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Equipment Discussions >> Refractors

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Eddgie
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Re: SW 100: Will I notice Difference? new [Re: Jon Isaacs]
      #6175372 - 11/04/13 12:36 PM

It's really far more than just the field curvature of the telescope itself.

For example, let's say that someone has drank the coolaid and is using a 10mm orto in their telescope for planets.

Or a Plossl for that matter.

Assume an f/5 reflector.

On the image axis of either a plossl or a an ortho, the eyepeice is capable of focusing the spot from a star into a one arc minute of appearent field circle.

But by time a planet has drifted to 10 degrees off axis (less than half the way to the field stop of even a 40 degree AFOV Ortho), due to astigmatims, the spot will have expanded to 5 arc minutes.

Any curvature will of course expand the blur even further.

This means that for fast telecopes the eyepeice itself is no longer diffraction limted once the target or detail has drifted less than half way to the field stop.

By f/10, you would have to go to about 20 degrees off axis for this same amount of astigatism, but really this is far to much astigmatism to generate a sharp image.

And again this is at a flat field.

Now the combination of the telescope and eyepecie field curvature may make this better (or perhaps worse) but most eyepeices that are praised for planets have very small off axis angles where they will be diffraction limited.

I can see that the detail on the sun looses definition when it drifts outside of an image circle that I would judge to be about 20 arc minutes in diameter in most eyeepcies I have tried.

The most difficult detail that is visible when I hold the detail at the center of the field is not available to me if I let the detail drift outside of this circle.

And even at f/7 and 75x, the detail drifts from this circle very quickly.

I don't know how people cannot see this difference. It is crystal clear to me that I only get the full performnce of the telescope and eyepeice when I place the target at or near the exact center of the field.

I could refocus of course as the object drifts out of this small area, but that gets tedious.

So, for me, I can't see how people can miss this image degradtion.

I see much more detial when I place the target at the exact center of the field and focus on it and track it than if I allow it to drift away from center.

How can people not see that? It is glaring to me.

On the sun, the preumbra simply falls apart and turns into a soft blur if I let it get more than about apparent degress off to the side of the eyepiece center.

It was driving me crazy using my mount without the motors. As soon as the sunspot drifted a little away from center, I was loosing the detail that I was workning hard to see even with the target perfectly centered.

I just don'd understand how peopel could miss this level of damage.

And for this kind of observing, Naglers will of course have far less astigmatism, but in a fast dob, the coma will still kill the contrast on the most difficult detail.

Honestly, anyone that takes the time to really study the image hard will see that as the target drifts off of the optical axis in most scopes either the eyepciece fails (on a fast scope) or the scope itself contributes enough off axis aberration (field curvature or coma or whatever) to render the image less than diffraction limited.

It just escapes me how people can miss this!

But there was a recent thread... An individual using a binoviwer in his EdgeHD 8".

I had been going on and on about the ills of a lot of back focus when using binoviewers with the EdgeHD 8".

Over the space of a month, HowardK measured his apeture and discovered he was loosing about .5".


But it did not seem to bother him...

Until....

When he took the time to really make a serious comparison using the configureation that was causing the apeture loss to a configuration with much less apeture loss, he quickly discovered that there was a meaningful difference in performance.

He had to see it for himself, and once he did, he abandoned the configuration he was using.

If people look for this, they are going to see it.

With the detail are of intrest at the exact center of the field, find the most difficult feature possible.

Now, let the target drift towards the edge.

My bet is that for most observers, using most telecopes/eyepeciece combinations, they will no longer be able to see that detail before the area has drifted half way to the field stop.

At least that is the way it works in almost every telescoep I have used for planets. I only get the full capability of the instrument when I keep the target very near the center of the field.

I don't really care what other people use, but if they really want to get the most out of their telescopes, they will take this test.

And if they decide that they still don't mind the performcnae falloff, then they get to make their own choice.

But many are going to perhaps be surprised.

Thre is nothing like seeing it for yourself.

Ask HowardK. He is now a believer about back focus in the SCT. He proved it to himself.


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Eddgie
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Re: SW 100: Will I notice Difference? new [Re: Eddgie]
      #6175412 - 11/04/13 01:02 PM

And this.. The book Telescope Optics has a great example of what I am talking about.

On page 194, they show how varius telescope and eyepiece combinations perform.

They show the blur circle for each combinations with a one and five arc minute circle so you can see how much the image suffers.

Remember, the scotopic eye can resolve about 2 arc mintues of damage.

They also show how much visual accomdation would be required in diopters for the observer to keep the blur at best focus (though the image will never be better than the blur allows).

Only one/eyepeiece combination kept the blur contained very well, and that was a 100mm f/8 APO using a 13mm Nagler, and this was ongly good to 20 degrees off axis, or half the way to the field stop of the eyepecie.

Now imagine how quickly a planet is going to get half the way to the edge of a Nagler at 150x.

And this was the best case.

Anyone that has this book can quickly and easily see this for themselves and once seeing this, my bet is that the will quickly understand the point I am making.

But if they have a telescope, they can test it for themselves.

I don't really care what other people do, but if they want to get the absolute best planetary detail with their telescope, they should strive to keep the point of interests as close to the center of the field as possible.

Edited by Eddgie (11/04/13 01:05 PM)


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Eddgie
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Re: SW 100: Will I notice Difference? new [Re: Eddgie]
      #6175422 - 11/04/13 01:08 PM

And I know.. Someone will say "Oh it is just a book.. Just a theory.. Ray traces don't tell all that much."

We all make our own choices of what data to believe.

But telescope optics is such a well known area that all of this was figured out many decades ago.

I encourage more people to read about it. Fun topic.

But anyone that reads Section 16.7 of Telescope Optics and studies the chart on page 194 will quickly conclude that for best performnce, it is best to kee the target at or near the center of the field in most telescope/eyepeice combinations.

And for fast dob users, they will quikly conclude that if they want to let the target drift, they are beter off using a Nagler than an Ortho. They still get the coma from the instrument, but at least the eyepiece behaves.

Most simple designs are incapeble of performing well outside of a pretty narrow angle.

Edited by Eddgie (11/04/13 01:12 PM)


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Jon Isaacs
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Re: SW 100: Will I notice Difference? new [Re: Eddgie]
      #6175456 - 11/04/13 01:20 PM

Quote:

And for this kind of observing, Naglers will of course have far less astigmatism, but in a fast dob, the coma will still kill the contrast on the most difficult detail




Eddgie;

I just provided you with the coma free field of view of a 10 inch fitted with a Paracorr as being significantly larger than the moon. Yes, orthos and Plossls are not sharp off axis. So.. Naglers are.. those who view the planets and track by hand typically use Nagler class eyepieces and a Paracorr.

Is that so difficult to understand?

A telescope is a system, the mount, the scope, the eyepiece and the observer. Making them all work together is what it's about. Dob mounts are solid and stable. If hand tracking is part of the equation, then the scope, accessories and eyepieces need to be chosen wisely.

It's just that simple.

Jon


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BillP
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Re: SW 100: Will I notice Difference? new [Re: saemark30]
      #6175521 - 11/04/13 01:53 PM Attachment (27 downloads)

Quote:

BillP,
Can you tell us the details of the drawing such as power, eyepieces, filters if any, and planet apparent size?




That sketch, rendered in a smaller scale to show how it appeared in the AFOV is pictured below. That was at 2010 Jupiter opposition so one really didn't need much magnification to get a good view. I was only operating at 100x. For Mars, I am generally much higher and usually around 140x to 220x just depending. I rarely use filtration when observing and tend to prefer cool toned eyepieces over warmer tones, even for Jupiter because I tend to focus on the lowest contrast challenge features when observing and find they get obscured more easily with warmer toned eyepieces. If I can do bino-mode I do as 2 eyes always lets me perceive a bit more detail and contrast is perceived better as well. The image says the eyepieces were RKE but that is an error. They were actually the older Edmund 1-1/8" eyepieces which are non-symmtrical Plossls. These provide a sharper on-xis image than their successor the 28mm RKE. It's just slightly better, but the small gain is important to me since I'm always after challenge features for the scope.


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BillP
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Re: SW 100: Will I notice Difference? new [Re: saemark30]
      #6175546 - 11/04/13 02:04 PM Attachment (23 downloads)

Quote:

A 5" can show some polar caps and dark regions on Mars at a close opposition.




You don't need a 5" to see great detail on Mars. Polar caps are an easy catch and lots of surface details including limb haze, orthographic clouds over Olympus Mons, and other very delicate details - my sketches do not do justice to what I can see as my sketching skills are not that honed. Contrast performance is really important I find. As you can see in the sketch below, the difference between a complex design eyepiece and on the right the as pure as you can get singlet eyepiece is quite substantial. Even though the left sketch is at a higher magnification, when I do use EPs where the magnifications are equalized, the singlet always provides a higher contrast view that is easy to notice when comparing it to anything other than a premium-level standard Abbe or the like. The singlet is a ball or sphere lens eyepiece that was ATMed. Both good views but the home made ball EP can keep up and sometimes even exceed a ZAO as its contrast is unparalleled. Tracking mount required with the ball EP as the usable FOV is only about 10 degrees or less. Worth it though to me as it brings out lots of subtlties within the various Mares, not to mention how blazing stark white the polar caps and limb haze appear when using the Ball or ZAO.


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PeterR280
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Re: SW 100: Will I notice Difference? new [Re: BillP]
      #6175578 - 11/04/13 02:17 PM

I have seen polar caps on Moars with 4"

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saemark30
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Reged: 02/21/12

Re: SW 100: Will I notice Difference? new [Re: PeterR280]
      #6175667 - 11/04/13 02:57 PM

Mars varies from 13 to 26 arc sec at opposition so I guess the instrument required must vary within that ratio too.

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t.r.
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Re: SW 100: Will I notice Difference? new [Re: saemark30]
      #6175714 - 11/04/13 03:17 PM

Quote:

A 5" can show some polar caps and dark regions on Mars at a close opposition...You don't need a 5" to see great detail on Mars.






I have read that 60mm is all that is needed to show some macro planetary detail on the usual targets of Mars, Jupiter and Saturn. I agree and indeed, many of us started planetary observing with a 60mm. Take a look here...

60mm

And here...

60mm too

To answer the OP's question...Will I notice a difference?...Well, I notice the difference when there is as little as 10mm increments in refractors until about 7". Is it "dramatic"? That is up to you! But rest assured the difference of just 10mm can be seen.





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saemark30
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Re: SW 100: Will I notice Difference? new [Re: t.r.]
      #6175784 - 11/04/13 04:02 PM

Yes but those sketches were made with a premium 4" APO.
The 100ED might have very difference Q&A.
In fact there was a triplet version of the 100ED that had problems with spherochromatism.
And apparently not all 60mm refractors are created equal. I never saw detail like those as the images were too faint by the time the magification was high enough.


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pdxmoon
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Re: SW 100: Will I notice Difference? new [Re: t.r.]
      #6175829 - 11/04/13 04:26 PM

I use a 60mm on the moon and planets all the time. Great for rocket fast 10 minute observing sessions!

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BKBrown
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Re: SW 100: Will I notice Difference? new [Re: saemark30]
      #6175951 - 11/04/13 05:41 PM

Quote:

Yes but those sketches were made with a premium 4" APO.
The 100ED might have very difference Q&A.
In fact there was a triplet version of the 100ED that had problems with spherochromatism.
And apparently not all 60mm refractors are created equal. I never saw detail like those as the images were too faint by the time the magification was high enough.




My SW100ED can detect those kinds of detail, and I have seen them on many occasions. BillP has a superb instrument in his TSA-102, and the Jupiter sketch (very nicely done Bill ) is quite representative of what an experienced planetary observer can pick out. That said, properly figured 100ED f/9 optics can pleasantly surprise you if you give them a chance...

Clear Skies,
Brian


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jag767
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Re: SW 100: Will I notice Difference? new [Re: t.r.]
      #6176236 - 11/04/13 08:31 PM

Quote:

Quote:

A 5" can show some polar caps and dark regions on Mars at a close opposition...You don't need a 5" to see great detail on Mars.






I have read that 60mm is all that is needed to show some macro planetary detail on the usual targets of Mars, Jupiter and Saturn. I agree and indeed, many of us started planetary observing with a 60mm. Take a look here...

60mm

And here...

60mm too

To answer the OP's question...Will I notice a difference?...Well, I notice the difference when there is as little as 10mm increments in refractors until about 7". Is it "dramatic"? That is up to you! But rest assured the difference of just 10mm can be seen.








I have to agree a lot can be seen with something so small. My 65mm nearly blew me off my feet the first time I looked at Jupiter with it. The level of detail was so far beyond what I had expected and in fact I had no intention of ever doing any planetary viewing with it. I just decided to take a peek on a whim, very very glad I did. I think a lot of people get caught up in the numbers with this hobby- something that is incredibly easy to do. The bottom line at least to me is if you put something in view, and seeing it makes you crack a smile, then it did the job!


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BillP
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Re: SW 100: Will I notice Difference? new [Re: BKBrown]
      #6176367 - 11/04/13 09:44 PM

Quote:

Quote:

Yes but those sketches were made with a premium 4" APO.
The 100ED might have very difference Q&A.
In fact there was a triplet version of the 100ED that had problems with spherochromatism.
And apparently not all 60mm refractors are created equal. I never saw detail like those as the images were too faint by the time the magification was high enough.




My SW100ED can detect those kinds of detail, and I have seen them on many occasions. BillP has a superb instrument in his TSA-102, and the Jupiter sketch (very nicely done Bill ) is quite representative of what an experienced planetary observer can pick out. That said, properly figured 100ED f/9 optics can pleasantly surprise you if you give them a chance...

Clear Skies,
Brian




And as you know (but others don't) I had an observing session with you with your 100 f/9 and my 102 f/8. The two were so darn close in their views on planetary, and no difference on other targets! That SW 100ED is optically very very good so quite a value. Put a Feathertouch or Moonlight focuser on it and....sweet!

The fun part of that evening though was placing multiple stacked Barlows under a 28mm RKE with your TEC140 and observing Saturn with eye relief in the neighbors yard

Edited by BillP (11/04/13 09:45 PM)


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BKBrown
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Re: SW 100: Will I notice Difference? new [Re: BillP]
      #6176627 - 11/05/13 12:05 AM Attachment (9 downloads)

Quote:

Quote:

Quote:

Yes but those sketches were made with a premium 4" APO.
The 100ED might have very difference Q&A.
In fact there was a triplet version of the 100ED that had problems with spherochromatism.
And apparently not all 60mm refractors are created equal. I never saw detail like those as the images were too faint by the time the magification was high enough.




My SW100ED can detect those kinds of detail, and I have seen them on many occasions. BillP has a superb instrument in his TSA-102, and the Jupiter sketch (very nicely done Bill ) is quite representative of what an experienced planetary observer can pick out. That said, properly figured 100ED f/9 optics can pleasantly surprise you if you give them a chance...

Clear Skies,
Brian




And as you know (but others don't) I had an observing session with you with your 100 f/9 and my 102 f/8. The two were so darn close in their views on planetary, and no difference on other targets! That SW 100ED is optically very very good so quite a value. Put a Feathertouch or Moonlight focuser on it and....sweet!

The fun part of that evening though was placing multiple stacked Barlows under a 28mm RKE with your TEC140 and observing Saturn with eye relief in the neighbors yard




I remember that session well Bill, that was also when you introduced me to ball EPs...a surprising revelation for me And yes, the Barlowed RKE 28mm experiment was a real treat! That was back before the observatory was built and Saturn was the star of the show, I would be more than willing to try it again as Jupiter approaches opposition...

Clear Skies,
Brian

Edited by BKBrown (11/05/13 03:15 PM)


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saemark30
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Re: SW 100: Will I notice Difference? new [Re: BKBrown]
      #6177264 - 11/05/13 12:08 PM

Where did you find the ball lenses and what size did you get?
I did try using doublets as eyepieces and the contrast was great for the 2005 Mars opposition.
I guess a long f/# 100mm refractor can show a lot of detail especially with a binoviewer and good seeing.
A larger scope will show the same detail with more contrast though.


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astrobug
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Re: SW 100: Will I notice Difference? new [Re: saemark30]
      #6177647 - 11/05/13 03:34 PM

Here's my $0.02:

I don't think you're going to see a huge difference in the view between the ED100 and ED120 for planetary work, assuming equal optical quality and seeing conditions. Yes, the image will be a little brighter in the 120 at the same magnification, but the theoretical difference in smallest observable details is only about 1/5 of an arc-second.

My experience supports this. I used to have a Burgess 1278 (125mm f/8 achromat). Despite the false color it was a fine planetary performer. I tried a bunch of different sized aperture masks, and frankly the planetary views at 150-200x were actually just a hair better at 100mm than they were at 110mm or full aperture (this is likely due to a slight reduction in chromatic aberration, while still having enough aperture to show fine details--Ronchi testing showed no edge problems at full aperture). Stopped down to 80mm, there was a much more obvious reduction in the amount of detail. (Interestingly the Burgess at 80mm outperformed my 80mm Skywatcher Equinox ED by a hair at the same magnification...but this could be attributed to differences in eyepieces/having to use a Barlow.)

So, in the end, for me, the extra aperture was basically just dead weight (I have other scopes for deep-sky viewing), but the difference between 80mm and 100mm was compelling, so I ended up picking up a Vixen ED100sf. So far, this seems to be a little bit better than the Burgess for planetary viewing (no noticeable loss in fine detail, and a cleaner view overall). The big pluses for me are the 2-speed focuser and lightweight OTA. I can mount the Vixen on my old-school Polaris for quick looks rather than having to use my Meade LXD-75 (which takes much more setup time).

All that said, on a night of very good seeing, I would want a larger instrument for planetary work. Considering this is the refractor forum, the ideal be an 8" or larger APO. Considering my bank account, I would have to be content with a similarly-sized Newtonian that has been optimized for planetary viewing: 8-12", focal ratio above f/6, tube flocked or baffled and extending well past the focuser, minimally obstructing secondary, curved-vane or wire spider, and fans on primary mirror to reach equilibrium faster.

To the OP: I live in Portland, OR, too, so if you'd like to try out an ED 100 variant before buying, feel free to PM me. (Though with the recent change in weather, we might not get a clear night before the sale ends ).

-Brett


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Sasa
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Re: SW 100: Will I notice Difference? new [Re: astrobug]
      #6177866 - 11/05/13 05:21 PM

My story is similar to Brett and Stephen. I was also considering buying either ED100 or ED120. Originally to complement my fairly good 250mm f/6.4 fully flocked Newton. Having that large telescope already I opted at the end for ED100. I wanted a telescope for quick sessions on light alt-az mount that would give me more optical power than my 80mm f/6 apochromat.

At the end I found out myself observing through 100mm refractor more and more. I was taking out the big dobson only occasionally, even if I knew that I would see more on DSO or planets with it. I did not care. What I saw on planets with this small telescope was good enough and still very interesting. (You can see my drawings on my web page, here for Jupiter

http://www-hep2.fzu.cz/~kupco/astro/sketches/obj/jupiter.html

here for Mars

http://www-hep2.fzu.cz/~kupco/astro/sketches/obj/mars.html )

With ED100 I was out every reasonably good night, most of them were nights when I would not bother to take the large dobson out. Finally, when I set up my dark-side observatory, I decided to sell the large Newton.

I think I did a right decision going with ED100 instead of ED120. I observed once with my friend's ED120 and it was clear that thermalization of ED120 was visibly slower. ED100 was providing crisp views of Saturn while ED120 was still a little bit mushy. Later I had also in my possession 130mm Vixen ED doublet with an excellent star test (it was supposed to be my main telescope for the dark-sky observatory, but it was replaced by Zeiss AS110 which I found at about the same time). In winter, it was almost unusable on planets without prior cooling. I remember one winter night when I started just to see Jupiter GRS after 30 minutes of being outside. With AS80/1200, I was done with drawing Jupiter in 20 minutes including Red Spot Junior...

Concerning the difference between 80mm, 100mm (and 130mm), the difference is clearly visible on first sight. However, with more time spent behind eyepiece, I'm not sure that it is indeed that large as it looks at the first sight. My experience on Jupiter, Saturn and Mars is that I record on my drawings very similar amount of detail. But I would say you need to work harder at 80mm to noticed them than in 100mm. Still of course I can find some details in 100mm which are not visible in 80mm.

Take for example this series of 3 Mars sketches made through 80mm, 100mm, and 250mm telescopes in 3 subsequent days (Mars diameter was about 9"):

http://www-hep2.fzu.cz/~kupco/astro/2012/2012_05_10/pic/orig/Mars_20120510_20...
http://www-hep2.fzu.cz/~kupco/astro/2012/2012_05_08/pic/orig/Mars_20120508_19...
http://www-hep2.fzu.cz/~kupco/astro/2012/2012_05_09/pic/orig/Mars_20120509_20...

The sketches look more similar than different.

On DSO objects, the difference between 80mm and 100mm is large. For example some galaxies start to reveal to my eyes some details in 100mm while in 80mm it is more about just seeing the galaxy (but still, here and there I run on unexpected surprises even in 80mm). And 130mm was another big step. At these sizes every centimeter counts. Already the difference between 11cm refractor AS110 and ED100 is clearly visible (for example on many globular clusters). But still I do not mind with what telescope I observe, being it just 63mm refractor or 250mm one. When I observe, I'm not bothered by thoughts what I would saw if I had larger telescope instead. I simply enjoy the view as it is.


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magnus
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Loc: Visby, Sweden
Re: SW 100: Will I notice Difference? new [Re: Sasa]
      #6178031 - 11/05/13 07:12 PM

Sasa!

Thanks for an informative post and amazing drawings; 100ED must be the way to go if I want to have a larger refractor than my C80ED!

Magnus 57N.

------------------
C80ED
MK66
C8
8"f/6 OOUK Newt.
LS35HaDX


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jrbarnett
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Re: SW 100: Will I notice Difference? new [Re: pdxmoon]
      #6178078 - 11/05/13 07:43 PM

"Will I notice a difference in my planetary and lunar observing between the 100ED and the two scopes I already have,..."

Yes over the 80mm. Not likely compared to the 102mm.

"Should I just wait until the day I can add a 120ED?"

YES!

Sure, a new mount is kind of a drag, but look at it this way. Once you have that 40-45# payload class mount, you're good for anything from a tiny refractor to a mid-to-large SCT (10"-11"). Having a versatile mid-capacity mount is quite liberating, actually. Celesrton's CGEM is on sale currently for $1300; less than a 120ED OTA at current prices. And it'll work just fine with the OTAs you already have until you're ready to take the plunge and move up to a ~5" glass.

While I agree with Bill that an experienced observer with keen eyesight and a quality 4-incher can see quite a lot on bright targets like planets, to me for a refractor to achieve "do everything" status, it needs to be around 5-inches in aperture or larger.

I love 'em all, but I could comfortably observe for the rest of my days with a quality 5-incher. I can't say the same for a 4-incher, though.

Regards,

Jim


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