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Help me pick a larger planetary scope

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#51 CHASLX200

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Posted 08 April 2018 - 09:18 AM

Whoa, there.  The loss of contrast due to diffraction is proportional to the area of the obstruction.  The spider vanes of a 10 inch will represent an area of about 0.5% of the total area.  The secondary on a 10 inch F/6 can be well under 20% by diameter, that would be 4% by area.  That means the total obstructed area would be less than 5% and it could be as low as 3%.

 

A finding a 7 inch Mak with a secondary obstruction that small will be difficult, in part because of the need for a secondary baffle.  Orion talks about the "secondary mirror obstruction" but neglects to include the secondary baffle diameter, at least in their smaller Maks.  They claimed a 39mm CO in my 127mm Starmax but when I measured it, the actual diameter of the baffle was 48 mm.. 

 

Jon

Yikes.  I may have turned into a Mak lover now.  After getting a $300 UHTC ETX125 i want a bigger Mak with super optics. 



#52 CHASLX200

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Posted 08 April 2018 - 09:25 AM

"Unfortunate it is coming up yet again."

It should come up in this forum whenever it is suggested a larger Newt would be a worse choice than a small refractor for planetary (no matter the location!) People that run a high quality Newt will want to see why you would say that. What you wrote doesn't make it clear.

I sure don't understand man. My bigger Newts have always crushed my APO's on the planets and deep sky every time. Not even close. I have owned 230 scopes and the Newt is the only way to go if it has top notch optics.   

 

I have had 6" APO's and they just lack the light needed with the power i use. Non of the 230 scopes i have had have come close to the Zambuto and OMI scopes in the 11 to 18" size and i avg the best seeing that most any place in the country.   Like i keep saying, why buy a 8" APO that would run $30k + with a big mount vs a old school 8" F/8 Cave or other top notch built Newt that cost $25 to 27K less? I mean sure that 8" APO will be a tiny bit sharper and have a little more contrast, but is the view worth 25K+ more vs the Newt.  And don't even bother with SCT's as they never come close for planet work.


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#53 CHASLX200

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Posted 08 April 2018 - 09:28 AM

I don't think that's the case here. Bill is trying to explain that there are practical considerations and I agree, some people jump into things without understanding what's involved. Aperture can be great, but we really don't know what the circumstances are with the OP and the OP admits not being familiar with dobs. Learning curve for planets and reflectors deserves some mention. OP is welcome to do whatever, but there are many factors that come into play with bigger scope and planets. It isn't just a set it up scenario and it works. Most amazing planetary images have been with large reflectors, but there are many factors involved. Also, a high quality Newt? From what it sounds like to me, this is just a basic, low cost dob, which may or may not be good, yes? 

The run of the mill Dob will not have a good mirror 7 out of 10 times and maybe 100% of the time to make me happy. I gotta have the best from the get go or it goes down the road.

 

As for the OP his seeing can limit what he can use or should buy.  My seeing has no limits so for me the big Newt is the clear winner.



#54 Richard Whalen

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Posted 08 April 2018 - 12:36 PM

Jon,

 

With spiders, the numbers dont tell the whole story from my experience. The effect on scatter/glare is much greater than the numbers would suggest. Most so on Jupiter. On the moon I see much less effect as high power views showing just a section of the moon tend to disperse the effects and hide them. On smaller objects like Planets, the glare from the spikes are quite noticeable.  The brighter the Planet, the larger the aperture the worse they are. 

 

As far as Dawes limit and others, they dont apply to observing planets and the moon much. They are for point source objects of equal brightness (double stars). Planets and the moon have color, shading, shadows, and varying degrees of brightness. An example is craterlots on the floor of Plato where many should not be resolvable according to the various formulas but are, often observers are seeing close to 1/2 those limits. 

 

There actually might be some validity to Chas maddness of very high magnifications (for him), dims the effects of the spiders, spreads out the glare? As long as the seeing/optics supports those high magnifications, he might actually be seeing more fine detail than at lower normal magnifications most use that have good eyesight. So for him it works.

 

Sure you can always go larger in aperture, but the so many other factors come into play such as seeing, cooldown, mechanics, mirror support, optical quality tracking smoothness etc. Not to mention setup/takedown etc.

 

I don't have enough fingers or toes to count the times I have seen smaller high quality scopes (apo's, MCT, MNT's) outperform much larger quality (well collimated) dobs on Jupiter under excellent (at least for the smaller scopes) seeing conditions on low contrast fine detail. 

 

Also what might be excellent seeing for a 8" might be considered poor to fair for a 18". This is always the case from my observing location. Same with a 5" verses a 12.5" from other locations. If the seeing is not dead still for your aperture/magnification fine low contrast detail is washed out. I have even seen this many times from WSP which is known for its excellent seeing and large numbers of high end telescopes of various designs.

 

Also I have found the higher the optical quality, the less effected it is by seeing unless it is horrible to average. Most common is when seeing is very good but not excellent. Just my observations, your experiences my differ....



#55 Daniel Mounsey

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Posted 08 April 2018 - 06:11 PM

The run of the mill Dob will not have a good mirror 7 out of 10 times and maybe 100% of the time to make me happy. I gotta have the best from the get go or it goes down the road.

 

As for the OP his seeing can limit what he can use or should buy.  My seeing has no limits so for me the big Newt is the clear winner.

 

At these levels, for a Planetary Newt, where contrast is a major factor at higher magnification, I’d have to concur with you from my experienced relative point of view.



#56 CHASLX200

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Posted 08 April 2018 - 06:50 PM

At these levels, for a Planetary Newt, where contrast is a major factor at higher magnification, I’d have to concur with you from my experienced relative point of view.

I am just not a APO fan in sizes over 4".  3 and 4" APO's are my fav all around small scopes.  Once you get into the 5" and bigger sizes cost become a problem for me and 7" and bigger the mount needed becomes pricey and big. A bigger Newt is many times cheaper and does what i need it to do.  No 7" or 8" APO would give me the image at 1100x+ like my 14.5" and 15" Zambuto and OMI optics have done time and time again.



#57 starman876

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Posted 08 April 2018 - 08:37 PM

The APO versus the newt rabbit hole.  Hard not to go down it.  However, some cannot resist and do their best to take as many with them as they canlol.gif 


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#58 GShaffer

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Posted 09 April 2018 - 09:03 AM

This is heading into the tired old APO vs Newt argument we have seen soooooooooo many times before. Reading the OP's original post and considering this is the reflectors forum it is pretty obvious the intent was to get recommendations on a Dob/Newt/Mak. Lets go back there and leave the APO's out of it or this topic will be considered as having run its course....


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#59 starman876

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Posted 09 April 2018 - 09:16 AM

darn. i forgot what we were talking aboutlol.gif



#60 scooke

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Posted 09 April 2018 - 06:49 PM

I have to say, I favor a combination of Bill and Chas.  Bill makes the argument that aperture doesn't override observing skill, experience, and convenience.  I agree.  Chas says that larger telescopes under good seeing conditions allow one to see more and use higher power than smaller telescopes.  I agree.  Why not do what I do?  Have a large telescope setup at ambient temperature that can be used on a moments notice.  Use it as often as possible and really spend some time with each object you view and concentrate on increasing your skill.  Best of both worlds.  So the answer to the OPs question depends on his personal situation.  If you can keep your scope in the garage or observatory and just start observing, do it.  If not, start deciding what is most important to you.  Convenience or horsepower.



#61 starman876

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Posted 09 April 2018 - 07:36 PM

My 12.5" porta ball has provided me views better than any other scopes I have owned. Of course that is on rare nights of awesome seeing.   On regular nights when seeing is not so good my smaller scopes perform. The bigger the bite you take out of the atmosphere the more noise you will see on nights of poor seeing.    However, in the end the figure of the optics rule.  I have seen some write ups that  say that average optics with large aperture will outperform smaller optics with better optics.  I think the comparison scale is rather large before that happens.



#62 Mel M

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Posted 09 April 2018 - 08:04 PM

I have to say, I favor a combination of Bill and Chas.  Bill makes the argument that aperture doesn't override observing skill, experience, and convenience.  I agree.  Chas says that larger telescopes under good seeing conditions allow one to see more and use higher power than smaller telescopes.  I agree.  Why not do what I do?  Have a large telescope setup at ambient temperature that can be used on a moments notice.  Use it as often as possible and really spend some time with each object you view and concentrate on increasing your skill.  Best of both worlds.  So the answer to the OPs question depends on his personal situation.  If you can keep your scope in the garage or observatory and just start observing, do it.  If not, start deciding what is most important to you.  Convenience or horsepower.

"Bill makes the argument that aperture doesn't override observing skill..."

True, but aperture is fun and the satisfaction of seeing your reflector respond as you learn how to use it can keep a person interested. 

I took the beginner advice to start with binos. Killed my interest.

Image was too small, too dim. I came back after a few years but I use binos only in daylight.



#63 scooke

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Posted 09 April 2018 - 08:36 PM

I have never understood the common concept that smaller telescopes deliver the best images during bad seeing and big telescopes during good seeing.  Boloney.  Big scopes deliver the best views always.  You just may have to wait longer for larger scopes.


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#64 Pinbout

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Posted 09 April 2018 - 08:46 PM

I have never understood the common concept that smaller telescopes deliver the best images during bad seeing and big telescopes during good seeing.  Boloney.  Big scopes deliver the best views always.  You just may have to wait longer for larger scopes.

 

If and only if the large telescopes have well corrected optics

 

And with imported scopes the bigger you go the more hit or miss getting good optics 

 

a lot of people have 1/2~ optics and blame the mushy views on the seeing


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#65 starman876

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Posted 09 April 2018 - 09:23 PM

If and only if the large telescopes have well corrected optics

 

And with imported scopes the bigger you go the more hit or miss getting good optics 

 

a lot of people have 1/2~ optics and blame the mushy views on the seeing

That is oh so true.  when I see that a mirror is rated at 1/4 wave than I know there is no way you will see 1/4 wave at the eyepiece.  If you start out with a 1/8 wave mirror you will have a good chance you will get 1/4 wave or better at the eyepiece. 



#66 CHASLX200

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Posted 10 April 2018 - 05:28 AM

If and only if the large telescopes have well corrected optics

 

And with imported scopes the bigger you go the more hit or miss getting good optics 

 

a lot of people have 1/2~ optics and blame the mushy views on the seeing

I would never touch any of these cheaper made Newts.  Just put a Zambuto or other top notch made Newt next to these cheaper scopes and it is like nite and day.  For a first timer they won't know any better.  



#67 starman876

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Posted 10 April 2018 - 06:32 AM

I will be testing my new ZWO 12" F4 newt agaisnt the 12.5" porta ball I have. The porta ball is F 4.8.  So there should not be too much difference in coma. Of course coma in a fast scope has nothing to do with looking at objects as small as a planet


Edited by starman876, 10 April 2018 - 08:16 AM.


#68 rolo

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Posted 10 April 2018 - 07:13 AM

Fortunately I've had both a Newtonian 12.5" f/6 and Mak 10" f/12.5. 

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#69 starman876

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Posted 10 April 2018 - 08:17 AM

Fortunately I've had both a Newtonian 12.5" f/6 and Mak 10" f/12.5. 

Looks to me like the Cave imaged better than the Mak.  It is clear that 2.5" extra aperture shows more detail. 



#70 Jeff B

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Posted 10 April 2018 - 08:58 AM

I have a takahashi fc100d on a cg4.  I want to get a larger scope to use on nicer nights when I am going to be out for several hours.  I care most about lunar and planetary.  I was thinking about getting a 10 inch dob or a cat that I can mount on my cg4.  What are my best options?

Always a lively discussion.

 

I see you live in Illinois so I assume that like around here, the seeing usually sucks...out loud sometimes.  But sometimes it can be quite good.  So, I recommend a Newt in the 8-10" range for planetary.  That aperture seems to work really well for me for solar system viewing here in the midwest.

 

Actually I have a local friend who is looking to do exactly the same thing, a 10" F6 Newt for the planets and looking for some advice so here was my response"

 

"Hey Nick.  I hope you are well.

 

I see you're getting ready for the planets this year.  10" is a great planetary aperture for around here.

 

My experience is that once you get below ~25% CO, the differences are really small.  I personally like ~ 17-18% if there is a standard diagonal size that will get me there.  In your case, that would be a 1.75" diagonal, which is a standard size.  However anywhere between 15-20% is fine but at 15%, it can be a bit more challenging to collimate the scope because the fully illuminated field is very small. 

 

By far the biggest factors to good planetary views with a Newt are:

 

1. Excellent optical quality of the primary and secondary.

2. The ability to get and maintain excellent collimation no matter where in the sky the scope is pointed.  This includes the focuser and primary & secondary.  At F6, the scope's sensitivity to,and keeping good, collimation is much easier than at F 5 or below.  I assume you will be using the bino-viewers so you need to make sure the focuser will not sag under the weight of the viewer.

3. Great thermal cooling ability which includes a fan on the back of the primary mirror as well as a boundary layer fan scrubbing the primary's surface.  It's best to have a thinner primary mirror to help with cooling.

4. Did I say excellent optical quality of the primary and secondary?

 

I also recommend that when the scope is pointed south, the focuser be put on the east side of the OTA.  That's because most breezes will come from the south, north and west and will not blow your body heat across the front of the scope if the focuser is on the east side. 

 

Some lessons hard won over the years.

 

Jeff"

 

I also suggested a solid tube.

 

So, I hope this helps and take what you like and leave the rest.

 

Jeff


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#71 Mike Spooner

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Posted 10 April 2018 - 09:12 AM

Most of the posts have touched on the desirability of quality optics. The first time getting a nearly perfect night with a nearly perfect scope on well placed planets or moon can be quite an experience. You have a nice refractor but as many here have stated, the increased aperture of a Dob with great optics will open up possibilities for improvement. In addition to the size and optical quality I would encourage some ergonomic considerations such as standing, sitting and focuser placement. Lots of this has been hashed over in the forums but determining your own preferences can remove a lot of distraction from the viewing experience. The very best nights can be relatively rare depending on location but having a great performing and comfortable scope can remove tiny aggravations that get in the way of getting the utmost out of those excellent conditions. For decades I never used an observing chair (a lot of my scopes needed ladders or were longer focus) but after getting one I don't find I missed as much as I had thought. With my height, I really prefer standing with an 8" or 10" Dob with approx 55" FL. Focuser placement can be critical for folks with back problems. I have an 8" f/6 and it is just too short for my observing style though it does transport a bit easier than longer scopes. For equatorial mounts I like shorter scopes and here I find the chair gets used quite a bit.

The major planets are fairly low in altitude for the US this year and that will impact the views available from here. 

 

While superior seeing conditions are rarer than most would think, if you never get hints of excellent definition then it may be time to do an honest evaluation of the scope and optics.

 

Mike Spooner 


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#72 scooke

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Posted 10 April 2018 - 08:28 PM

My apologies.  In my earlier posts I assumed but did not state that my opinions and experience assumed highest quality optics.  I've been doing this for over 40 years now and have no place for average to low quality optics.  At this point, I would prefer to go without rather than settle.  Get the highest quality, largest aperture that you can support logistically often and you will be happy with the results.  It's a slight tangent but if you are looking at large quality aperture that probably means a newtonian.  I grew up in Kansas City, lived in Omaha for a few years and have lived in Florida for the last 4 years and I can tell you that what most people think is bad seeing is really bad thermal management.  Get the thermal issues managed and the larger the quality aperture the better the view.  In ALL seeing conditions.



#73 scooke

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Posted 10 April 2018 - 08:36 PM

, if you never get hints of excellent definition then it may be time to do an honest evaluation of the scope and optics.

 

Mike Spooner 

What Mike states is what I was saying with my earlier post.  Large high quality optics win in ANY seeing condition in my experience.  If seeing is bad you just have to observe for longer times so your brain can build a good image from the moments of clarity.  If the optics are good and thermal issues are good then you will have moments of clarity in ALL seeing conditions.  If the stars are always bloated, it is not seeing, it is thermal issues.


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#74 scooke

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Posted 10 April 2018 - 08:37 PM

Or your quality optics are not the quality you think.


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#75 Not Here Anymore

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Posted 11 April 2018 - 05:01 PM

If and only if the large telescopes have well corrected optics

 

And with imported scopes the bigger you go the more hit or miss getting good optics 

 

a lot of people have 1/2~ optics and blame the mushy views on the seeing

Kind of a given.

 

And the other problem is that people fail to recognize the "seeing" inside their telescope.  Large scopes take longer to cool, and in some cases will never keep up.  Boundary layer fans are a necessity in my experience, and their impact was not subtle.

 

Also, seeing limits all scopes the same in my experience.  If seeing is 1 arcsecond, than it is one arcsecond for all scopes (assuming they are large enough to resolve to 1 arcsecond).  Granted, larger scopes, which of course are capable of greater resolution, will likewise show more clearly the impact on that resolution, but they do not magically lose detail because they are larger.


Edited by JCAZ, 11 April 2018 - 05:20 PM.

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