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Help me pick a larger planetary scope

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#26 barbie

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Posted 03 April 2018 - 07:32 AM

Star test and the optics were tested on the bench.  I was told that there was nothing more that needed to be sensibly done to the optics to make them any better.  Planetary views bear this out.  I don't use binoviewers either!grin.gif


Edited by barbie, 03 April 2018 - 07:36 AM.

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#27 Pinbout

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Posted 03 April 2018 - 08:03 AM

Star test and the optics were tested on the bench.  I was told that there was nothing more that needed to be sensibly done to the optics to make them any better.  Planetary views bear this out.  I don't use binoviewers either!grin.gif

that sounds like you sent them to Carry  lol.gif

 

more so than not the 2ndries sux ...a lot


Edited by Pinbout, 03 April 2018 - 08:28 AM.


#28 Steeveaux

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Posted 03 April 2018 - 11:11 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?


You will need to be real conscious of weight. What will a cg4 carry?
Using an old Cave (Parks?) fiberglass tube I put together a 10" f7 Newt that rotates on a Losmandy C-11. I got the weight of the cradle, rings and OTA down to less than 25 lbs.

https://www.cloudyni...-1520634622.jpg

#29 Pinbout

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Posted 03 April 2018 - 12:37 PM

You will need to be real conscious of weight. What will a cg4 carry?
Using an old Cave (Parks?) fiberglass tube I put together a 10" f7 Newt that rotates on a Losmandy C-11. I got the weight of the cradle, rings and OTA down to less than 25 lbs.

https://www.cloudyni...-1520634622.jpg

how about getting some cut vinyl graphics for that blank canvas...grin.gif



#30 Steeveaux

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Posted 03 April 2018 - 01:32 PM

how about getting some cut vinyl graphics for that blank canvas...grin.gif


It's "tat-free" isn't it?

I did that with my refractor (that's a different kind of telescope) and sorta regretted it after a while. My daughter is an artist. I might let her pinstripe it up a little.

Steve O.
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#31 barbie

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Posted 03 April 2018 - 06:36 PM

Danny

 

I did not send my optics to Carey and my secondary is just fine, unlike your spelling!!lol.gif


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#32 Jeff B

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Posted 03 April 2018 - 06:52 PM

I find that the ideal planetary telescope is the largest quality aperture that you will use frequently.

 

It can be fast or slow, in terms of f/#, so long as the optics are good.  Ideally the primary is not too thick so it can cool and be cooled in a reasonable time.  Proper mirror support and achieving and holding collimation are also very important.

Can I "like" this a couple of dozen times?  Especially the text in bold.


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#33 barbie

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Posted 03 April 2018 - 10:18 PM

Can I "like" this a couple of dozen times?  Especially the text in bold.

Then my scope qualifies as the most important criteria that Mike Lockwood mentions are more than easily met.lol.gif lol.gif lol.gif



#34 Daniel Mounsey

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Posted 03 April 2018 - 11:32 PM

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?

 

Your location for seeing is questionable. How long have you been observing and how many bands have you seen on Jupiter with your scope for example?


Edited by Daniel Mounsey, 03 April 2018 - 11:40 PM.

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#35 BillP

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Posted 05 April 2018 - 03:08 PM

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?

 

A 4" doublet is small, light, nimble, effortless, and a joy to wield.  If that is what you are used to, then be cautious as you go larger as things become larger, heavier, unwieldy, and effort prone very quickly.  So you need to be in the mindset for all that, especially thermal management of the optic (which there is none with your Tak).  So more work, not effortless, more planning, sometimes frustration, less observing, less fun -- aperture is great, but it is not free.  Given you are used to a 4", I would say you should go with 8" instead of 10" as overall it is more ergonomic and less of a hassle for thermal management/acclimation with its lesser mass.  An 8" aperture also gets you .57 arcsec resolution, and that is well below what extremely good seeing conditions will show you (the 4" gets you 1.14 arcsec).  I tend to like my minimum exit pupil for Jupiter to be around .65mm.  Smaller than that and the dimming loses too much of the low contrast details on the planet.  So in your 4" that exit pupil gets you to 154x, with the 8" it lets you have more image scale and gets you 313x!  A nice jump in capability for when the seeing will support that magnification at your location.  With Saturn or Mars, I find these can take a smaller exit pupil well so will routinely go to a .3mm or even smaller with them (350x in the 4", way higher with an 8" but need some really special evenings for a steady view at my location over 400x).

 

Now at many locations, seeing does not often support sub-arcsec seeing.  However, when I lived near D.C. I used to observe most every clear night.  Observing that often I would find that in general, I could expect at least 2 to 3 observing sessions a month where I encountered very excellent seeing for steady high magnification planetary (i.e, 350x-450x is rock steady).  Now most of my observing is with a 4"...and taking a 4" out every clear evening for a few hours is no chore at all.  Hauling my 10" out that often though is something I would never ever do as just too much prep and care and feeding to manage the thermal behavior and collimation and cleanliness of the mirrors, etc.  So if all I had was my 10", well then I would not be out as often, and that means I would not encounter those really good seeing times as often.  So just food for thought as how much planning and effort you are willing to undertake with your equipment can impact not only how much you observe, but also how likely it will be that you encounter the best seeing.  If extra effort, bigger carry loads, etc. are not an issue for you, then you are golden and not a worry.

 

A premium mirrored 8" Newt/Dob would be nice IMO.  I would go with that over an SCT simply because it will also be great as a generalist since it can achieve a lot larger TFOV than an SCT can. IME the Newt is also a little easier to thermally manage than an SCT and the focuser can be had that is much nicer/precise (no mirror flop), and has a smaller CO.  But if you don't care about TFOV and want the smaller footprint over other considerations, then I've had some excellent planetary views through an 8 SCT.  But if I were to get one, would go through Company 7 or the like so at least you are assured of getting a mirror up to specs.  My preference though would be the Newt.  Who would make a premium 8" mirror?  I'm sure other will chime in as to who.


Edited by BillP, 05 April 2018 - 03:39 PM.

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#36 BillP

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Posted 05 April 2018 - 05:29 PM

Your location for seeing is questionable. How long have you been observing and how many bands have you seen on Jupiter with your scope for example?

 

This is a good question.  As long as the scope is collimated well and diffraction limited, you do not need a lot of aperture for a lot of detail on Jupiter.  So if you are typically only getting the 2 major bands in your views, that means most likely poor seeing so more aperture will not help much.  With my TSA-102, on one of those "good" planetary evenings which I get 2-3 times a month, I will get a view similar to this except that the GRS is more defined for me and also has visible structure within it:

 

http://www.lpl.arizo...97/jup0910f.jpg

 

Then a few times a year I can find windows of seeing good enough to get a steady view like this from the little 4", showing more fine intermediate details between the bandings.  My views though show higher contrast than these sketches though:

 

http://wwwcdn.skyand..._painting_l.jpg

 

So I'm not talking "moments" of steady seeing like you hear many say, these views will be stable so can enjoy extended views with the image not letting go of any details.  So I know I have windows of good steady seeing to support larger apertures, but I'm pretty happy with what the 4" will do for me during these good seeing/transparency windows.


Edited by BillP, 05 April 2018 - 05:31 PM.


#37 CHASLX200

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Posted 05 April 2018 - 06:08 PM

A 4" doublet is small, light, nimble, effortless, and a joy to wield.  If that is what you are used to, then be cautious as you go larger as things become larger, heavier, unwieldy, and effort prone very quickly.  So you need to be in the mindset for all that, especially thermal management of the optic (which there is none with your Tak).  So more work, not effortless, more planning, sometimes frustration, less observing, less fun -- aperture is great, but it is not free.  Given you are used to a 4", I would say you should go with 8" instead of 10" as overall it is more ergonomic and less of a hassle for thermal management/acclimation with its lesser mass.  An 8" aperture also gets you .57 arcsec resolution, and that is well below what extremely good seeing conditions will show you (the 4" gets you 1.14 arcsec).  I tend to like my minimum exit pupil for Jupiter to be around .65mm.  Smaller than that and the dimming loses too much of the low contrast details on the planet.  So in your 4" that exit pupil gets you to 154x, with the 8" it lets you have more image scale and gets you 313x!  A nice jump in capability for when the seeing will support that magnification at your location.  With Saturn or Mars, I find these can take a smaller exit pupil well so will routinely go to a .3mm or even smaller with them (350x in the 4", way higher with an 8" but need some really special evenings for a steady view at my location over 400x).

 

Now at many locations, seeing does not often support sub-arcsec seeing.  However, when I lived near D.C. I used to observe most every clear night.  Observing that often I would find that in general, I could expect at least 2 to 3 observing sessions a month where I encountered very excellent seeing for steady high magnification planetary (i.e, 350x-450x is rock steady).  Now most of my observing is with a 4"...and taking a 4" out every clear evening for a few hours is no chore at all.  Hauling my 10" out that often though is something I would never ever do as just too much prep and care and feeding to manage the thermal behavior and collimation and cleanliness of the mirrors, etc.  So if all I had was my 10", well then I would not be out as often, and that means I would not encounter those really good seeing times as often.  So just food for thought as how much planning and effort you are willing to undertake with your equipment can impact not only how much you observe, but also how likely it will be that you encounter the best seeing.  If extra effort, bigger carry loads, etc. are not an issue for you, then you are golden and not a worry.

 

A premium mirrored 8" Newt/Dob would be nice IMO.  I would go with that over an SCT simply because it will also be great as a generalist since it can achieve a lot larger TFOV than an SCT can. IME the Newt is also a little easier to thermally manage than an SCT and the focuser can be had that is much nicer/precise (no mirror flop), and has a smaller CO.  But if you don't care about TFOV and want the smaller footprint over other considerations, then I've had some excellent planetary views through an 8 SCT.  But if I were to get one, would go through Company 7 or the like so at least you are assured of getting a mirror up to specs.  My preference though would be the Newt.  Who would make a premium 8" mirror?  I'm sure other will chime in as to who.

Zambuto makes 8" in many speeds of choice. I think once in a while Mike Lockwood has smaller mirrors for sale. I would never worry if i bought mirrors from Zambuto or Mike as i know they would be the best you can buy..

 

Since i have some super steady seeing more than most anyone else i think the bigger Newts really shine in 10" to 15" sizes and would destroy any 6" and smaller APO's.



#38 BillP

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Posted 05 April 2018 - 09:04 PM

Since i have some super steady seeing more than most anyone else i think the bigger Newts really shine in 10" to 15" sizes and would destroy any 6" and smaller APO's.

 

It's not really about what scope destroys a 6" Apo (which I have btw), or what aperture is "better" as there are no such things.  Observing, and planetary observing in particular IMO, is a process that starts with the observer and ends with the planetary target with the many things and procedures in between like paying attention to water vapor, humidity, altitude, scatter suppression in the OTA, main objective, diagonal or secondary, eyepiece, and leveraging peculiarities of the human perception system for the observation, etc.  So any scope type or aperture is not that entire process and therefore cannot be the determiner of the entire system and process of planetary observing.  My 4" Apo, for me, will destroy your 15" Newt on many levels, for me.  Apertures that large and all the extra things they require of the observer to operate to the same exacting and refined levels that I operate my 4" Apo and 6" Apo are a show stopper for me (I still prefer the 4" btw).  That being said 10" and 15" Newts would be a worst choice planetary for me, no matter observing location.  So realize that what makes a scope optimum for planetary observing has nothing to do with aperture and has everything to do with the observer, their skill, their process, their goals.  This topic has been argued ad nauseam.  Unfortunate it is coming up yet again.


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#39 barbie

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Posted 05 April 2018 - 11:01 PM

There is no way a 4" apo will destroy a larger Newt!!  lol.gif Simple laws of physics are at play here.  My 6" Newt with a 20% obstruction consistently shows more planetary detail than ANY of my 4' Apos ever did!!  My former Takahashi TSA102 never performed as well on the planets as my large dobsonians did.  Aperture wins, every time! Small apos really shine in the portability department so they are eminently well suited to quick setup and teardown times.


Edited by barbie, 05 April 2018 - 11:25 PM.

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#40 Jon Isaacs

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Posted 06 April 2018 - 04:58 AM

Given you are used to a 4", I would say you should go with 8" instead of 10" as overall it is more ergonomic and less of a hassle for thermal management/acclimation with its lesser mass.  An 8" aperture also gets you .57 arcsec resolution, and that is well below what extremely good seeing conditions will show you (the 4" gets you 1.14 arcsec).

 

 

The Dawes limit is a measure of the ultimate double that can be resolved.  In a Dawes limit split, the Airy Disks are over lapping and there is a small minima in the brightness between the two central disks.  This is what a Dawes limit split looks like:

 

Airydisks_dawes_sqrt.png

 

I think it's quite apparent at the Dawes limit that there is essentially zero contrast.

 

The Rayleigh Criterion (5.45 in/D) is another measure of resolution but the disks are still very much overlapping. In the image below, the image in the center represents the Rayleigh Criterion.  The overlapping disks still dramatically affect the contrast.  It is really only the top image which represents approximately twice the Rayleigh Criterion that the pair is fully resolved.  And even there, there is a loss of contrast because of the diffraction rings.

 

Airy_disk_spacing_near_Rayleigh_criterio

 

For a 4 inch telescope, twice the Rayleigh Criterion is 2.75", for an 8 inch scope, it's 1.36" and for a 12 inch scope it's 0.91".  This is really just visual way of thinking about the mechanism behind the MTF and contrast transfer.  A larger aperture provides much better contrast transfer.  This is why larger scopes can show more planetary detail even when the seeing is not sub-arcsecond,  

 

Consider the thought experiment of drawing a digital image where the pixels represent the Dawes limit, the Rayleigh Criterion and Twice the Rayleigh Criterion.  I think it's clear which image would be the sharpest, show the most fine scale contrast. 

 

Jon Isaacs


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#41 CHASLX200

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Posted 06 April 2018 - 05:30 AM

It's not really about what scope destroys a 6" Apo (which I have btw), or what aperture is "better" as there are no such things.  Observing, and planetary observing in particular IMO, is a process that starts with the observer and ends with the planetary target with the many things and procedures in between like paying attention to water vapor, humidity, altitude, scatter suppression in the OTA, main objective, diagonal or secondary, eyepiece, and leveraging peculiarities of the human perception system for the observation, etc.  So any scope type or aperture is not that entire process and therefore cannot be the determiner of the entire system and process of planetary observing.  My 4" Apo, for me, will destroy your 15" Newt on many levels, for me.  Apertures that large and all the extra things they require of the observer to operate to the same exacting and refined levels that I operate my 4" Apo and 6" Apo are a show stopper for me (I still prefer the 4" btw).  That being said 10" and 15" Newts would be a worst choice planetary for me, no matter observing location.  So realize that what makes a scope optimum for planetary observing has nothing to do with aperture and has everything to do with the observer, their skill, their process, their goals.  This topic has been argued ad nauseam.  Unfortunate it is coming up yet again.

I just don't see much detail on a smaller disk. While the APO does have the cleanest image it runs out of light at the powers i like using.  Give me that golf ball size image in the face at 650x to 1100x anytime vs a cleaner small image under 350x if the seeing allows.  My best seeing comes in Feb when it very warm at nite and no temp drops and sea fog just starts to come on shore. I am right on the gulf and Feb with warm nites always beats any other month of the year.  You would think summer is better but that is not the case for my area.


Edited by CHASLX200, 06 April 2018 - 05:30 AM.


#42 Richard Whalen

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Posted 07 April 2018 - 09:22 PM

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?

For Planetary or lunar observing you want a scope with well corrected smooth optics and excellent contrast. If you want to use your current mount a 7" MCT will show much more than a 4" refractor if it has very good optics. If you dont mind contrast robbing vanes, a 10" f6 newtonian is a good choice if it tracks. 

 

The thing is if you go big, you may find you use it less and miss those rare nights of excellent seeing. For me, observing comfort, ease of setup  keeps me at the eyepiece longer and more often. That means sitting. Tracking is a must for me to keep the planet in the center of the fov, the sweet spot, especially with a newtonian.

 

Another great scope for your needs might be a MNT. Many have excellent, low scatter optics with great contrast. The 7" to 10" range is a sweet spot at many locations, and can show tremendous detail on nights of excellent seeing.  Whatever you get, go quality.


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#43 Jon Isaacs

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Posted 07 April 2018 - 09:42 PM

For Planetary or lunar observing you want a scope with well corrected smooth optics and excellent contrast. If you want to use your current mount a 7" MCT will show much more than a 4" refractor if it has very good optics. If you dont mind contrast robbing vanes, a 10" f6 newtonian is a good choice if it tracks.

 

 

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



#44 Mel M

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Posted 07 April 2018 - 10:14 PM

It's not really about what scope destroys a 6" Apo (which I have btw), or what aperture is "better" as there are no such things.  Observing, and planetary observing in particular IMO, is a process that starts with the observer and ends with the planetary target with the many things and procedures in between like paying attention to water vapor, humidity, altitude, scatter suppression in the OTA, main objective, diagonal or secondary, eyepiece, and leveraging peculiarities of the human perception system for the observation, etc.  So any scope type or aperture is not that entire process and therefore cannot be the determiner of the entire system and process of planetary observing.  My 4" Apo, for me, will destroy your 15" Newt on many levels, for me.  Apertures that large and all the extra things they require of the observer to operate to the same exacting and refined levels that I operate my 4" Apo and 6" Apo are a show stopper for me (I still prefer the 4" btw).  That being said 10" and 15" Newts would be a worst choice planetary for me, no matter observing location.  So realize that what makes a scope optimum for planetary observing has nothing to do with aperture and has everything to do with the observer, their skill, their process, their goals.  This topic has been argued ad nauseam.  Unfortunate it is coming up yet again.

"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.



#45 Daniel Mounsey

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Posted 07 April 2018 - 11:01 PM

"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 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? 


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#46 BillP

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Posted 07 April 2018 - 11:02 PM

There is no way a 4" apo will destroy a larger Newt!!  lol.gif Simple laws of physics are at play here.

 

I think you missed most of what I said.  Simple, or complex, laws of physics are actually not much involved in what makes one scope vs. another a killer tool for planetary observing.  If all that's necessary for killer planetary observing is simply a biggest stick, well then not much more to be said about that amateur and skill-less approach lol.gif  But alas it does not work that way, never has.  For the OP my advice is to not fall into the aperture hole as it has shown itself in my experience to be bad advice, and unfortunately common advice.  Good luck I guess shrug.gif 



#47 barbie

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Posted 07 April 2018 - 11:05 PM

I couldn't disagree more with you Bill!lol.gif



#48 Mel M

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Posted 07 April 2018 - 11:54 PM

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? 

drD, I drifted from the OP and was quoting from what Bill said.


Edited by Mel M, 08 April 2018 - 02:44 AM.

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#49 fred1871

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Posted 08 April 2018 - 02:57 AM

I find the illustrations of diffraction images by Jon Isaacs (above, #40) somewhat worrying. The Dawes Limit pic does not resemble, in my experience, the appearance of a Dawes Limit double in a good telescope. The human eye doesn't see things the way computer constructed images show them. I'll add that I've looked at thousands of double stars, a reasonable sub-set of them at Dawes for the scope in use. 

 

The series of images that follow in #40, including the Rayleigh split, seem to me to resemble the images Dick Suiter has in his star-testing book as examples of poor optical quality combined with large central obstruction (CO). Not typical for good optical quality, with either no CO or a smallish CO. In summary, the illustrations I find unhelpful because they don't show what can be expected with good optics in good seeing, for the human eye.

 

I find Rayleigh to appear like the first image in the set of three, not the central one. The second one is closer to Dawes; the third may be taken as Sparrow.


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#50 Daniel Mounsey

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Posted 08 April 2018 - 09:10 AM

drD, I drifted from the OP and was quoting from what Bill said.

 

For me, it’s just a bit difficult to say what’s good for the OP without a better understanding of the observers expectations and the circumstances. It may be that the OP gets hold of a larger 10” scope and is satisfied, but a good deal of learning is going to need to come with it. To me, there’s just not enough information in the OP and I think that’s what leads to these debates. If someone came to me with a question like the OP, and really wanted to dedicate more time to the solar system with a large aperture reflector, I’d want to ask what’s already been learned from using a small scope. There are some observers who claim they literally can’t see much in a small 4” and that to me already is a red flag. Even in small 4” if the observer is as dedicated and interested as they think they are, then they should already have the skillset to see multitudes of detail, which for those less interested is very possible, even in just a measly 4” as some would call it. Bill expressed a list of factors. When I view the solar system, there’s lots of factors that go through my mind, lots. 


Edited by Daniel Mounsey, 08 April 2018 - 09:11 AM.

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