of a Cat Cooler) the equalization can be managed.  When I say "planning ahead", I'm talking about 3 hours before session start.

 

Optically, the MakNewt is design is wonderful, I would take it over an equal aperture standard Newt any day. While the mathematical contribution of spider vanes to diffraction is small, it is still amazing what getting rid of those spikes does to the qualitative presentation of Jupiter or Saturn.

 

Obviously the refractor is more versatile, but within the province defined the MakeNewt is a strong choice.

[Cotts]

Check out this graph of Modulation transfer:  (from Suiter....)

 

 

Now, it is for comparing a 10-inch unobstructed optic to a 12-inch 20% obstructed optic.  But the curves look exactly the same for a 5 inch unobstructed vs a 6 inch 20%. ( The cycles per arc second numbers at the bottom will have to be reduced by 1/2 as well...)  You can scale the diagram for any two apertures where one is 1.2 times bigger than the other..... (3" vs. 3.6", 4" vs. 4.8", 5" vs 6", 6" vs. 7.2", 7" vs 8.4", 8" vs 9.6".......) Scale the numbers at the bottom as well...

 

What does it say?   It says that a an unobstructed scope of aperture X cannot keep up with a 20% obstructed scope of aperture 1.2X at any spatial frequency you care to name.  The bigger, obstructed scope will have better resolution and transfer more contrast to the eye.

 

The 'one-inch-bigger' rule of thumb is not really true if the central obstruction of the larger scope is 20% or less.  

 

enjoy your popcorn!

 

Dave 

 

 

 

 

[Jon Isaacs]

.

 

 

What does it say?   It says that a an unobstructed scope of aperture X cannot keep up with a 20% obstructed scope of aperture 1.2X at any spatial frequency you care to name.  The bigger, obstructed scope will have better resolution and transfer more contrast to the eye

 

 

Dave:

 

I like that graph and it makes the point I often try to make regarding the central obstruction, contrast and aperture.

 

But those three hours Jeff talks about and Erik's comments about thermal equilibrium are important practical aspects in a comparison like this.  Refractors are very reliable and perform their very best nearly all the time.  And when they are cooling, they're still quite solid performers.

 

I not sure if insulating the Mak Newt helps with thermal stability but it seems to be something people are doing with SCTs.  The Dutch seem to have been doing this for quite sometime but a Dutch ATM recently introduced the idea here on Cloudy Nights and people seem to be finding it effective.

 

Jon

[Cotts]

.

Dave:

 

I like that graph and it makes the point I often try to make regarding the central obstruction, contrast and aperture.

 

But those three hours Jeff talks about and Erik's comments about thermal equilibrium are important practical aspects in a comparison like this.  Refractors are very reliable and perform their very best nearly all the time.  And when they are cooling, they're still quite solid performers.

 

I not sure if insulating the Mak Newt helps with thermal stability but it seems to be something people are doing with SCTs.  The Dutch seem to have been doing this for quite sometime but a Dutch ATM recently introduced the idea here on Cloudy Nights and people seem to be finding it effective.

 

Jon

Jon, I agree about the thermal management.  My habit is to put the scope (any scope, actually) outside in the late afternoon the moment when the sun can no longer hit the tube.  This has never let me down - I rarely have to deal with tube currents.  

 

The thermal wrap strategy is gaining traction for those who have to take the scope out and begin observing more or less immediately...

 

Dave

[Darren Drake]

A classic take of this very question by Ed Ting.  Enjoy..

http://scopereviews.com/best.html

[Jeff Morgan]

What does it say?   It says that a an unobstructed scope of aperture X cannot keep up with a 20% obstructed scope of aperture 1.2X at any spatial frequency you care to name.  The bigger, obstructed scope will have better resolution and transfer more contrast to the eye.

 

“In theory, there is no difference between theory and practice. But in practice, there is.”

 

― Benjamin Brewster

 

Ultimately, I suspect the OP's answer will come down to price, availability, and ergonomics regardless of theory.

Edited by Jeff Morgan, 07 January 2019 - 03:00 PM.

[TG]

For planetary details, which tend to be low contrast, the rule of thumb often used for comparing obstructed vs. unobstructed scopes is to simply subtract the central obstruction from the aperture. I-M Mak-Newt f/6 scopes tend to be 20% obstructed, so the equivalent refractor would be a 4.8" (120mm one). This, however, ignores other important variables which may not actually make the "120mm equivalent refractor" perform better:

 

• Cooldown: as others pointed out, cooldown is critical for reflectors. A 120mm refractor is ready to use in a very short amount of time, perhaps instantly. Even if there are small thermals in the tube, they affect the reflector far more than the refractor.
• Smoothness of optics: this hurts mirrors much more than lenses, so the smoothness has to be top-notch to compete with refractors. Scatter is a particular concern.
• Mirror reflectivity: with modern optical glass and coatings, objective lenses transmit about 97% of light. A run-of the mill M-N with Al-coating will do only 0.88 (primary) *0.88 (secondary) * 0.97 (corrector) =75%. Big difference and one that needs to be factored into the equivalency comparisons.
• Body thermals: with a refractor, the object tends to be away, and also off the ground, lessening thermals. With a M-N, you are practically jammed against the front aperture
• Collimation: this needs to be perfect in the M-N to compete with a refractor which, if it's the Fraunhofer kind, doesn't care about collimation at all.
• Mounting requirements: I-M 6" Maks tend to be about 20lbs, I suspect several lbs heavier than 120mm refractors. Having a stable mount is really important for fine planetary viewing (unless your name is Jon Isaacs who can track ISS at 1000x by hand )
• Binoviewing: You need enough back focus and OCAs etc. with a M-N, which when inserted can actually protrude into the light path and produce diffraction effects and further contrast loss. With a refractor, you may already have enough back focus and an OCA even if used, doesn't obstruct anything. This point is especially important as binoviewing is one of the most effective ways to observe more on the planets.

 

Apart from the above, a refractor will provide better field illumination if wide-fields are used. The OP's aim is planetary viewing but this is something to keep in mind.

 

I don't mean to convince the OP not to get a Mak-Newt. They're great instruments and I have owned 3 myself. But, it's hard to beat a 20% smaller refractor, as I indicated above. I used to have a 5" f/15 D&G achromat that routinely provided better Mars viewing than a 6" f/6 0.984 Strehl M-N. Maybe a 7 or 8" M-N would outdo a 125mm apo but a 6" one would be fighting a losing battle.

 

Tanveer

 

[Jon Isaacs]

For planetary details, which tend to be low contrast, the rule of thumb often used for comparing obstructed vs. unobstructed scopes is to simply subtract the central obstruction from the aperture. I-M Mak-Newt f/6 scopes tend to be 20% obstructed, so the equivalent refractor would be a 4.8" (120mm one). This, however, ignores other important variables which may not actually make the "120mm equivalent refractor" perform better:

 

 That is a rule of thumb that tries to capture a complex phenomenon with a linear approximation.  The graph Dave posted is a more exact solution and shows that the 12 inch obstructed scope has better contrast transfer characteristics than the 10 unobstructed scope.  It's my understanding that the "clear aperture approximation" over estimates the effect of COs in smaller sizes (20% for sure) and underestimated them in the larger sizes.  

 

But, I do agree that there are many factors and as Erik said, refractors generally operate at near peak performance, reflectors, for a variety of reasons (as you documented) tend to be more finicky.

 

Jon

[JohnH]

I own a pair, an MN 76 Intes Micro and an M809 Intes Alter.

They are over 20 lbs each, so need a substantial telescope Mount hold them steady. To keep the thermal problem to a minimum, I keep both scopes in an unheated basement or garage in their cases or in a dust-free area. The one thing I did notice witj the MN76 is that the usable field is much sharper by and large, over do a lot of Newtonian especially the faster ones due to Coma becoming a problem

[Jon Isaacs]

I own a pair, an MN 76 Intes Micro and an M809 Intes Alter.

They are over 20 lbs each, so need a substantial telescope Mount hold them steady. To keep the thermal problem to a minimum, I keep both scopes in an unheated basement or garage in their cases or in a dust-free area. The one thing I did notice witj the MN76 is that the usable field is much sharper by and large, over do a lot of Newtonian especially the faster ones due to Coma becoming a problem

A Paracorr II will correct the field so that even at F/3.5 the coma is substantially less than the Airy Disk 20mm off-axis, that would be the edge of the field of a 31mm Nagler. 

 

 

 

http://www.televue.c...2_spotsizes.pdf

 

The yellow area is the radius of the Airy Disk, the black line is the spot size of the coma.  From what I can find,  Mak-Newts are not nearly as well corrected..  An F/3 Newtonian with a Paracorr II operates at F/3.45 and has the coma of an F/11 Newtonian.  At F/3.5 the coma is less than the Airy disk over the entire field.

 

A Paracorr II costs slightly less than $500 and is a one time purchase.  I have 5 Newtonians and I use it in all of them.  It's almost like you could buy a device and easily swap it between achromats and it would turn them in the apochromats.

 

Something to think about anyway.

 

jon

[Eddgie]

One must remember that no telescope made is perfect.   Even the best MNs and the best Apos are going to be somewhat short of perfection, so any attempt to plot MTF is going to overstate reality.

 

That being said, it is almost assured that an Intes Micro MN66 Deluxe will have a manufacture Strehl in the .97 range.

 

What is also true that unless the 5" is a triplet of maybe f/7.5 and utilizing something like ZKN7/FPL53/ZKN7, the Polychromatic Strehl will by itself be lower than the .96 or so Strehl you would get from that combo.

 

This means that you have to step up with a really expensive 127mm Apo made to Astro-Physics levels of perfection to get to the perfect 127mm aperture line depicted on this chart.

 

Also, this chart shows something that is typically ignore in these kinds of dialogs, which is the observer's contrast sensitivity threshold, and this is sown on this chart.   Now the 6" will have some transmission loss, so that is in fact likely to slighly lower it from what is shown here, but it would almost for sure be no worse than a 5" Apo, assuming the MN66 is a later model with the improved coatings. 

 

My experience is that the Intes MNs are outstanding.  It will take an exceptional 5" Apo to equal it.

 

For what it is worth:

 

 

My tolls do not allow me to actually plot the smaller scope, so these curves are generated by hand to reflect the fact that the 5" scope is 83% of the fsmax of the 6" scope.  It is pretty easy to freehand that kind of line and while it might not be perfect, it is close enough I think. 

 

 

 

[Eddgie]

And regarding the chart from Suiter's Book.  This chart is accurate, but it ignores the eye contrast sensitivity threshold.  A camera used in the 12" scope in Suiter's book would be able to exploit that contrast transfer at the higher frequencies, but the human eye gives up much earlier than this.

This is why when you see MTF plots, they will typically have something like "at the important visual frequencies" with reference to the chart.   The eye simply can't see the lowest contrast, finest resolution detail that the scope can provide.  For the case of a 10" vs 12" obstructed though, this level is considerably to the right and lower of the point indicated on the plots I have drawn.

Notice on those plots that the 5" scope would have a slighly higher threshold for visual use than the 6".  Again, as has been stated, the transmission of the 6" would be reduced enough that in the real world, the 6" might not have the indicated limit, but if the scopes were 10" and 12", the threshold would be both to the right and below the threshold of these two scopes.

Again, CN dialogs almost never consider the benefit that luminance provides to the viewer.  In my own experience, one of the greatest benefits of ever larger aperture (when it comes to planetary observing) is the tremendous gains in luminance, which improves the obsever eye contrast sensitivity.  If you make no other change but to make the image brighter, the eye will see see the details more easily, and may see details in the brighter image that escape the dimmer image.

 

With that being said, neither of these would by my choice for a planetary scope, but my experience is that the MNs are quite excellet and hold their own against similar size refractors.   They really are superb. 

[glend]

I owned an original spec (with the small 50mm secondary) MN190, and with a Moonlight focuser it was an exceptional visual and AP scope. It is pitched as an Astrograph and Focus is setup for AP but it can be used visually with high magnification. I also have APOs.  In my experience the MN190 is the equal of say a 130mm APO, and of course a Mak-Newt will always have perfect colour correction, which gives it a price advantage over many APOs. The old adage is that "a good Mak-Newt is as good as an APO at 1/3 the cost".

 

Note that the current production version of Skywatcher MN190 has a larger secondary ( to provide for a larger image circle for larger sensors, whereas the original version was fine up to APS/C sized sensors). Personally I would not buy the larger secondary version over an APO. The same applies to the Orion version of the MN190, the secondary size is a problem.

Edited by glend, 14 January 2019 - 03:35 PM.

[MalVeauX]

How much larger does a good Maksutov-Newtonian need to be before it's on par with a good apochromatic refractor for planetary observing? I am curious about at what point the two are close to being equivalent? For example, is a 150mm Intes MN about the same as a Takahashi FC-125 at showing planetary details? What have been your experiences when it comes to comparing the two types of telescopes?

 

I am asking because I had very interesting results when doing such a comparison and I want to see if others have made similar observations. After letting this thread run for a while without biasing it with my own experiences, I'll post my results.

For planetary observing, strictly, a well made Mak-Newt (such as Intes) will be as good as a well made APO refractor with the same aperture. You won't even notice the small obstruction difference. If you were getting critical about it, about "an inch" is safe to say, without getting into extreme subjective opinions. Note that observing is very much a practiced skill, in addition to using an instrument. Another observer can have a totally different, or lack there of, experience observing with these. Keep that in mind. A critical observer with experience and skill would have to really struggle to find an appreciable, pre-producable for others, difference between a good Mak-Newt and a good APO with the same aperture on planetary observing, assuming thermal acclimation and equivalent properties are all optimal.

 

In reality? Well, the APO is better. No collimation fuss. No thermal woes. Instant point and look.

 

To make fussing with collimation and thermal acclimation and all that worth while? I would say the Mak-Newt needs to be at least 2 inches more aperture to really become an obvious choice. Meaning, a 150mm Mak-Newt would be a great choice, over a 102mm APO, for this purpose. But fighting over a 1 inch difference? Truly a subjective difference, if any, and likely not.

 

There's a lot to be said about the comfort of the eyepiece too. For that, the Mak-Newt wins in my book. Being able to use a longer eyepiece that is more comfortable due to the naively much longer focal length can make a difference in observing simply by being comfortable.

 

Very best,

[Cotts]

For planetary observing, strictly, a well made Mak-Newt (such as Intes) will be as good as a well made APO refractor with the same aperture. You won't even notice the small obstruction difference. If you were getting critical about it, about "an inch" is safe to say, without getting into extreme subjective opinions. Note that observing is very much a practiced skill, in addition to using an instrument. Another observer can have a totally different, or lack there of, experience observing with these. Keep that in mind. A critical observer with experience and skill would have to really struggle to find an appreciable, pre-producable for others, difference between a good Mak-Newt and a good APO with the same aperture on planetary observing, assuming thermal acclimation and equivalent properties are all optimal.

 

In reality? Well, the APO is better. No collimation fuss. No thermal woes. Instant point and look.

 

To make fussing with collimation and thermal acclimation and all that worth while? I would say the Mak-Newt needs to be at least 2 inches more aperture to really become an obvious choice. Meaning, a 150mm Mak-Newt would be a great choice, over a 102mm APO, for this purpose. But fighting over a 1 inch difference? Truly a subjective difference, if any, and likely not.

 

There's a lot to be said about the comfort of the eyepiece too. For that, the Mak-Newt wins in my book. Being able to use a longer eyepiece that is more comfortable due to the naively much longer focal length can make a difference in observing simply by being comfortable.

 

Very best,

Which of the three do you really mean?

 

Dave

[MalVeauX]

Which of the three do you really mean?

 

Dave

Each, exactly in the the context provided that you left out.

 

Strictly observing planets, it would be difficult to know commonly if the apertures were not the same.

 

For a critical observer with experience, the conventional wisdom of 1" aperture difference can apply without too much subjective design bias.

 

To make it really obvious and worth it, 2" aperture difference. Even gave an example of 150mm vs 100mm.

 

So yes, all three. I think it's way understated in these forums the experience and observation skill of the observer, is often totally left out of context and people take it to the hair splitting level.

 

Very best,

Edited by MalVeauX, 16 January 2019 - 08:01 AM.