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Visual observing: big APO refractor VS. big Dobson

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#126 Bomber Bob

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Posted 28 September 2020 - 12:30 PM

I had a very regrettable experience with a craptastic CG5 and have stayed off mainland exports as much as possible ever since.  Still, some of the optics are pretty good.  It is difficult to pick out which will be which.  I think the servoCAT dob systems are pretty reliable.  But they do add considerable complexity to set up.

 

GN

The mirrors on the Orion XT12g were above average.  No complaints with the views.  At.  All.  On a better DOB mount, I probably would've kept it -- and added casters to the base, or a caster platform for it to rest on.  (I routinely wheel out my 220# antique Tinsley pedestal EQ, so weight on wheels doesn't bother me.)

 

IME, the optical quality of the Big DOBs has gone up a lot since I saw the first ones back around 1990.  At that time, they really were cheap Light Buckets.  Now...  Poor Man's Real APO?  Yeah.



#127 Mitrovarr

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Posted 28 September 2020 - 12:37 PM

I just find the whole effort to keep a dob thermally equalized and collimated to be a mess. My 12" dob is basically never fully equalized - dropping nighttime temps will do it even if the scope starts equalized. My 10" SCT's thermal issues are trivial by comparison. And the collimation doesn't even last the whole night, and can vary just by pointing the scope at a different part of the sky. Bleh.

IMO if you want to use a large dob (over 10") on planets, assume you will have to either buy a premium dob or mod a commercial one extensively (install boundary layer fans, de-strain and possibly replace the secondary, etc.)
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#128 Garyth64

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Posted 28 September 2020 - 12:53 PM

I just find the whole effort to keep a dob thermally equalized and collimated to be a mess. My 12" dob is basically never fully equalized - dropping nighttime temps will do it even if the scope starts equalized. My 10" SCT's thermal issues are trivial by comparison. And the collimation doesn't even last the whole night, and can vary just by pointing the scope at a different part of the sky. Bleh.

IMO if you want to use a large dob (over 10") on planets, assume you will have to either buy a premium dob or mod a commercial one extensively (install boundary layer fans, de-strain and possibly replace the secondary, etc.)

Maybe it's because of its construction.  I never had any problems with my 10" f/7 newt in an aluminum tube on a GEM.  I rarely had to collimate it, and it held looking at different parts of the sky.  I really didn't have any thermal issues, and it equalized quick enough.  With no fans.


Edited by Garyth64, 28 September 2020 - 12:54 PM.

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#129 Ihtegla Sar

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Posted 28 September 2020 - 01:14 PM

Maybe it's because of its construction. I never had any problems with my 10" f/7 newt in an aluminum tube on a GEM. I rarely had to collimate it, and it held looking at different parts of the sky. I really didn't have any thermal issues, and it equalized quick enough. With no fans.


May also have a lot to do with where you live and how big the ambient temperature swings are, whether you store the scope indoors, and whether ambient temperature levels off quickly after sunset or continues to drop, etc.

Edited by Ihtegla Sar, 28 September 2020 - 01:15 PM.

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#130 BillP

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Posted 28 September 2020 - 01:19 PM

Now my question is: (And forget about the cost for a minute. I realize it can costs thousands on top of the scope for a big refractor) Purely visually. What will be the difference looking at planets and dso’s through a 6k, let’s say 130-140mm, APO refractor compared to a good 12-16” dobson?

Regardless of which scope type you get, you will be making compromises.  So up front wanted to make it clear that no single scope or design does everything well -- and by everything I mean everything and not simply the view under those hypothetical perfect condition (which rarely if ever happen in reality so they are just for the theoretical at heart).

 

1) First let's consider working magnifications for planets.  Will only be addressing planets as that was the question.  IME it is first and foremost about exit pupils.  When the exit pupil gets too small, the lower light level makes to more subtle contrast features on a planet vanish from view.  Examples would be some lighter and smaller festoons on Jupiter, small not so dark barges on Jupiter, the Equatorial Belt si sometimes ethereal so needs a brighter view, the edge details on Martian maria, etc.  IME about as low as I like to go on the planets so I can retain brightness for those low contrast features is usually around 0.65mm.  I will bracket that as 0.75mm to 0.5mm.  Mars as example can usually hold up better at the dimmer 0.5mm whereas Jupiter does not and needs a bit of a brighter view. 

 

So you need to be cognizant of this because the exit pupil will drive what magnification you can use. So  @0.65mm exit pupil here are the magnifications you will get by aperture:

102mm - 157x

130mm - 200x

152mm - 234x

203mm - 312x

254mm - 391x

305mm - 469x

 

Now for planets, I personally can live with 135x and be quite happy as I have trained myself to be adept at the very small image scales.  The details are there to be seen so nothing missed, just a more strenuous endeavor.  150-200x is certainly more comfortable and I rarely go above this even when the seeing allows it as 200x is a walk in the park once you are used to 135x to see details.  But many folks have posted that they seem to prefer the 250x mark, or at least get excited when they can reach or exceed that.  So you need to find out what you need in terms of magnification so you can choose an aperture wisely.  Btw, the 0.75-0.5mm exit pupil rule I use does not work on the Moon as it is so bright and high contrast that the views look great even as low as a 0.25mm exit pupil.

 

Another consideration for the observer is how many floated they have in their eyes.  You generally get more with age and they can be annoying when on-axis.  So if you have a lot of floaters when working at small exit pupils, then shoot for the larger apertures so you can be at the image scale magnification you prefer at a larger exit pupil.

 

2) Second you need to consider ergonomics of the scope in the field.  This is a BIG issue because it is a BIG topic as it covers things like size, weight, mounting needs, thermal acclimation, etc.  So affects everything from if it will break your back getting it outside, if it will take hours to cool, if it needs assistance cooling so active devices like fans, if it also needs special boundary layer scrubbing which mirrored designs can need, if the mounting needs to be heavy and costly to keep the scope from shaking, if the entire setup will take multiple trips and significant time, if collimation is needed with each outing (truss Dobs many times), etc.  So generally, the larger the aperture the larger all these p.i.t.a. items become more and more troublesome and time consuming.  SCTs win on size but lose on thermal stability so you "can" be always futzing wit that. Dobs with their mirror mass need cooldown time and usually fans to help that and boundary fans also for really steady views.  Refractors need the least care and feeding and at 102mm and smaller are fairly ready when you are and can be carried mounted.  Get up to 130-152mm and now a bit of a heavier chore and needing a more substantial mount and taking 30-60 minutes and sometimes longer for optimum cool down.  Some SCTs I've had never cooled down and the baffle tube plume was visible all evening.  My 6" Dob would cool fairly quickly and well, but my 10" one would need fans on it for an hour to get it ready for planetary and then re-need them every so often if the temps were not stable outside.  If you live in humid conditions then you might need optional dew heaters for secondary mirrors and have issues with the primary if it is just in a short rocker box.  SCTs of course will need them as the corrector plate is not shielded.  No refractor I've had has ever needed them as the standard dew shield has always been sufficient for 4-5 hours of observing even in the worst conditions here in Northeast.  Collimation is usually a one time affair with a refractor and SCT, but a more regular thing with Dobs.  And if a large truss Dob with a heavy secondary, may need tweaks during the observing session!  Anyway, lots of issues to contend with here.  As far as a p.i.t.a. factor, IME it is highest for Truss Dobs, less for solid tube Dobs, tiny bit less for SCTs, a lot less for refractors (even 6" ones).  So important to know yourself here and how much futzing you enjoy as part of the process.  Some like to tinker preparing for observing session and during the session, others hate it (I am the latter).

 

3) Next you should consider focal ratio and design.  If working less than f/7 then you will need to be more careful with eyepiece selection if you want sharp to the edge or near edge.  So your expense goes up with shorter focal ratios.  At f/8 most everything works quite nicely and at f/9 really nicely.  Now SCTs are f/10 usually but they are a special case as unless the Edge or ACF variety they have enough coma and field curvature that they behave IMO like a short focal ration Dob.  Only problem is that little you can do to correct the views as no coma correctors and visual flatteners, although the standard 0.63x reducer helps a a lot.  With Dobs they all have coma so upi really need a coma corrector, and these are pricey.  But without them then you will not have much drift time as more than half the FOV is too aberrated for a clean planetary image.  So I put them as a necessity when the focal ratio is f/6 or less.  With refractors the off-axis will have field curvature especially if shorter than f/7, so the wider field longer focal length eyepieces will need to be better corrected varieties to get a completely clean off-axis.  With Dobs the mirrors get dirty/dusty easy IME.  So after a month or so of observing there is enough dust build up to visibly show more scatter - dust on a mirrored surface will show 4x the scatter as the same dust level on a refractive element (based on study of observatory optics to determine cleaning schedules), so why the mirrors need cleaning more often.  So mirror cleaning is IMO a regular need, 2x a year minimum in my book.  Refractors seem to accumulate less dirt and the front surface is an easy clean.  SCTs usually stay pristine unless you use cooling fans for the interior without filters, and they they get dirty fast and much harder to clean.  But I have had closed system SCTs for 20 years or more and the mirror is pristine looking and just the corrector plate needs cleaning as often as a refactor's objective, so not too much, sometimes as little as yearly.  When my scopes are freshly cleaned and fully thermally stable, the views are scaled by aperture so one not really better than the other if all the same aperture, although the refractor will be a little brighter.  However, the effort to get and keep them in that condition is way different, so why I just use refractors for planetary and do not want the hassle of the care and feeding the other designs need.  My 4" goes outside and is generally ready in 15 minutes.  My 152 is ready for planetary in 45 minutes or so.  My SCT was never ready except twice in my life.  My Dobs were ready half the time, other half could not get the mirrors fully tamed.  So my compromise to refractors is mainly due to all the ergonomics of their field operation.  Nothing to do but put them outside and let them sit for just a little and ready to go with no other attention.  Dobs and SCTs need much more care and attention and not my cup of tea.  Others though have no issues.  Also why I shied away from Harley Davidson's and kept to Japanese motorcycles as the only tool kit I want to worry about on a long trip is whether I packed my ATM card for gas as nothing else will fall off or break on a 1000 mile trip.  With the Harley though, even 2-3 hour runs with my friends would often need stop to fix or adjust this or that.  As I said, not my cup of tea lol.gif but others love futzing with their equipment.

 

4) Finally let's talk a little about seeing.  There are probably other things we can talk about but these are the major ones IMO.  Seeing is the last as it plays into the ergonomic issues of the scope choice.  I do a LOT of planetary observing!  And a pattern for me at several homes now in the Northeast, is that before 2am the seeing is an iffy thing for planetary.  So if my viewing is only in the early evenings then I know I probably have about a 1-in-5 chance of the seeing being good enough to support what I consider productive planetary observing.  By that I mean that the planet is detailed and stable for periods of time that are much longer than it might be unstable.  So no "moments of best seeing" in my operating book because if the seeing is such that there are only moments, then the seeing is bad!  So in my observing the planet is mostly rock steady most of the time and only the occasional few seconds of instability over a few minutes of observing.  So for me about a 1-in-5 chance of getting that in early evening.  From 2-5 am however, more like a 3/4-in-5 chance of getting that prime planetary seeing.  So given all this, if you are getting a scope that needs some work to get it ready for an observing session, it can be a real bummer most times as more often you will set up and find the seeing is not there for planetary.  So if you do that then get an 80-100mm Apo also so at least you can tell in 15 minutes if the seeing is good enough for wherever the planet is to put all the work into a setup of a rig that needs more care and feeding.  Another reason why I go with refractors as I just put one outside and no hassle to bring it back in if the seeing is off.  So they are just no hassle instruments and for that big pro in my book, I can live with the con of smaller aperture and less details.  A larger aperture does get one nicer details, as example the eddies around the GRS and structure inside the GRS are easily seen and detailed in a 10" when it is magically freshly cleaned and collimated and actually fully thermally stable, and those things are still there but less detailed in my 4", but then I don't have to worry about the magic being there as the 4" needs to magic of all conditions needing to be just right.  So again, does not mean one scope/aperture is better than another, just a matter of where you do and don't want your poison with whatever you choose as they all come with some level of poison.

 

Bottom line for me, a person that wants their equipment to get out of the way so all I worry about are the sky conditions and placement of the planet, a 100-130mm refractor is the best instrument out there as it is ready when I am for a 15 minute look or a 4 hour session.  If I need to see more details, I can look at Hubble or Planetary Probe pictures as they will always show more details than any consumer scope.  IMO a 152 Apo is not worthwhile better than a 130 for planets.  Where the 152 shines is on DSO as it goes almost as deep and satisfying as an 8 SCT.  So it is a nice mix instrument for both planetary and DSO, so a better general purpose refractor.  For SCTs 6-8" are the least hassle IMO, 5" MAK and they are getting to behave more like refractors than SCTs.  For Dobs 8-10" are the least issues, with the 8" being preferred as IMO they are much more thermally stable than 10" and be quite fine even without active cooling.  So for least hassle planetary scope, least hassle first: 100-115 Apo, 5" Mak, 120-152 Apo, 6-8" Dob.


Edited by BillP, 28 September 2020 - 01:20 PM.

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#131 Ihtegla Sar

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Posted 28 September 2020 - 01:20 PM

I just find the whole effort to keep a dob thermally equalized and collimated to be a mess. My 12" dob is basically never fully equalized - dropping nighttime temps will do it even if the scope starts equalized. My 10" SCT's thermal issues are trivial by comparison. And the collimation doesn't even last the whole night, and can vary just by pointing the scope at a different part of the sky. Bleh.

IMO if you want to use a large dob (over 10") on planets, assume you will have to either buy a premium dob or mod a commercial one extensively (install boundary layer fans, de-strain and possibly replace the secondary, etc.)

I have the same issues. For the most part the only times I have been able to use my dob for planetary is early morning when ambient temperatures have stabilized or are beginning to rise. It's only then that my dob outperforms my 4" apo for planetary.

I may add a boundary fan to see if that improves the situation.

Edited by Ihtegla Sar, 28 September 2020 - 01:20 PM.


#132 mikeDnight

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Posted 28 September 2020 - 01:32 PM

This thread inspired me to do a side by side comparison last night between my 200mm F6 Orion Optics Newtonian and my humble little 100mm apo. Initially I intended to only use the 200mm, as it was already set up in the observatory and was thermally stable. Looking at Mars through a light mist, the planet immediately put on quite a show with Syrtis Major, Hellas, Syrtis Minor, Margaretifer Sinus and Sinus Meridiani, as well as a sharply defined Libya were all seen without any difficulty.  I made a sketch of the detail and then decided to compare the view through the 100mm apo. So I rushed into the house and brought out the little Tak which felt decidedly warm to the touch. I thought the scope would need time to cool but after mounting the refractor onto the observatories pier mounted GP, the view was amazingly sharp and the detail well defined. The detail appeared to be on an equal footing with that seen through the 200mm but with a noticeably smaller image scale and noticeably dimmer. However, the refractor was undeniably sharper than the Newt', which I initially put down to the lower magnification. So to make things as equal as I could in this unequal comparison, I increased the magnification to be as close as I could to that of the 200mm. In the Newtonian I'd used a 5mm Ultrascopic which gave a power of X240, while the same eyepiece in the 100mm apo gave X160, so I changed to a 3.4mm Vixen HR giving X235 in the refractor, which was close enough for a fare comparison.  Even at X235 the refractor gave a sharper image than the 200mm, and despite the view being dimmer, the darks appeared darker and more sharply defined. Again I sketched what I saw and concluded that the view in each scope was more alike than different, and that the 200mm revealed nothing not also seen in the 100mm apo. If anything, the 200mm with its softer view was the least pleasing of the two scopes, and the ease of use offered by the little refractor added to its pleasure. The sketches below are an attempt to illustrate the difference at the eyepiece between the two scopes. The reflector was brighter but softer, and the refractor was sharper with better definition. Even at X333 the refractor remained sharp!

 

IMG_7736.jpg


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#133 Garyth64

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Posted 28 September 2020 - 01:47 PM

May also have a lot to do with where you live and how big the ambient temperature swings are, whether you store the scope indoors, and whether ambient temperature levels off quickly after sunset or continues to drop, etc.

SE Michigan, so we get all that.  And I stored the scope in a detached, unheated garage, year round.

 

If someone keeps the newt in the house and then takes it outside for viewing, they would have to wait at least an hour or more for equalization.



#134 turtle86

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Posted 28 September 2020 - 02:10 PM

Hard to beat a Zambuto mirror wink.gif

 

It is.  grin.gif   And my best planetary views ever have been with my 18” Starmaster/Zambuto.  It helps of course that I live in Florida, which usually has good seeing.  


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#135 gnowellsct

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Posted 28 September 2020 - 03:07 PM

I just find the whole effort to keep a dob thermally equalized and collimated to be a mess. My 12" dob is basically never fully equalized - dropping nighttime temps will do it even if the scope starts equalized. My 10" SCT's thermal issues are trivial by comparison. And the collimation doesn't even last the whole night, and can vary just by pointing the scope at a different part of the sky. Bleh.

IMO if you want to use a large dob (over 10") on planets, assume you will have to either buy a premium dob or mod a commercial one extensively (install boundary layer fans, de-strain and possibly replace the secondary, etc.)

 Most of the stuff about SCT thermal issues comes from our brethren in the southwest.  They insist on generalizing to everyone everywhere.  I've had some good views in dobs that did not have the mods you suggest, though the mods aren't necessarily bad ideas.

 

Unfortunately there are some SCTs out there that do not hold collimation all sky.  I can't believe Celestron hasn't figured this out after 50 years.  In the age of the internet failure to hold collimation is the biggest killer of its reputation as a company.  It's a mechanical issue.

 

Greg N


Edited by gnowellsct, 28 September 2020 - 03:08 PM.


#136 gnowellsct

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Posted 28 September 2020 - 03:29 PM

(...)

 

Bottom line for me, a person that wants their equipment to get out of the way so all I worry about are the sky conditions and placement of the planet, a 100-130mm refractor is the best instrument out there as it is ready when I am for a 15 minute look or a 4 hour session.  If I need to see more details, I can look at Hubble or Planetary Probe pictures as they will always show more details than any consumer scope.  IMO a 152 Apo is not worthwhile better than a 130 for planets.  Where the 152 shines is on DSO as it goes almost as deep and satisfying as an 8 SCT.  So it is a nice mix instrument for both planetary and DSO, so a better general purpose refractor.  For SCTs 6-8" are the least hassle IMO, 5" MAK and they are getting to behave more like refractors than SCTs.  For Dobs 8-10" are the least issues, with the 8" being preferred as IMO they are much more thermally stable than 10" and be quite fine even without active cooling.  So for least hassle planetary scope, least hassle first: 100-115 Apo, 5" Mak, 120-152 Apo, 6-8" Dob.

Whew!  Every now and again someone is more prolix than me.  

 

A key element here is taking the scope out of the house and setting it up.    In my world the scope goes into a car and driven 35 minutes out to the observing field.  

 

I have been able with alt-az to do more around-the-house observing.  That's because when something goes behind a tree I can pick it up and move it.  Or just decide I've had enough and bring it all in.

 

Greg N


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#137 turtle86

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Posted 28 September 2020 - 04:18 PM

Whew!  Every now and again someone is more prolix than me.  

 

A key element here is taking the scope out of the house and setting it up.    In my world the scope goes into a car and driven 35 minutes out to the observing field.  

 

I have been able with alt-az to do more around-the-house observing.  That's because when something goes behind a tree I can pick it up and move it.  Or just decide I've had enough and bring it all in.

 

Greg N

 

 

I recently picked up a Stellarvue M2 alt-az mount for that very reason when using my refractors.  Very easy to move around in the backyard when trees come into play.  My 12.5 and 10 inch Dobs set up pretty quickly and the 10 inch at least is easy enough to move around in the backyard.


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#138 BillP

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Posted 28 September 2020 - 04:53 PM

Whew!  Every now and again someone is more prolix than me.  

 

A key element here is taking the scope out of the house and setting it up.    In my world the scope goes into a car and driven 35 minutes out to the observing field.  

 

I have been able with alt-az to do more around-the-house observing.  That's because when something goes behind a tree I can pick it up and move it.  Or just decide I've had enough and bring it all in.

I would argue that if all we are talking is planetary, not necessary for you to drive anywhere.  So no particular advantage for doing planetary at a dark site, only if it was significantly higher in elevation.


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#139 Jon Isaacs

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Posted 28 September 2020 - 05:17 PM

Along with the general takeaway for the visual astronomer that if seeing is as indicated at some observatories that have exceptional seeing, how might that correlate to the typical seeing in my backyard? The article also addresses issues that are of interest to the visual astronomer.

 

Here is just one such quote from the article…

 

"As a simplification most useful to the visual astronomer, the optical effects of this turbulence layer can be contrasted as three types of distortion in the star diffraction artifact (diagram at right)."

 

Bob

 

Seeing is not a constant, it varies. The eye can catch moments of good seeing.  Alan French who's located north of you points this out. Greg is making this same point.. patience..

 

Goodtostargaze.com rated the seeing from my backyard at 0.6" for the last two nights. Tonight they're saying it will be 1.2"-1.3".  My 10 inch will still outperform my 120 mm apo. 

 

My guess is your backyard isn't that steady...

 

I have come to realize there are two types of planetary scopes..

 

- Those that make the best of a very good situation. These are generally large aperture scopes that can take advantage of excellent seeing. Evidence suggests such a scope would be a good fit for my situation.  

 

- Those that make the best of a less than ideal situation. Thermal issues from harsh climates, jet streams overhead, unstable seeing, clouds and short periods of clear skies.  You have to be ready and you don't want to wait for your scope. Refractors of moderate size are very often preferred.

 

I suspect that Ruben's situation is more like the second than the first.

 

Jon


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#140 CHASLX200

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Posted 28 September 2020 - 06:22 PM

I just find the whole effort to keep a dob thermally equalized and collimated to be a mess. My 12" dob is basically never fully equalized - dropping nighttime temps will do it even if the scope starts equalized. My 10" SCT's thermal issues are trivial by comparison. And the collimation doesn't even last the whole night, and can vary just by pointing the scope at a different part of the sky. Bleh.

IMO if you want to use a large dob (over 10") on planets, assume you will have to either buy a premium dob or mod a commercial one extensively (install boundary layer fans, de-strain and possibly replace the secondary, etc.)

All my Newts never had a cool down prob. But i only view on warm winter nites and have the best seeing in Feb when it is super warm with sea fog rolling onshore.  I am sure winters up north would kill my mood no matter what the scope.
 


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#141 CHASLX200

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Posted 28 September 2020 - 06:24 PM

Yes, after I upgraded the 826 focuser to the Lumicon helical, I put it up against my APM152ED and saw for myself with each scope on a StarFinder EQ.  The ED can go 75x per inch, and the Galilean are sharper disks; and, it presents tight doubles with better clarity.  But, the 826 shows more belt colors on Jupiter & Saturn, more belt to zone intrusions, and overall both disks are brighter.  For DSOs the 826 pulls way ahead.  $300 investment vs. $3000...

You saw the same things i told you guys i saw. Dead on.
 


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#142 CHASLX200

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Posted 28 September 2020 - 06:26 PM

For you people that see tons of detail on planets at Sub 200x you sure got me beat. I need at least 500x up to see good detail. I need a big in your face disk. Must be my eyes.


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#143 Bomber Bob

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Posted 28 September 2020 - 07:20 PM

You saw the same things i told you guys i saw. Dead on.
 

Yeah, and I really haven't had many 7+ / 10 nights since I got it -- partly cloudy most nights if I can see the stars at all...  Meade hit it out of the park with this 826.  I'm glad that I nabbed it.

 

Unfortunately there are some SCTs out there that do not hold collimation all sky.

 

And, let's be honest, some owners don't take the time / make the effort to nail collimation in the first place.  My old '76 C5 does surprisingly well -- after cleaning & collimation.  Got some other performance improvements on the list for it.  Meanwhile, it's an all-round Fun Scope...


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#144 Echolight

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Posted 28 September 2020 - 07:54 PM

I don't like tabletops.  They tend to be unsteady or too low to the ground.   I wouldn't mind one that was higher up on a reasonable tripod.  I like the Vixen RS130 for this reason.

 

I think an ED refractor is a good investment if one puts it on a good mount.  But I add that qualifier to everything.  

 

Greg N

I agree. I have this Nexstar HD 2 inch tripod that my C8 came on that I think would be great for a tabletop dob.

20200801_081032~2.jpg

But a little ED refractor is a more versatile and compact grab and go. And much more likely to last and stick around for my lifetime.



#145 turtle86

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Posted 28 September 2020 - 08:10 PM

I would argue that if all we are talking is planetary, not necessary for you to drive anywhere.  So no particular advantage for doing planetary at a dark site, only if it was significantly higher in elevation.

 

I agree. I do almost all of my own planetary observing from the backyard.  



#146 gnowellsct

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Posted 28 September 2020 - 10:16 PM

I would argue that if all we are talking is planetary, not necessary for you to drive anywhere.  So no particular advantage for doing planetary at a dark site, only if it was significantly higher in elevation.

I like to look at other stuff.  When the moon is up etc., I stay around here.  GN



#147 BillP

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Posted 29 September 2020 - 10:05 AM

All my Newts never had a cool down prob. But i only view on warm winter nites and have the best seeing in Feb when it is super warm with sea fog rolling onshore.  I am sure winters up north would kill my mood no matter what the scope.
 

lol.gif  You have no idea exactly how odd, almost surreal, that phrase you said that I bolded above is!! lol.gif  It stopped me in my tracks and I had to re-read it several times as just could not figure out what planet you were on!!! lol.gif 


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#148 gnowellsct

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Posted 29 September 2020 - 12:17 PM

I recently picked up a Stellarvue M2 alt-az mount for that very reason when using my refractors.  Very easy to move around in the backyard when trees come into play.  My 12.5 and 10 inch Dobs set up pretty quickly and the 10 inch at least is easy enough to move around in the backyard.

Looks sturdy and very much to the point.  There doesn't seem to be a provision for encoders, which I do use on my alt-az (not every time, but often).

 

I actually do deep sky with my refractors, even the small ones, the computer is a boon.

 

Greg N



#149 Ihtegla Sar

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Posted 29 September 2020 - 01:12 PM

Well all the praise for SCTs and big newts in this thread inspired me to break out my old C-8 yesterday and set it up at sunset for cooling.  Three hours later I decided to give it a try on the Moon and Mars. It performed adequately on the Moon but the views of Mars it was giving left a lot to be desired and weren't up to my recollection of viewing Mars with my 4" Tak.

 

So I went inside and grabbed my Tak and plopped it down next to the C-8 and did a side by side comparison using the same eyepieces and different eyepieces that provided the same magnification.  The results were that the Tak won hands down. 

 

I don't use the C-8 often but it has on occasion given some stunning views of Jupiter and Saturn, so I chalked up its poor performance last night to cooling, so I let the C-8 cool for another three hours while using the Tak and going inside for a while to take a nap and by 3:00 am, the C-8 seemed to finally have cooled off but still the seeing was not great (2 arcsecond seeing at best, typical for my location) and the views it was giving were not satisfying.

 

Went back to the Tak and it was noticeably sharper/better focused and the albedo features as well as the south polar ice cap were much more easily visible.  Even at the same magnification as the C-8 the views of Mars in the Tak were still very bright at 180x, which was the maximum usable magnification  given the seeing, and the clear hands down winner was the Tak.


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#150 Simoes Pedro

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Posted 29 September 2020 - 01:20 PM

Hi there,

I’ve been a proud owner of a small 6” dobson and enjoy using it very much. Being a little smaller and looking at other scopes online, naturally the mind starts to wonder what a bigger scope might reveal through the lens. I might’ve caught some aperture envy!

Looking online i started developing an interest for refractors. Not exactly sure why, but something draws me to their form factor and it might be due to the stories about crisp, contrasty views you’ll only get with a high end APO triplet refractor. Visually as wel as for AP.

I’m very aware that larger dobsons deliver the best bang for buck views and aperture possibilities. Their big aperture lends well for viewing both DSO and planets and the dob mount lends itself perfectly for easy visual observing. But when i read high-end APO triplet refractor (~6k $) product descriptions, it tells me they are capable of also showing details in faint DSO’s and sharp images of planets at very high magnification.

Now my question is: (And forget about the cost for a minute. I realize it can costs thousands on top of the scope for a big refractor) Purely visually. What will be the difference looking at planets and dso’s through a 6k, let’s say 130-140mm, APO refractor compared to a good 12-16” dobson?

I only observe visually so far, but might take up AP in the future.

Best,

Ruben

Straight answer: you cannot beat aperture. The difference is so big that the differences will be overwhelmingly obvious. Even mediocre dobs of that size will be perfect APOs in that range.

 

 

On planets: you will see detail within the detail you can see with the APOs.

 

DSOs: objects will become structured. As you go up in size, "faint fuzzies" are no longer that fuzzy nor faint.




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