83 replies to this topic

[Arkade]

I've watched Ed Ting rate the 8inch dob as his "if i could only have 1 scope", even more than the 10 inch due to size,awkwardness etc.

I have never looked through one before.
Biggest Ive used is a 5inch dob and I found my 80mm ed apo more contrasty and sharper on planets and moon.

Does anyone have any stories or comparisons between an 8 inch dob and smaller apos. Was it a wow moment viewing the moon, planets or dso after using a smaller scope (which scope)

I had no idea or may have forgot a 8inch dob is only £370. Going to keep my 80mm apo and get a dob as opposed to 120mm skywatcher ed ( would have cost over £2000 incl a mount).

[triplemon]

Why not go somewhere there are other astronomy minded folks. I'm pretty sure they will show you what it looks like.

 

And that look for yourself will do you much more good than any words we could type here.

[Jay_Reynolds_Freeman]

Given good seeing, a well-made eight-inch Dobson can be expected to outperform any telescope of five inch or less aperture on any astronomical object. A five-inch might come close on low-contrast medium-scale planetary detail, but the eight inch will have a stellar limiting magnitude about one magnitude fainter than the five-inch, and will have resolving power greater in proportion to its aperture.

 

An eight-inch Dobson is likely more difficult to store and transport than many smaller telescopes, but it is extremely simple to set up and use.

 

Small refractors make decent grab-and-go telescopes, and in poor seeing they may show less wiggly views than larger instruments, which is perhaps an advantage for aesthetics of viewing, but in astronomy, aperture wins, and it wins big.

 

 

Clear sky ...

[Arkade]

Given good seeing, a well-made eight-inch Dobson can be expected to outperform any telescope of five inch or less aperture on any astronomical object. A five-inch might come close on low-contrast medium-scale planetary detail, but the eight inch will have a stellar limiting magnitude about one magnitude fainter than the five-inch, and will have resolving power greater in proportion to its aperture.

An eight-inch Dobson is likely more difficult to store and transport than many smaller telescopes, but it is extremely simple to set up and use.

Small refractors make decent grab-and-go telescopes, and in poor seeing they may show less wiggly views than larger instruments, which is perhaps an advantage for aesthetics of viewing, but in astronomy, aperture wins, and it wins big.


Clear sky ...

True. I have literally one step to move it outdoors so moving shouldnt be an issue. The 250mm is just over £500. That looks interesting too.

[Arkade]

Why not go somewhere there are other astronomy minded folks. I'm pretty sure they will show you what it looks like.

And that look for yourself will do you much more good than any words we could type here.

I am not good socially so i find it easier here, peoples views are also helpful here.

[vtornado]

I am not good socially so i find it easier here, peoples views are also helpful here.   -- I do understand this as my brother has social anxiety also, but as they say a picture is a worth a 1000 words.  I bring my 8 inch dob to public star parties and I let any one who wants to steer it, focus it and find things with it.  It's really hard to communicate this tactile info.

 

I could never own just one scope, just like I could never own one hammer.   (I need a good 16 oz claw hammer, rubber mallet,  and a tack hammer, ... maybe others).   But if I could own just two scopes it would be an 80mm refractor and an 8 inch dob.

 

Speaking of 120mm telescope.   I compared my 120mm SW ED to my 8 inch Dob on Jupiter and the image was so much better in the dob as far as the details I could see.  Because the dob was $400 and the 120 ED was $1000+ for just the tube,   I could not justify keeping the refractor.  The tripod and mount for the refractor makes it just as difficult to setup.   The refractor did cool faster, and I miss that.

Edited by vtornado, 24 March 2025 - 08:41 PM.

[Arkade]

I am not good socially so i find it easier here, peoples views are also helpful here. -- I do understand this as my brother has social anxiety also, but as they say a picture is a worth a 1000 words. I bring my 8 inch dob to public star parties and I let any one who wants to steer it, focus it and find things with it. It's really hard to communicate this tactile info.

I could never own just one scope, just like I could never own one hammer. (I need a good 16 oz claw hammer, rubber mallet, and a tack hammer, ... maybe others). But if I could own just two scopes it would be an 80mm refractor and an 8 inch dob.

Speaking of 120mm telescope. I compared my 120mm SW ED to my 8 inch Dob on Jupiter and the image was so much better in the dob as far as the details I could see. Because the dob was $400 and the 120 ED was $1000+ for just the tube, I could not justify keeping the refractor. The tripod and mount for the refractor makes it just as difficult to setup. The refractor did cool faster, and I miss that.

Thank you, appreciate the words.
You answered all the Qs, as i have a 80mm apo, and was going to spend £2000 on a 120mm apo + mount...if the 8 and even 10, which isnt much more, is that good, why waste 2 grand.
I also stress using my cheaper 80mm ed incase it falls. The dob seems great choice. Probably changed my mind at least 10 times but the collomation was a big thing which I guess will have to be learnt.
Thanks ,hope your brother is ok too

[RalphMeisterTigerMan]

A great deal will depend on what type of skies you observe under. The darker the skies and the less light pollution the better. However, an 8-inch doby is fairly decent for visual.

 

Clear skeis and keep looking up!

RalphMeisterTigerMan

[Arkade]

A great deal will depend on what type of skies you observe under. The darker the skies and the less light pollution the better. However, an 8-inch doby is fairly decent for visual.

Clear skeis and keep looking up!
RalphMeisterTigerMan

Im in a city so a lot of light pollution but im guessing it will help having more aperture

[Dave Mitsky]

Im in a city so a lot of light pollution but im guessing it will help having more aperture

A larger aperture allows more magnification to be used at a given exit pupil, which darkens the background field more and helps to mitigate light pollution, at least to some degree.

It will also make extended objects a bit easier to see.

https://clarkvision....-mag/index.html

https://clarkvision....pert/index.html

[Arkade]

A larger aperture allows more magnification to be used at a given exit pupil, which darkens the background field more and helps to mitigate light pollution, at least to some degree.

It will also make extended objects a bit easier to see.

https://clarkvision....-mag/index.html

https://clarkvision....pert/index.html

Thank you for the article link, appreciated

[PKDfan]

Given good seeing, a well-made eight-inch Dobson can be expected to outperform any telescope of five inch or less aperture on any astronomical object. A five-inch might come close on low-contrast medium-scale planetary detail, but the eight inch will have a stellar limiting magnitude about one magnitude fainter than the five-inch, and will have resolving power greater in proportion to its aperture.

An eight-inch Dobson is likely more difficult to store and transport than many smaller telescopes, but it is extremely simple to set up and use.

Small refractors make decent grab-and-go telescopes, and in poor seeing they may show less wiggly views than larger instruments, which is perhaps an advantage for aesthetics of viewing, but in astronomy, aperture wins, and it wins big.


Clear sky ...

NO this is incorrect information.

I'm surprised at this statement Jay. I had thought you had looked through a supreme Strehl optic.

On DSO globs and maybe even most nebulae but an 4inch apo with a divine figure will eat an 8inch Dob for lunch on planetary fine scale contrast.

Nature of the beast mirror SCATTER jeesh.


There is a very good reason a good apo is two or three times the cost of a 8inch Dob.


CSS
Lance

[WillR]

I’m surprised to hear your views are sharper and more contrasts in the 80mm as compared to the 130 mm. I wonder if your collimation is off? I compared views in my 5” dob and my friend’s 100mm Televue Genesis, and the views were brighter in the little dob even though it cost 1//10 th maybe of the frac.

 

i don’t have an 8” but I upgraded to a 10” and the improvement from the 5” was dramatic. The big wow objects were globular clusters. Instead of fuzzy balls they were resolved into stars. Those and certain open clusters improved dramatically, specifically M11, M37, M46, M67, and NGC 7789. These are all dense clusters of faint stars.

 

But every class of object looks better including the moon and planets.

Edited by WillR, 24 March 2025 - 10:56 PM.

[scotsman328i]

NO this is incorrect information.

I'm surprised at this statement Jay. I had thought you had looked through a supreme Strehl optic.

On DSO globs and maybe even most nebulae but an 4inch apo with a divine figure will eat an 8inch Dob for lunch on planetary fine scale contrast.

Nature of the beast mirror SCATTER jeesh.


There is a very good reason a good apo is two or three times the cost of a 8inch Dob.


CSS
Lance

But if I could choose both, which I have, it’s well worth having an 8” Dob to compliment the 3” or 4” APO refractor. I agree that a well figured APO will destroy an 8” Dob on sharpness and contrast, but I also love having my 8” Dob for its light gathering ability for really digging deeper into the fainter objects that my APO can’t resolve. Things like planetary nebs, other fainter nebs and some dimmer galaxies. You’re right though, even though an 8” Dob will pull in more light to make a planet brighter and bigger in the eyepiece, the APO still wins out in sharpness, contrast and most of the time a wider field of stars. A short tube APO like my Televue 76 is fantastic for sky scanning. 

[Inkie]

I've watched Ed Ting rate the 8inch dob as his "if i could only have 1 scope", even more than the 10 inch due to size,awkwardness etc.

I have never looked through one before.
Biggest Ive used is a 5inch dob and I found my 80mm ed apo more contrasty and sharper on planets and moon.

Does anyone have any stories or comparisons between an 8 inch dob and smaller apos. Was it a wow moment viewing the moon, planets or dso after using a smaller scope (which scope) If the scope is in good shape, collimated, good optics, yewbetcha that Dob will blow the doors off an 80 mm APO.

I had no idea or may have forgot a 8inch dob is only £370. Going to keep my 80mm apo and get a dob as opposed to 120mm skywatcher ed ( would have cost over £2000 incl a mount). IF you are youngish, can deadlift 90 pounds, or carry a 40 pound suitcase held between your arms, you might change your direction and look for a 10" instead.  They're not much more, modern 'commercial' market Dob optics are in the B- to A- range (so pretty solid), and you'll be able to carry the two components easily.  Storage might be a consideration, and you'll want a cover of some sort to keep dust out of the tube when it's not being used, and the mirror free of the same....dust. That will be another $80?  Haven't looked. 

 

One last consideration: A Dob is a Newtonian, and they need frequent collimation.  If you go with any Dob, any size or make, you should start reading the threads here on collimation.  Your scope may come with a laser collimator, so check the purchasing info to see what comes with the scope.

 

 

[SeattleScott]

The biggest difference is on DSO, especially globulars and galaxies, but nebulae and open clusters benefit too. There won’t be as much difference on planets but given proper collimation and cooling, the larger Dob should prevail. My 6” Mak is roughly equivalent to my 4” Apo on lunar/planetary. A Dob will have diffraction spikes though, so there’s that.

[Arkade]

I’m surprised to hear your views are sharper and more contrasts in the 80mm as compared to the 130 mm. I wonder if your collimation is off? I compared views in my 5” dob and my friend’s 100mm Televue Genesis, and the views were brighter in the little dob even though it cost 1//10 th maybe of the frac.

i don’t have an 8” but I upgraded to a 10” and the improvement from the 5” was dramatic. The big wow objects were globular clusters. Instead of fuzzy balls they were resolved into stars. Those and certain open clusters improved dramatically, specifically M11, M37, M46, M67, and NGC 7789. These are all dense clusters of faint stars.

But every class of object looks better including the moon and planets.

A few people are saying similar here about better views in the apo. I feel they look better than the 127 mak too. I dont mean more detail but the sharpness and the look. Reminds me of watching an old television then buying a brand new tv where everything looks so crisp

[Jay_Reynolds_Freeman]

NO this is incorrect information.

I'm surprised at this statement Jay. I had thought you had looked through a supreme Strehl optic.

On DSO globs and maybe even most nebulae but an 4inch apo with a divine figure will eat an 8inch Dob for lunch on planetary fine scale contrast.

Nature of the beast mirror SCATTER jeesh.


There is a very good reason a good apo is two or three times the cost of a 8inch Dob.


CSS
Lance

My remarks in posting 3 of this thread were based both on well-known Strehl-ratio calculations and on personal experience: My telescopes have included both several Dobsons -- including an eight-inch f/5 -- and several triplets from Astro-Physics -- including a 130 GT (130 mm f/6.3) (and I don't believe they come any better). My conclusion remains, that for visual use, given excellent fabrication and excellent seeing, the 8-inch Dobson will win every time. Aperture wins, and it wins big.

 

In fairness, it does take good mirrors -- with excellent coatings over an excellent polish -- but they do exist. If reflective scattering were an incurable problem, my ten-inch Astro-Physics Maksutov-Cassegrain (two reflections and 23-percent central obstruction) would have under-performed a 180 mm Astro-Physics triplet in a side-by/side test observing fine planetary detail on Mars and chasing down Phobos and Deimos. In fact, it at least equaled the 180 on low-contrast medium-scale detail, and of course out-resolved it on fine detail and had a limiting stellar magnitude more than half a magnitude fainter.

 

Clear sky ...

Edited by Jay_Reynolds_Freeman, 25 March 2025 - 03:57 AM.

[Tony Flanders]

A 4inch apo with a divine figure will eat an 8inch Dob for lunch on planetary fine scale contrast.

That appears to be true for you -- and perhaps a few other observers. For me and the great majority of people I know, a standard commercial 8-inch Dob shows vastly more planetary detail than any 4-inch scope assuming that the mirror isn't outright defective and the seeing isn't truly abysmal.
 

There is a very good reason a good apo is two or three times the cost of a 8inch Dob.

Indeed there is. The APO costs more to make. And the APOs sell well because both instruments are quite cheap compared to (say) a car, and APOs have various useful qualities that Dobs lack, including photographic capability.

[Jon Isaacs]

I've watched Ed Ting rate the 8inch dob as his "if i could only have 1 scope", even more than the 10 inch due to size,awkwardness etc.

I have never looked through one before.
Biggest Ive used is a 5inch dob and I found my 80mm ed apo more contrasty and sharper on planets and moon.

Does anyone have any stories or comparisons between an 8 inch dob and smaller apos. Was it a wow moment viewing the moon, planets or dso after using a smaller scope (which scope)

I had no idea or may have forgot a 8inch dob is only £370. Going to keep my 80mm apo and get a dob as opposed to 120mm skywatcher ed ( would have cost over £2000 incl a mount).

 

Personally, I have had my 10 inch F/5 GSO Dob since 2003.  At the time, I had an XT-8 but soon I gave it to a local astronomy club.  Since then, I have at least five 8 inch F/6 Dobsonians and they are all gone.  Why:

 

I could never figure out a reason to observe with an 8 inch F/6 when I had the 10 inch F/5 right there waiting.  Size and awkwardness are very individual and depend on many things. I am 76 years old and not as strong as I once was but I still find my 10 inch F/5 as very manageable.  My recommendation is to take Ed's recommendations with a grain of salt... Does lifting 35 pounds instead of 25 seem difficult?

 

Performance wise, a decent 8 inch Dob will require collimation, attention to thermal equilibrium but under stable seeing, the planetary views will be more colorful and more detailed than possible with the most perfect 4 inch refractor.  In terms of deep sky, bright globulars will be reasonably well resolved in an 8 inch, you will see more nebulae and galaxies, you will see more stars in open clusters, smaller clusters.  

 

Jon

[Tony Flanders]

Does anyone have any stories or comparisons between an 8 inch dob and smaller apos. Was it a wow moment viewing the moon, planets or dso after using a smaller scope (which scope)

The story of my astro career is that after fuddling around with various instruments in a desultory kind of way, I finally started to do productive observing with a high-quality refractor, the Tele Vue Ranger 70-mm f/6.9 "semi-APO."

My first major project was observing all the Messier objects, which took me a little less than a year. I started out with the bright winter Messiers looking out the window of my city apartment, then started commuting to my local city park, then various suburban sites, and finally located the faintest of the Messier objects from my country home, which has much darker skies. I must note in passing that for deep-sky observing, there's absolutely no substitute for dark skies.

During that time I also observed many non-Messier objects, including all the planets (not including Pluto), various asteroids and comets, a bunch of non-Messier deep-sky objects (DSOs), and so on.

After completing the Messier list I rewarded myself by buying a premium-quality 7-inch Dob. It was a revelation. With the possible exception of the Pleiades, each and every object that I viewed appeared vastly more detailed in the bigger scope. It's really hard to convey just how big the difference is. Globular clusters might be the most dramatic example. My 70-mm refractor can indeed resolve a small number of individual stars in a few globular clusters when used under dark skies. Through the bigger scope under the same conditions, those same clusters are blazes of dozens or hundreds of stars.

 

In some ways I notice the difference most on the planets, especially Jupiter, my favorite. Perhaps that's because even though aperture makes a huge difference on DSOs, it doesn't make as much difference as dark skies do. My 7-inch Dob from the city competes well on star clusters with my 70-mm refractor when used under dark skies, but no telescope, no matter how big, can show really satisfactory views of galaxies under bright skies. I see M33 much better with 7x35 binoculars under dark skies than I do with my 12.5-inch Dob under city skies.

 

Planets, however, are immune to light pollution. And the amount of detail my 7-inch Dob shows on Jupiter on a decent night is staggering. On the same night I would have to struggle to see even the boldest features with the little refractor.

 

If I could keep just one telescope out of the many that I own, it would be the 7-inch Dob. My 12.5-inch Dob is obviously far more powerful, but it's much more of a production to set up and break down, and its field of view is insufficient for a fair number of important objects. The 7-inch Dob is just about the most potent scope that I can carry in a single trip for reasonable distances, and I can set it up and break it down in a few seconds -- far faster than my refractor, in fact. And it still has a luscious 2.5-degree true field of view.

Edited by Tony Flanders, 25 March 2025 - 05:54 AM.

[Jon Isaacs]

The 7-inch Dob is just about the most potent scope that I can carry in a single trip for reasonable distances, and I can set it up and break it down in a few seconds -- far faster than my refractor, in fact. And it still has a luscious 2.5-degree true field of view.

 

 

For years, I carried my 10 inch Dob in two pieces... Within the past year or two, I started using a hand truck.. It is quick and easy... 

 

 

Jon

[scotsman328i]

If I could keep just one telescope out of the many that I own, it would be the 7-inch Dob. My 12.5-inch Dob is obviously far more powerful, but it's much more of a production to set up and break down, and its field of view is insufficient for a fair number of important objects. The 7-inch Dob is just about the most potent scope that I can carry in a single trip for reasonable distances, and I can set it up and break it down in a few seconds -- far faster than my refractor, in fact. And it still has a luscious 2.5-degree true field of view.

…and this is exactly why I’ll be letting my 12” Explore Scientific Discovery truss Dob go this year. It just doesn’t get used. It’s upstairs in the office room broken down. My 8” Dob is in the garage and ready to go at a moment’s notice. The 8” mirror cools significantly quicker than the 12” mirror and I don’t have to lift the 8” Dob piece by piece down to the garage, assemble it, collimate it, wait for cool down that takes a while even with the fans running. The 8” is a fixed tube, so collimation is done in a snap with a tweak or two, however collimating the 12” can be a chore at times. I do miss the larger and brighter images that the 12” presents, but with a trashed back and the constant bending to assemble and disassemble, I just avoid the 12” now…and any telescope that anyone ends up avoiding, is a scope that needs to find a new home with someone who won’t avoid it and use it.

Edited by scotsman328i, 25 March 2025 - 06:27 AM.

[Russell Swan]

NO this is incorrect information.

I'm surprised at this statement Jay. I had thought you had looked through a supreme Strehl optic.

On DSO globs and maybe even most nebulae but an 4inch apo with a divine figure will eat an 8inch Dob for lunch on planetary fine scale contrast.

Nature of the beast mirror SCATTER jeesh.


There is a very good reason a good apo is two or three times the cost of a 8inch Dob.


CSS
Lance

I would say it’s a quality versus quantity issue.  The difference in resolving power between 4 and 8 inch aperture is significant. At a given magnification there will be more detail available from the  8 inch dob’s primary mirror even if not as sharp, due to the newtonian’s obstructed optics, than the lesser detail in the 4 inch apo.

 

In terms of glass/mirror scatter, the Newtonian has only 2 surfaces while the apo triplet will have 6. The apo really needs those expensive coatings. 

Edited by Russell Swan, 25 March 2025 - 10:17 AM.

[PKDfan]

But if I could choose both, which I have, it’s well worth having an 8” Dob to compliment the 3” or 4” APO refractor. I agree that a well figured APO will destroy an 8” Dob on sharpness and contrast, but I also love having my 8” Dob for its light gathering ability for really digging deeper into the fainter objects that my APO can’t resolve. Things like planetary nebs, other fainter nebs and some dimmer galaxies. You’re right though, even though an 8” Dob will pull in more light to make a planet brighter and bigger in the eyepiece, the APO still wins out in sharpness, contrast and most of the time a wider field of stars. A short tube APO like my Televue 76 is fantastic for sky scanning.

Yrs For Sure scotsman, its not an either or.

An 8inch Dob is a Potent performer definitely !

Its just got mirror reflective scatter and that hobbles it for extreme FSC detection.

For many its a Great choice !!


And the refractor needs to be very Good !


CSS
Lance