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Do double stars have more seperation and look better through larger scopes

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#1 kingsbishop

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Posted 25 January 2023 - 08:34 AM

Hello i am wondering do double stars look better and have more seperation through larger scopes or does seeing block that no matter what appeture you use?

#2 TOMDEY

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Posted 25 January 2023 - 08:55 AM

The wavefront quality of the scope (as built, aligned and thermally stabilized that night) and the seeing as presented that night. If both of those are superb --- then the presentation is indeed proportionally improves with aperture. Most often, especially for common backyard sites, seven inches (or smaller) is the cut-off aperture... with occasional moments where larger aperture clearly and decisively outperforms smaller. Which is to say - if you wait long enough - quality big to giant aperture will eventually win out. But the wait could be minutes, hours, or months. You can improve your odds at a site known for its superb seeing. Similar comment regarding planets and the moon.   Tom


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#3 kingsbishop

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Posted 25 January 2023 - 09:04 AM

The wavefront quality of the scope (as built, aligned and thermally stabilized that night) and the seeing as presented that night. If both of those are superb --- then the presentation is indeed proportionally improves with aperture. Most often, especially for common backyard sites, seven inches (or smaller) is the cut-off aperture... with occasional moments where larger aperture clearly and decisively outperforms smaller. Which is to say - if you wait long enough - quality big to giant aperture will eventually win out. But the wait could be minutes, hours, or months. You can improve your odds at a site known for its superb seeing. Similar comment regarding planets and the moon. Tom

but if larger appeture reduces the size of thr airy disk then it should be better no matter how bad the seeing is?
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#4 Jon Isaacs

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Posted 25 January 2023 - 09:07 AM

The wavefront quality of the scope (as built, aligned and thermally stabilized that night) and the seeing as presented that night. If both of those are superb --- then the presentation is indeed proportionally improves with aperture. Most often, especially for common backyard sites, seven inches (or smaller) is the cut-off aperture... with occasional moments where larger aperture clearly and decisively outperforms smaller. Which is to say - if you wait long enough - quality big to giant aperture will eventually win out. But the wait could be minutes, hours, or months. You can improve your odds at a site known for its superb seeing. Similar comment regarding planets and the moon.   Tom

 

As the relator's say, Location, Location.Location. 

 

My urban backyard is probably better than most for observing close doubles. It 4 miles from the cool Pacific Ocean, normally south of the jet stream in a area with a very mild climate. My primary double star instruments are my 10 inch and 13.1 Dobs.  

 

It's an effort... 

 

Jon


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#5 rhetfield

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Posted 25 January 2023 - 09:29 AM

but if larger appeture reduces the size of thr airy disk then it should be better no matter how bad the seeing is?

The airy disk will be smaller, so the potential will be there, but if the seeing is so bad that you can't get to high enough magnification to see the large airy disk of the smaller aperture, then the bigger aperture is not going to help.

 

As an example, it has been cloudy for weeks/months in my location, so no stars.  Doesn't matter if I use my small scope or if one of the other local guys uses their large scope.  There will be no star splitting.  We will all see greyish yellow skies of the same quality.  Right now it is snowing - advantage small scope (smaller thing to scrape the snow off of).


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#6 Ihtegla Sar

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Posted 25 January 2023 - 09:45 AM

but if larger appeture reduces the size of thr airy disk then it should be better no matter how bad the seeing is?


The seeing will make the smaller airy disc dance around and look fuzzy while the larger airy disc produced by a smaller scope will look round and stable, providing a more aesthetically pleasing view.

You should take a look at this thread discussing why double stars look more aesthetically pleasing in smaller apertures.

https://www.cloudyni...tars-on-a-diet/

Larger scopes make the airy discs smaller, allowing closer doubles to be split, but smaller scopes generally provide a more aesthetically pleasing view of those double stars that are within the capabilities of smaller scopes to be split.
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#7 CowTipton

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Posted 25 January 2023 - 10:59 AM

My anecdotal experience:

 

With my previous 8" dob I never successfully split the D and E stars of the Trapezium.

With my 14" dob I always see the E star and occasionally the F as well as long as M42 is above 35° or so.

 

D and E are only about 4 seconds apart (about the same separation as Algieba) so you would think that should be a fairly easy split with an 8" but it never was.

 

Porrima and Izar are about 2.8 seconds apart and my 14" splits them without a problem on any average night.  They're tight but separated.

I haven't pushed my 102mm refractor on binaries very much but I intend to.  4" vs 14"


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#8 TelescopeGreg

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Posted 25 January 2023 - 01:30 PM

Once you get beyond being diffraction limited, the quality of the scope becomes the major factor.  We did an impromptu "bake off" between my garden variety Celestron 8" f/5 Newtonian and an exquisite Stellarvue SV115 triplet refractor at a Star Party a few years ago.  To the question, the Stellarvue was about half the diameter of the Newtonian. 

 

We picked eyepieces for a similar magnification and pointed both to the "Double Double" in Lyra.  In the Stellarvue I saw two pairs of clearly separated stars, crisp and clear.  In my Newt I saw two blobs.  They were much brighter, as would be expected (f/5 vs I think f/7), but only their oblong shape let you know there were two stars in each.

 

The difference was so stark that a few years later when I saw a Stellarvue SVA130EDT show up on their Certified Pre-Owned site it followed me home the next day.  Very happy with that upgrade in so many ways.


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#9 luxo II

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Posted 25 January 2023 - 10:39 PM

Yes and yes.

 

Now that's a direct answer.


Edited by luxo II, 26 January 2023 - 02:30 AM.

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#10 quilty

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Posted 26 January 2023 - 09:55 AM

may I state the opposite, partially?
Yes in perfect seeing conditions larger apertures show smaller star patterns and the separation appears more.
But a good star pattern you find more likely with smaller apertures. And to me the beauty of a split is not told in terms of separation
but quite the opposite: To me the most beautiful split consists of two Ariy disks (the bigger the more beauty) just touching
each other or separated by a narrow gap.
That way you are more likely to find beautiful splits with small apertures.
But no doubt at good seeing a larger aperture is capable to closer splits.
What double is more beautiful, Epsilon Lyrae through 4 inch or 52 Ori through 10 inch?
Answer: The former more often. At good seeing 52 Ori looks the same through 10 inch.

"look better and have more separation" is just pears and apples

Edited by quilty, 26 January 2023 - 10:01 AM.


#11 Starman1

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Posted 26 January 2023 - 10:10 AM

It all depends on Seeing.

In the 12.5", ε Lyrae appears to be 4 stars close together at 60x--an easy split.  In good seeing.

In poor seeing (like I had the other night), it's obviously two doubles, but barely so at 200x, and the stars blur and merge and bounce around a lot.

So it's more than aperture that matters.

In good seeing, I have split a 0.5" separation star in the 12.5".

In extremely poor seeing (one night when a front passed overhead), I could not split Albireo, which is 18" separation!  It was oblong mush.

 

Seeing is king.  But in the main, if both scopes are used in good seeing, the larger aperture will always resolve a closer double than a smaller aperture.


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#12 Eddgie

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Posted 26 January 2023 - 11:04 AM

Most of the answers here are oversimplified or anecdotal and there is no hard and fast answer. 

One important point.  The "Separation" never changes with aperture. Separation is the distance between the peak intensity of the two Airy Disks and this never changes with aperture, scope design, or anything else.  If a double is a two arc second double, it is always a two arc second double.  What changes is the angular size of the split which is the distance between the edges of the spurious disk. Even this will change because some people with more contrast sensitivity than others may see the same spurious disk as being slightly bigger than someone with less contrast sensitivity.

 

Next, there are several different kinds of "Splits"  A classic split is where the spurious disks do not touch. The second, one that I will use here, is a Raleigh Criterion split. Here, the spurious disks are slightly overlapping and you will see this in the first simulation.  Here is what this is important. When people talk about the separation a particular size aperture is capable of, this is based on the size of the Airy Disk (which is wider than the spurious disk). With this kind of split, you are not fully resolving the Airy Disks as individual disks, which is what the formula for resolution is doing but you can tell without question that it is a double star. The stars are not truly "Split" with angular separation but we can see the two distinct peak intensities infer that there are two stars, so we say that they are "Split". In this case, we are passing what would be considered the classic angular resolution of the telescope. The stars are not separated, but we can see that there are two of them. We call it a split but there is no space between the edges of the disks.  (Here, we are dealing with linear resolving power, which is something most people on CN have a poor understanding of, but when you get much past a Raleigh Criterion Split, you are pretty far past angular resolution, which is base on Airy Disk size. 

With a Dawes split, we have long passed the classic angular resolving power of the instrument.  Here, the peak intensities have almost merged. The human eye might not be able to tell that there are two peaks, but we cans see the elongation but if the instrument is excellent, we may still be able to separate the points of peak intensity. 

 

Finally there is the Sparrow Criterion split. Here, the peaks are so close that only a camera can tell see that the stars are elongated in the direction of the position angle (PA)  

 

I do recommend that you read this link because it further explains what I am saying here. 

 

https://en.wikipedia...esolution_limit

 

This sound like little nit picks, but to really explore the issue, it is important that everyone be on the same page with respect to terminology.

Let's start with the first oft made mistake, which is the statement that a larger scopes can always split closer doubles than smaller scopes. Some people will be surprised to hear this, but a smaller SCT or SCT can sometimes split stars that a slightly larger refractor may not spilt. This is because the central obstruction in the SCT or MCT drains energy out of the Airy Disk and puts that energy into the diffraction rings and when it does this, it makes the spurious disk smaller than it would be in a perfect aperture. This means that if you used a 127mm MCT against a 130mm Apo, you might find that the 127mm MCT can "split" very close doubles better. Because the spurious disk has a smaller diameter in the slightly smaller instrument with the large obstruction, it will appear that there is a wider gap between the disks in the smaller scope. So, anyone that says a larger aperture will always be able to spit star better than any smaller aperture is not entirely correct. That being said, this example the 127mm MCT or SCT vs the 130mm refactor, is the rare kind of combination where this works out.  If the refractor in this case was 140mm, it would likely show a wider split. 

 

Another case where a smaller scope may split a double that a slight larger scope cant is a Dawes split. Here is another case were a smaller unobstructed telescope can sometimes "Split" stars that a slightly larger telescope can't and this time it is the central obstruction that can work against the larger scope. The obstruction drains energy out of the peak of highest intensity so that it does not stand out as sharply as it does in the unobstructed scope. When you see a faint double, you are not seeing the full diameter of spurious disk that you would see if you were viewing a bright star. That tiny little pinprick of light you see when viewing very faint stars in a telescope is that tiny tip at the center of the Airy Disk. In the slightly smaller Apo, you can still see this point when the larger obstructed scope has made it fall below the background brightness, but with respect to doubles, this can allow the smaller telescope to more positively identify this as a Dawes split.

 

 

 

 

Hello i am wondering do double stars look better and have more seperation through larger scopes or does seeing block that no matter what appeture you use?

This is of course totally subjective because one, if you can't spit a double star in one scope and you can split it in another, how can it "look better" in the scope that can't split it.  Also the aesthetics of a split are highly personal. If you can split it in both scopes though, some people might say that if there is any meaningful aberration, the split "looks better" in the scope that show less aberration. 

 

Here is an example. On the left is a double seen in a perfect 4" telescope. On the right is that same double as seen in a perfect 10" telescope with a 25% obstruction. I have thrown in some small amount of atmospheric disturbance.  The view in the left scope looks beautiful because there is no aberration to the stars, but the stars are not fully resolved. This is the Raleigh Criterion split I mentioned earlier. We can tell there are two stars because we can see the peak intensity but the Airy Disks are overlapping but there is no separation between the spurious disks.  In the right scope, the aesthetics are different (most would say not as good) because the first ring is in motion and perhaps breaking up, but this is a very wide split and one that the smaller scope is incapable of doing even on a night of perfect seeing.  Once again, a smaller unobstucted Apo can often do a better job than a larger instrument because of the Apo's exceptional capablitiy of focusing as much energy as possible into the peak intensity of the Airy Disk.

 

split.jpg

 

Same two telescopes but a different double on a different night. This time the stars are about 2.5 arc seconds apart (I think they were 2 arc seconds in the first simulation.)  I have added more seeing disturbance though, and this might be typical of a winter night where there is a lot of heat rising from rooftops.   Here, even though the larger scope still shows much wider split between the stars, almost everyone would say that the smaller scope has produced a more aesthetic split.    means that the darker space between the stars is wider with the larger instrument.  

 

 Split 2.jpg

 

 

 

Also, the above assumes that the stars are equal in brightness and this is rare, but if there is few amplitudes between the stars, the result can be completely different. If it is below the threshold of the smaller scope you may not even see the secondary star or if it is close, it might fall under the first diffraction ring of the smaller telescope and be outside of the first diffraction ring of the larger telescope, so even if seeing is not great, once again, the larger telescope could show you the secondary even though the view is aberrated. Conversely, if the star is bright enough to be seen in both scopes, and the seeing is poor, the star might show in the much smaller scope while the aberrated blur might make it hard to see in the larger scope. 

 

Subjectively, I would say that a telescope like a 4" of 5" Apo will provide more aesthetic splits far more often than much larger apertures, and on many stars, may even be able to split closer doubles if you are usingRaleigh or Dawes criterion but the much larger apertures can split stars that are much closer even when seeing aberrates the stars so that they are not showing even a hint of a first ring. 

 

If seeing was not particularly good and I wanted to split doubles, I used to use a 4" to 5" Apo and simply settled for the doubles that I could split, but when seeing was good, I would use the larger telescopes to split the doubles that the smaller scope could never split. In such conditions, it was often possible to split closer doubles than larger scopes. Again, nothing concentrates energy the way an Apo does, and this is why refractor people love them. I use all types of telescopes and my own opinion is that doubles are most enjoyable in a 5" to 6" Apo. I can split closer doubles in my larger scopes, but the Apos would always surprise me with how well they could do.

 

 

 

This does not mean having to use a smaller telescope and a larger telescope! This is one of those rare places where I recommend using aperture masks.  A sub aperture mask is just a piece of material that covers the aperture and has a smaller hole in it. The hole makes the aperture smaller and once you make it smaller, the effects of seeing will be reduced down to what they would be if you were actually using a smaller telescope. If you were using a 12" telescope, you could mask it down to get about 5" of aperture.  The secondary would slightly intrude into the circle, but the diffraction artifact it would produce would be very small and probably hard to see. This though, limits you to the same resolution and same size split as the smaller scope. 

The second kind of mask you could use is called an apodizing mask. This can greatly improve the aesthetics of a larger aperture when seeing is less than very good.  https://csastro.org/...podizing-mask/ 

 

So, it is not a question that has answers as simple as yes and no because when you introduce all of the variables it can be quite complex. There are different kinds of "splits" different and telescopes are not always the same in terms of design, so making a blanket statement that a larger scope can do better than a smaller scope is not always accurate.

 

 

 

 



#13 Eddgie

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Posted 26 January 2023 - 11:50 AM

And a bit on peak intensity.  When you look at the faintest star you can see in any telescope, what you are seeing is the very center of the Airy disk or the area of peak intensity. If this area is brighter than the surrounding sky you can see it. This is what we do when we test a telescope for its limiting magnitude. We find the faintest possible stars visible, and we look at a chart to see the magnitude.  When we do this though, we don't see the whole Airy Disk. We see only the brightest central peak.

 

Suppose you had two apertures, both perfect but one with an obstruction.

 

Airy (the man that the Airy Disk is named for) calculated that in a perfect aperture, 84% of the light would be concentrated in the Airy Disk and most of that light is concentrated in the central part of that disk with the energy quickly falling off as you approach the edge, where there is no more light falling. This is the point roughly between the edge of the spurious disk and the peak intensity of the first diffraction ring.

 

In the perfect system, the first ring would receive 7% of the remaining energy, the second ring would have 3%, and the outer rings would have the remaining 6%. (9% total outside of the AD and the first ring). 

 

In the obstructed system, as I mentioned in my previous post, the diameter of the Airy Disk is very slightly smaller because energy has been removed and redirected to the rings. If the obstruction is 25%, only 73% goes into the Airy Disk and a whopping 18% goes into the first ring but once again, the other rings all together have 9% just as with the perfect unobstructed system. 

 

 limiting magntude.jpg

 

And the above drawing is purely conceptual and just intended to show how the energy distribution of different systems can affect their performance on double stars. 

 

As I recall, the Airy Disk Diameter for a perfect 8" system would be 280 arc seconds per millimeter, for a 25% obstruction it would be 262, and for a 50% obstruction, 230. Pointing back to my earlier post, this is why a smaller MCT can sometimes split (no contact) doubles closer than a larger perfect Apo. The Airy Disk is smaller. 

 

The refactornistas demonize the central obstruction but will ignore the effects of choromatic aberration and spherochromatism, but these too lower the energy intensity of the Airy Disk but through other means, but the point is that for a perfect unobstructed system, the Airy Disk will contain 11% more energy than the unobstructed system and if you are not using an Apo, which can come very close to that figure, the total energy in the Airy Disk will be less, and if the scope is an achromat, it can be even less than a 24% obstruction.  Don't tell them this though, because it hurts their feelings.

 

Anyway, if they were the same aperture and you ran limiting magnitude tests, this is what would happen. The left is the perfect system with its better energy intensity, the right is the obstucted system with its slightly smaller Airy Disk, and the line repesents the sky background glow. If the peak intensity of the AD is bright enough, the perfect system might just have enough energy at its peak intensity point to raise it above the sky glow level while the obstructed system might not have quite enough to "Punch through" and be unseen.

 

This carries over in many ways to your post about doubles.  I keep saying Apos because again, these scopes are concentrating a super high percentage of light into the Airy Disk. If the theoretical maximum for the aperture is 84%, the best APOs can achieve 83.16% total while the best a 25% obstructed system can ever achieve is 73%. Once again, this assumes that the apertures are otherwise perfect. 

 

I will re-emphasize this. If the scope is not an Apo, it is not going to achieve this kind of excellent energy concentration.  The figure you would use would be the Polychromatic Sthrel of the telescope. If the polychromatic Strehl were .87 as an example, which could be the case for some fast ED doublets, then you  would multiply the 84% maximum possible by  the polychromatic Strehl (.87 in this case) and the total energy in the AD would be (cover your eyes refractor lovers!) 73%! 

 

As a side rant, refractor people simply either chose to ignore or simply can't grasp the importance of a polychormatic Strehl figure. This is exactly what it is telling us. It is telling us how much energy is failing to go into the Airy Disk.  Only the Apo can achieve anything close to the theoretical 84% of the energy going into the Airy Disk. 

 

Anyway, here is how it would look on paper.  Left is perfect (let's say 5" Apo) and right is 25% obstructed (and we assume perfect transmission in each scope, but the Apo would have an advantage here as well so in reality, the difference would be even better in the apo) 


Edited by Eddgie, 26 January 2023 - 11:51 AM.


#14 Jon Isaacs

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Posted Yesterday, 02:21 AM

Once you get beyond being diffraction limited, the quality of the scope becomes the major factor.  We did an impromptu "bake off" between my garden variety Celestron 8" f/5 Newtonian and an exquisite Stellarvue SV115 triplet refractor at a Star Party a few years ago.  To the question, the Stellarvue was about half the diameter of the Newtonian. 

 

We picked eyepieces for a similar magnification and pointed both to the "Double Double" in Lyra.  In the Stellarvue I saw two pairs of clearly separated stars, crisp and clear.  In my Newt I saw two blobs.  They were much brighter, as would be expected (f/5 vs I think f/7), but only their oblong shape let you know there were two stars in each.

 

The difference was so stark that a few years later when I saw a Stellarvue SVA130EDT show up on their Certified Pre-Owned site it followed me home the next day.  Very happy with that upgrade in so many ways.

 

The double -double is a Dawes limit double for a 2 inch scope. It's quite easy n a 70 mm. An 8 inch has a Dawes limit of 0.57". The key to doubles is scope preparation and seeing.  I suspect the 8 inch Dob was not fully cooled, it may not even have had a fan..

 

When comparing mages of doubles, equal exit pupil maybe required as the larger exit pupil of the larger scope when equal magnifications are used may not be fully resolved by the eye.

 

A challenge for a 4 inch is something around 1.2".

 

I split doubles in the 0.5"-1.0" with my commercial 10 inch Dob on nights of excellent seeing. I have managed doubles under 0.5" with my 13.1".. Excellent seeing and careful scope preparation are the keys. Refractors are easy scopes, pushing a large reflector to the limit requires attention to detail.

 

StarSplitter with Drum Fan.jpg
 
Jon

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#15 Jon Isaacs

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Posted Yesterday, 02:44 AM

 

Anyway, here is how it would look on paper. Left is perfect (let's say 5" Apo) and right is 25% obstructed (and we assume perfect transmission in each scope, but the Apo would have an advantage here as well so in reality, the difference would be even better in the apo) 

Eddgie:

 

A nice analysis.

 

When discussing the effect of the central obstruction, the operative word is slightly. When comparing telescopes people might own, a 10 inch reflector and a 5 nch apo refractor, the concentration of energy is so much greater in the larger scope that the central obstruction has little meaning. The Airy disc of the larger scope is 1/2 the diameter of the smaller scope.

 

In my experience, the two important factors in resolving close doubles are the seeing.and the preparation of the scope. Refractors are easy. Reflectors require attention to thermal equilibrium and optical alignment/collimation.

 

In my experience, a larger reflector will push deeper into the seeing as long as it's decent. In 1" seeing, a 120 mm refractor will not split a 1.2" double because because the overlapping Airy disks are being further smeared by the seeing. A much larger reflector will not be fighting the overlapping Airy disks.

 

Jon


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#16 havasman

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Posted Yesterday, 04:13 AM

I commonly use brighter ~equal mag doubles well positioned earlier in a night to gauge conditions and with the Starmaster have a separation range starting around 1.9" down through 0.5" and 1 or 2 below that ready to go for most sessions. Stelle Doppie makes selection easy. I usually haven't chosen to go to a dark site if the widest of those don't split well and easily. Conditions aren't going to be what I expect if clean easy splits aren't available in the 0.7" range and if I can go much below 0.5" with good splits then it's going to be quite good. I don't spend much time observing double stars except for that little exercise but it has been a valuable tool for me. And it is fun. I do enjoy the standard colored showpieces for my own pleasure (gamma Del is a long term fave) and the double-double for outreach. I have never considered that one particularly diff.

 

So I guess my experience is that aperture + good seeing > less aperture + good seeing and w/o the good seeing it's washed out anyway.


Edited by havasman, Yesterday, 04:20 AM.


#17 Ihtegla Sar

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Posted Yesterday, 04:35 AM

 

Refractors are easy scopes, pushing a large reflector to the limit requires attention to detail.

Attention to detail and moving somewhere with a mild climate and good seeing. 


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