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AT102/F7 planetary telescope?

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#1 paulsky

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Posted 12 May 2021 - 09:27 AM

Hello,

Yes, I have "supposedly" a good planetary telescope, one C8 .... for which I have never had a sharp, clear and stable observation, so and thinking of changing instruments I would like to know your opinion about this refractor as an instrument for planets and lunar initiation ...
very kind.
paul


Edited by paulsky, 12 May 2021 - 09:28 AM.

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#2 Mark Lovik

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Posted 12 May 2021 - 11:20 AM

Some questions on your current system

 

1. Have you collimated the C8?   This is critical for decent planetary views.

2. Have you tried any filters?   Sometimes it is a simple issue of blinding surface brightness.  The moon is terrible at typical magnifications in a C8 ... it's too bright.  Crossed polarized filter, or a neutral density filter is great for the view.

3. Do you observe when the planets are near the horizon?  The views will be poor in almost any telescope most of the time under these conditions.

4. Do you let the scope cool before observing? ... I keep mine in a storage tub outside for an hour or 2 before observing.  Checking slightly out of focus stars can detect tube current issues causing you problems.

 

I use a slightly smaller refractor than the AT102 and have an 8" SCT.  The images in the refractor are nice ... but a collimated 8" SCT will outperform a small refractor when seeing permits.


Edited by Mark Lovik, 12 May 2021 - 11:25 AM.

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#3 GUS.K

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Posted 12 May 2021 - 04:36 PM

Moved to refractors for a better fit.


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#4 ris242

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Posted 12 May 2021 - 05:49 PM

I would like to know your opinion about this refractor as an instrument for planets and lunar initiation ...

very kind.
paul

Would you consider F/11?

(it does make the focal length 400mm longer)



#5 CHASLX200

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Posted 12 May 2021 - 06:27 PM

Mine did fine at 400x on the moon. Jupiter after 350x is getting dim for a 4". But 300x is fine in my super steady seeing.

 

I rated the scope as best bang for the buck when i had mine around 3 years ago.  Most SCT 's are just lack luster at best and never do as well as a frac or Newt.


Edited by CHASLX200, 12 May 2021 - 06:28 PM.

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#6 mjt24073

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Posted 12 May 2021 - 06:53 PM

I had a different version of the 102 f7 refractors and a C8.  My personal experience was that if the C8 was collimated AND well cooled (takes a little planning), it was as good or better on the planets, and much better for everything else in the sky.  On very rare nights of great seeing, you can push the C8 farther than a 102mm.  Since the two scopes had similar mount requirements, I ended up keeping the C8 and picked up a more portable refractor for when I didn’t properly cool the C8 and wanted to take a quick look.  
 

I think personal preference will certainly impact a decision between a 4” refractor and a C8.  A C8 does take a little more TLC to extract the most out of it.  
 

Mike


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#7 gwlee

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Posted 12 May 2021 - 11:13 PM

I often use a small refractor for viewing planets, but I wouldn’t buy one for that purpose. The seeing here often supports 200x, and I prefer observing with at least a 1mm exit pupil. Meeting both requirements takes at least an 8” scope. An 8” refractor is an observatory class instrument. To me, a small reflector is ok for lunar observing. 


Edited by gwlee, 12 May 2021 - 11:20 PM.


#8 bobhen

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Posted 13 May 2021 - 05:52 AM

Hello,

Yes, I have "supposedly" a good planetary telescope, one C8 .... for which I have never had a sharp, clear and stable observation, so and thinking of changing instruments I would like to know your opinion about this refractor as an instrument for planets and lunar initiation ...
very kind.
paul

What is you location? Seeing can vary on average depending on location.

 

If you have done everything you can with the C8 (like suggested in post number 2) and still want a planetary refractor “with some budget limitations”, consider…

 

In the 4” class: The Sky Watcher 100mm F9 ED
Larger than 4”: The Sky Watcher 120mm F7.5 ED

 

Bob


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#9 Bomber Bob

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Posted 13 May 2021 - 06:29 AM

Mine did fine at 400x on the moon. Jupiter after 350x is getting dim for a 4". But 300x is fine in my super steady seeing.

 

I rated the scope as best bang for the buck when i had mine around 3 years ago.  Most SCT 's are just lack luster at best and never do as well as a frac or Newt.

Same here with my AT102ED F7 -- a great all-rounder & a Great Deal, too.  I could judge the planetary seeing based on its limiting magnifications:  60x per inch on Jupiter = 7 / 10; 75 x / in = 8 / 10; etc.  4 Plato craterlets most times; and, excellent Terminator contrast w/o fringing.

 

Pair the AT102ED with an 8" or larger Newt, and you cover a lot of objects, including planetary.


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#10 Spikey131

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Posted 13 May 2021 - 08:21 AM

I frequently observe with a 4” APO and a C8 in tandem.  They both give very good views of Solar System objects.  But I see far more detail with the C8, observing side by side, in all but the worst seeing conditions.  The AT 102 is a fine instrument at a more than fair price, but you have a better planetary scope already.

 

As others have noted, the C8 needs to be collimated, and you will need to deal with thermal issues to realize its full potential.


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#11 erin

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Posted 13 May 2021 - 08:22 AM

The AT102ED is a great scope and does well on the planets, but the faster focal length means a smaller image scale and more glass required to get there. The SW100ED would do better with the f9 or a larger mak. I have an 8” ACF and it is hands down my best planetary scope, but if it wasn’t, I would still want more aperture than 4 inches for the planets. Insulating a mak or sct really helps!


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#12 Mark Lovik

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Posted 13 May 2021 - 12:46 PM

Just to confirm -- most APO or semi-APO refractors are great.  I am considering purchasing a similar scope to the AT102ED.  This is about the edge (size and weight) for my manual alt-az grab and go setup and would compliment my imaging refractor / SCT combination.

 

My 8" SCT was originally setup on a manual mount EQ mount before I added the motors and controls for EAA imaging.  It was great for fast setup when I just wanted to view the sky.  Adding a 2nd refractor and make it a dedicated grab and go setup instead of shuffling my systems around would be useful.  It could do low power sweeping and casual planetary viewing.  My eyes no longer do well with smaller exit pupils (1mm is about my exit pupil limit before eye artifacts become annoying -- so my planetary limit is around 100x for a 4" refractor).  The chromatic aberration at these powers would not be a real problem on the AT102ED, or use a blue buster filter on the scope.



#13 t.r.

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Posted 13 May 2021 - 02:04 PM

120ED will do what you seek.
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#14 MortonH

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Posted 13 May 2021 - 06:38 PM

Hello,

Yes, I have "supposedly" a good planetary telescope, one C8 .... for which I have never had a sharp, clear and stable observation, so and thinking of changing instruments I would like to know your opinion about this refractor as an instrument for planets and lunar initiation ...
very kind.
paul

I found a Skywatcher 100ED (doublet) was rather dim when side by side with my C8 but a 120ED (doublet) was much closer.  If you want the simplicity of a refractor then a 120ED would be a good choice (assuming you have a suitable mount as the tube is relatively long).


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#15 BillP

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Posted 13 May 2021 - 09:21 PM

Hello,

Yes, I have "supposedly" a good planetary telescope, one C8 .... for which I have never had a sharp, clear and stable observation, so and thinking of changing instruments I would like to know your opinion about this refractor as an instrument for planets and lunar initiation ...
very kind.
paul

Larger aperture telescope will do better on planetary in many respects.  From a resolution perspective the aperture gives you more resolution but of course the atmosphere will generally limit you to about what a 4" will see anyway (just over 1 arcsec).  The extra light gathering is IMO the bigger benefit as it allows you to go higher in magnification and keep a brighter and consequently more contrast transferred image.  So more aperture is better to a point in the resolution/brightness/contrast realm, but to a point, especially with resolution.  Where the kicker will always come in is in thermal management.  No matter what way you want to cut it, the larger aperture will have more mass and have more heat to shed and be more problematic keeping up with temp changes through the course of observing.  So it will always add a chore to manage to the equation, and some folks can handle that chore well and others not so much.  So there is no "trick" that works consistently and you will easily be able to find folks that tried the tricks and the tricks did not work.  Not saying the tricks did not work for others, just pointing out it is not as safe of a best as keeping the aperture down so the mass is smaller and can handle acclimation passively without issue.  Optical design also comes into play as when the optics are folded, it means light can pass through the same internal thermal multiple times accentuating the disturbance to the view.  So the folds add complications that require extra effort in taming the thermals.

 

I have or had 4, 6, 8, and 10" instruments that I use on planetary and it is my primary observing passion.  I use a 4" because there is no thermal management as it passively does it all on its own very quickly and stays that way, plus given the seeing not being sub 1 arcsec most anyplace for anything other than fleeting fractions of a second, and that I care more about consistent stability rather than fleeting stability for my observing, the 5" and under class of refractors just makes life easier so I can just focus on the observing and not the equipment.  I have gotten superlatively detailed planetary views from my 6, 8, and 10" scopes, but for the most part those views were fleeting seconds during the observing session, whereas with the 4" the views are mostly rock steady with fleeting moments of instability.  So I just prefer the more stable view and ease of thermal management (i.e., no thermal management). 

 

So IMO 4-5" is an incredible aperture for planetary as it has so many benefits over larger mass instruments.  So I understand fully how you characterize your experiences with your 8" as while it is not common for all it is surely an experience many can empathize with.  But as others point out, these issues can be tamed.  Question is if that is what you want your focus to be or do you just want to get out and observe and instead put the work into developing observing skills rather than thermal management skills?  Personally, I feel nothing lacking in the resolution or magnification realm using a 4" for planetary...and that feeling is from someone who has seen exactly what apertures up to 10" can see after managing those larger beasts to views that they are best capable of.  I have seen some planetary details out of my 10" that my 4" will never in its wildest dream be able to reproduce, but all things considered I still choose the 4" as its views are fantastic, I have gained the skills to see things from it that I rarely read that others have seen with their larger apertures, and it is a no-effort platform for planetary.

 

Just wanted to give you the other perspective as any celestial observing is never about aperture and always about preferences as any aperture is limiting so no magic all of a sudden makes 6" or 8" or 10" or 32" all of a sudden "best" because there is always a better not matter what size you mention.

 

ps - If you really want a kickarse planetary view, or a DSO view, take any aperture up to 6000 ft altitude so your transparency and contrast is thru the roof and you will quickly realize that aperture is not the key to some jaw dropping capabilities!  Context is everything!!


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#16 CHASLX200

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Posted 14 May 2021 - 06:05 AM

I have a 18" Obsession for planets. But would like another 4" fract for just all around viewing. FL is not so hot for deep sky but i have super steady seeing so i can use every drop of that 18" mirror for planets at 400 to 1000x.  The AT102 makes a great sweeper and can take the power for the moon and planets for a cheap price.


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#17 gnowellsct

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Posted 15 May 2021 - 07:58 PM

Some people when they say they have a c8 mean they have a late 1970s c8. other people mean they have a 1980s c8. Other people mean they have a 1990s c8. Or you could have an early 2000 c8 or a c8 from the recent teens.

On the refractor group if you say you have a bad c8 then people will take it for granted that it's a bad c8 no matter what vintage because all c8s are bad. If you go over on to the SCT group and ask the question you will get a few people who will talk to you about scts but you will also get pounced on by all the people here who are saying bad things about scts and stroll over into the SCT group to offer opinions about a telescope they wouldn't be caught dead owning just for the chucks and giggles.

It's like in high school where some people thrived on passing bad gossip, over and over and over again.

One thing to bear in mind with a 100 mm refractor at f7 is that you will have 100x with a 7 mm ocular and 200x with a 3.5 mm ocular. 100x you will have a 1 mm exit pupil and at 200x you will have a 0.5 mm exit pupil.

In a c8 you will have 200x with a 10 mm ocular at a 1 mm exit pupil.

The reason to care is that as exit pupils shrink to 1 mm and below the telescope will begin to illuminate defects in your eye. If you are middle-aged or older they can be quite annoying. This is an organic limitation of the eyeball and has nothing to do with Roland Chisten or Mr yoshida or any of the other apostles of apochromatic refractors. A 4-in telescope at 200x is going to be a 0.5 mm exit pupil and that is going to do things to your eye. It will also be relatively dim which will diminish color.

So actually a small refractor as far as I'm concerned is not the weapon of choice for planet viewing. I have four such apos ranging from 81 mm to 130 mm. I do look at the planets with them. But I see more in the c8 and c14.

As for whether you should get a 4-inch refractor I think of course you should they are great to have. But I wouldn't call them planet viewing scopes. I call them beautiful luxuriously well-made scopes that are good for planets if I want to strain my eyeball with a smaller exit pupil. I ain't dead yet so every now and again I do that.

It is to be noted that Galileo would have considered a modern inexpensive 4 in achromatic telescope to be a fantastic planet viewing instrument. So there is that to consider. All you have to do is put yourself into the mindset of Galileo. Heck, even Messier would have jumped for a modern $400 achromat. Let alone the triplet and doublet apochromatics that we have.


Greg N

Edited by gnowellsct, 15 May 2021 - 08:05 PM.

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#18 SeattleScott

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Posted 15 May 2021 - 08:40 PM

Mine did fine at 400x on the moon. Jupiter after 350x is getting dim for a 4". But 300x is fine in my super steady seeing.

I rated the scope as best bang for the buck when i had mine around 3 years ago. Most SCT 's are just lack luster at best and never do as well as a frac or Newt.

Remember he lives in FL.

#19 ewave

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Posted 15 May 2021 - 09:52 PM

Some people when they say they have a c8 mean they have a late 1970s c8. other people mean they have a 1980s c8. Other people mean they have a 1990s c8. Or you could have an early 2000 c8 or a c8 from the recent teens.

On the refractor group if you say you have a bad c8 then people will take it for granted that it's a bad c8 no matter what vintage because all c8s are bad. If you go over on to the SCT group and ask the question you will get a few people who will talk to you about scts but you will also get pounced on by all the people here who are saying bad things about scts and stroll over into the SCT group to offer opinions about a telescope they wouldn't be caught dead owning just for the chucks and giggles.

It's like in high school where some people thrived on passing bad gossip, over and over and over again.

One thing to bear in mind with a 100 mm refractor at f7 is that you will have 100x with a 7 mm ocular and 200x with a 3.5 mm ocular. 100x you will have a 1 mm exit pupil and at 200x you will have a 0.5 mm exit pupil.

In a c8 you will have 200x with a 10 mm ocular at a 1 mm exit pupil.

The reason to care is that as exit pupils shrink to 1 mm and below the telescope will begin to illuminate defects in your eye. If you are middle-aged or older they can be quite annoying. This is an organic limitation of the eyeball and has nothing to do with Roland Chisten or Mr yoshida or any of the other apostles of apochromatic refractors. A 4-in telescope at 200x is going to be a 0.5 mm exit pupil and that is going to do things to your eye. It will also be relatively dim which will diminish color.

So actually a small refractor as far as I'm concerned is not the weapon of choice for planet viewing. I have four such apos ranging from 81 mm to 130 mm. I do look at the planets with them. But I see more in the c8 and c14.

As for whether you should get a 4-inch refractor I think of course you should they are great to have. But I wouldn't call them planet viewing scopes. I call them beautiful luxuriously well-made scopes that are good for planets if I want to strain my eyeball with a smaller exit pupil. I ain't dead yet so every now and again I do that.

It is to be noted that Galileo would have considered a modern inexpensive 4 in achromatic telescope to be a fantastic planet viewing instrument. So there is that to consider. All you have to do is put yourself into the mindset of Galileo. Heck, even Messier would have jumped for a modern $400 achromat. Let alone the triplet and doublet apochromatics that we have.


Greg N

Greg, out of all of your posts I've read here on CN, this was one of my favorites. bow.gif


Edited by ewave, 15 May 2021 - 09:52 PM.

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#20 Bomber Bob

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Posted 15 May 2021 - 10:34 PM

Hello,

Yes, I have "supposedly" a good planetary telescope, one C8 .... for which I have never had a sharp, clear and stable observation, so and thinking of changing instruments I would like to know your opinion about this refractor as an instrument for planets and lunar initiation ...
very kind.
paul

Paul:  At medium & high-power planetary, the AT102ED will be like a good 6" SCT for fine detail, but won't display all the subtle belt colors of Jupiter & Saturn of the larger CAT.  My excellent 1976 C5 Astro was about like a high quality 3" refractor.  My APM 152ED competed with my excellent Vixen VMC200L Field Maksutov  Et cetera, et cetera, et cetera...

 

This is the Refractor Forum, and I don't think you'll be disappointed with the AT102ED, but if your C8 is collimated, yet you're unhappy with its performance, I'd look at an 8" Newt.  Easier to align, same aperture, and most likely better overall performance than your C8.  It would trounce any 4" refractor.  And, it's a cheap option to test -- got mine used for $150 total.

 

Another plus for the AT102ED -- Portability:

 

AT102ED S24 LS (Mizar SP & FC-50).jpg

 

This was my ultimate Grab & Go Duo-Scope with the Tak FC-50 riding shotgun.  I had tons of fun with this kit -- with NO back strain!


Edited by Bomber Bob, 15 May 2021 - 10:35 PM.

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#21 Jon Isaacs

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Posted 16 May 2021 - 03:43 AM

Larger aperture telescope will do better on planetary in many respects.  From a resolution perspective the aperture gives you more resolution but of course the atmosphere will generally limit you to about what a 4" will see anyway (just over 1 arcsec).

 

 

The Dawes limit of a 4 inch scope is 1.14".  The Dawes limit is transformation of a high contrast pair to an extremely low contrast view, the Airy disks are overlapping, it takes high magnifications. In 1 arc second seeing, a much larger scope will be optimal.. Vladimir Sacek's simulations concluded that in 2" seeing, an 8 inch was optimal..

 

The diameter of the Airy disk to the first minimum of a 4 inch is 2.7", to avoid contrast loss, I want disks significantly smaller than the seeing..

 

This is not to say that a 4 inch cannot provide quality planetary views. Rather it's to say that the Dawes limit is a poor measure for matching aperture with seeing. 

 

Jon


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#22 Erik Bakker

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Posted 16 May 2021 - 05:32 AM

A good 8* SCT will need good circumstances, both of the skies above and the instrument itself, collimation, thermal equilibrium, to perform well at higher magnifications and the planets. If all of that is taken care of, the C8 is great.

 

If not all is taken care of, and is difficult to address, than the forgiving nature of a good apo and especially a good doublet come into play. It will perform to the best of it’s ability on any night after a half hour or so, many times even a bit sooner.

 

As an example, I have had absolutely stunning nights on the 3 big planets with my 7” Questar 7, some nights. And I have had it outperform my 4” fluorite doublet on quite a few nights. On most nights, the 4” fluorite showed quite similar detail, unless the seeing momentarily improved.

 

Where the Q7 always performed was in the ease of it’s views, always in the range of it’s 24-12mm Brandon eyepieces, never needing anything beyond that. And it always just cruised at 180-230x, where the refractor would already be strained. Having more of the 140-160x sweet range.

 

In return, the 4” fluorite always performed and only shoed the limits of the seeing. The bigger instrument showd that too, but many a night compounded by showing it’s own limitations of thermalvequilibrium or the lack thereof. Collimation was fixed at the factory and perfect.

 

But the real take from it all for me was, that both the 4” and 7” are limited by their apertures to really bring in  the finer planetary detail. My custom Matthias Wirth 16” f/5 with outstanding optics made that clear the very first night I used it on Jupiter. After decades, it was an astounding jump in observable detail. But it comes at the price of a much bigger instrument that is sensitive to thermal conditions and the quality of the seeing. At times taking 2 hours to cool and reaching (near) optimum performance. But when it does that, it is well worth the wait. As a consequence, I always start with a smaller refractor to judge the seeing when observing the planets and take out the 16” when seeing looks promising. 


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#23 Jon Isaacs

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Posted 16 May 2021 - 07:58 AM

But the real take from it all for me was, that both the 4” and 7” are limited by their apertures to really bring in  the finer planetary detail. My custom Matthias Wirth 16” f/5 with outstanding optics made that clear the very first night I used it on Jupiter. After decades, it was an astounding jump in observable detail. But it comes at the price of a much bigger instrument that is sensitive to thermal conditions and the quality of the seeing. At times taking 2 hours to cool and reaching (near) optimum performance. But when it does that, it is well worth the wait. As a consequence, I always start with a smaller refractor to judge the seeing when observing the planets and take out the 16” when seeing looks promising.

 

:waytogo:

 

 

My own approach is somewhat different than Erik's.  If my gut feeling is that the seeing will support a larger aperture, I bring it out an hour or more before sunset and begin the cool down process, this gives me a head start and I can be observing the planets not too long after sunset.  If I were to wait to begin the cool down until I'd verified the seeing, the wait would begin then rather than earlier.

 

Two things are relevant probably.  First, at our place in the city, good or better seeing is expected, it's not a rare occurrence. Part of the reason is that the cool Pacific is about 5 miles to the west, it moderates the climate and laminar breezes off the ocean make for stable seeing.

 

The second reason the result of the first. During a good portion of the year, those gentle breezes often the ocean will often carry low and very local clouds in that quickly end the time evening. If I wait to setup, I'm losing observing time.

 

If I am going to use a refractor, I still put out early but more like sunset.. 

 

Jon


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#24 clearwaterdave

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Posted 16 May 2021 - 08:02 AM

I have to say it's a matter of opinion.,When I showed folks Jupiter in my 70/600 achro they were amazed.,as was I.,And seeing the rings of Saturn took my breath away.

 Small scopes are not the best planetary scope's but they can still give a thrill.,waytogo.gif .


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#25 CHASLX200

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Posted 16 May 2021 - 08:33 AM

I have to say it's a matter of opinion.,When I showed folks Jupiter in my 70/600 achro they were amazed.,as was I.,And seeing the rings of Saturn took my breath away.

 Small scopes are not the best planetary scope's but they can still give a thrill.,waytogo.gif .

I love the smaller fracts for fast grab and goes in the 90mm range. I have a 90mm F/10 and ST80  to go along with my 18" Obsession. I have some super steady seeing so i can use all of that 18" and really see some crazy detail on the planets.

 

I would rather view deep sky in the fracts.




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