31 replies to this topic

[Dave Novoselsky]

I know this will be terribly subjective and perhaps it has been chewed on numerous times at this group, but I must confess that I have been away from reflectors for many years, and much more familiar with and own refractors.

 

i have now reached the stage where I am getting old enough that I must cut back and reduce the number of my herd of telescopes. I need go address my stability and balance issues that make mounting larger refractors an issue. It is my intention to sell off all of my 5”+ and other refractors and retain a small group of 50-100mm lighter APOs that will be easy to move 

 

I am familiar with the Dobsonian telescope having owned a variety of Starmaster  and Obsession dobs ranging from the very tiny up through the 24+ inch stage. However, I have not owned any of those or any other reflectors other than a 6 inch Newtonian F6 and even that is a hassle for me to mount in the dark at nearly 77 with poor balance.  So I am contemplating replacing my 5”+ APOs with a 10”.  I can mount the entire scope on a wheeled platform, leave it inside the enclosed porch adjacent to my observing deck, and just wheel it in and out.  It may need occasional collimation tweaking but I can deal with that.

 

The specific question is assuming a good mirror on the 10 inch cob, what is the equivalent visual image compared to a top of the line APO refractor?  

 

i have had APOs up to including a 203 TMB, now long gone and several other 5 to 8” top end APOs.  Can somebody who has had the opportunity to use one of the larger well corrected APOs  me a comparison of how they compare with a 10 inch DOB? The scope will be used primarily for planetary and with occasional deep sky.

 

Again, I realize this is very subjective but would appreciate opinions from those who have had the opportunity to compare these two very different forms of telescope.

 

Thank you. 

[Jethro7]

Hello Dave,

The views though a quality Dob that is properly collimated and properly cooled down are quite stunning. If there is going to be issues, I would think it would be the Dobs focuser placement as opposed where a refractors focuser is positioned. I am pretty much refractor fan and really enjoy my Taks and find the Newts focuser placement always feels somewhat awkward. However, I deal with it just fine when using my 12.5" Dob or 6" Newt. The other issues are the defraction spikes that occurs with bright stars and planets. The defraction spikes are caused by the Newts Spider veines. I don't have an issue with defraction spikes and find them rather beautiful but other folks do not. Another issue that can occur is, If you purchase a fast Dob, one with a focal ratio faster than F/5 you will probably experience Coma shaped stars caused by the parabolic mirror. The good thing is, the later two issues can be corrected. If the defraction spikes bother you, I have left you a link to a fix and a link to a Coma corrector. The former issue with the Newt focuser arrangement is what it is. 

 

Here is the link

https://www.cloudyni...nian-telescope/

 

I like the TV Paracorr 2, however there are cheaper ones out there

https://www.highpoin...type-2-vip-2010

 

HAPPY SKIES AND KEEP LOOKING UP Jethro

Edited by Jethro7, 29 September 2024 - 07:34 PM.

[sevenofnine]

For a high quality 10" Dob, look no further IMO 

 

https://www.newmoont...s.com/shop/p/10.

[ShaulaB]

Why not have a large refractor on its mount + tripod set on Wheelybars or a scope buggy?

 

Here is a Cloudy Nights thread discussing such options.

https://www.cloudyni...ptions-or-wait/

[Dave Novoselsky]

Why not have a large refractor on its mount + tripod set on Wheelybars or a scope buggy?

 

Here is a Cloudy Nights thread discussing such options.

https://www.cloudyni...ptions-or-wait/

Still a bit of a hassle.  The DOB will sit much lower to the ground and its weight is down low.  I have used scope Dollie’s with refractors but the larger/longer refractor sitting at the top makes the setup inherently tippy as the weight is concentrated in the worst possible balance point.

[havasman]

Dave,

 

At 73 I have looked at this for myself and may choose a different solution. I do not currently expect to continue via remaining a DSO observer while retaining significant apertures. I certainly do not expect to part with my 4" refractor at all. I have seen what the scope can do from a dark or even dark-ish place on a push-to DM4 so I expect to remain primarily a DSO observer. I figure the sacrifice in aperture is not too large a concession for still being able to observe from good sites at all.  

 

I have little experience with big APO's. None beyond a week with an available TOA130 and other instruments from a great place. But I have some time with a good 10" and agree with others that it is a powerful instrument. Also not too big and not too heavy. In fact I looked the other day and found that for @ $3000 I could put a top Zambuto mirror riding an Aurora Precision cell in it. So you never know. Very certainly diffraction spikes are a minor matter in use and coma can be dealt with. It's the aperture that matters with Dobs.

 

Whatever you pick I'll bet it'll work out well.

Edited by havasman, 29 September 2024 - 07:54 PM.

[ausastronomer]

A high quality newtonian that is properly set up, with a Central Obstruction (CO) < 20%, proper cooling and equilibration, good collimation and good baffling, will essentially equal a high quality APO refractor of the same aperture.  Once the CO gets down to 20% (or less) the effect on the MTF curves of the Central obstruction is almost the same as an unobstructed scope. Refer to Star Testing Astronomical Telescopes Richard Suiter pp 163 - 168. On page 168 Suiter demonstrates that a 12" Telescope with a 20% CO easily outperforms an unobstructed 10" telescope.

 

I've used Newtonians to 36" and Refractors to 15" and the best lunar planetary views that I have had over 53 years of observing have been with Newtonians in the 10" to 18" class.  Newtonians larger than this can also give exceptional views, but they are affected more by variable and fluctuating seeing conditions than medium aperture scopes and they usually only deliver their best a handful of times each year.

 

If I was to choose a dedicated lunar / planetary scope it would be either a 12" or 14" / F4.5 to F5 Truss dob with a premium "thin" mirror, GOTO and Tracking.

 

My 14"/F4.5 Zambuto powered SDM with Argo Navis and Servocat is perfect for that job.  It's more portable, storable and transportable than a 5" refractor and optically it blows any refractor under 12" aperture out of the water.

 

The problem is;  a refractor is easy, a newtonian for the inexperienced, can be a lot harder.  With a refractor you plonk the mount down, put the scope on it, give the scope a short time to cool and away you go and it delivers it's peak performance each and every time.  A medium aperture Newtonian is a whole different ball game.  It needs a thin mirror to cool properly, it needs cooling and boundary layer fans, it needs to be properly collimated and set up. A 12" Newtonian with a premium Zambuto mirror that's not properly set up cooled and collimated will always deliver poor views.

 

Cheers

Edited by ausastronomer, 29 September 2024 - 10:19 PM.

[Jeff_Richards]

Dave,

 

I would suggest this if looking for solar system use:

 

https://www.teeterstelescopes.com/pk

 

The issue is because it is f7, the eyepiece is going to be fairly high but these scopes are pretty well known for planet (and planetaries  ) use.

 

Good luck,

Jeff

[Jeff_Richards]

Never mind, just saw on the home page they are no longer taking orders...what a shame.

Edited by Jeff_Richards, 30 September 2024 - 05:56 AM.

[Dave Novoselsky]

Obviously my Borgs, being featherweight Jeff, are not at risk

[vtornado]

Not exactly what you are asking but I had a 120 Skywatcher ED, and compared it side to side with an 8 inch synta dob, on Jupiter.  The difference was so large (favoring the dob), the 120 ED was offered for sale the next day.   I know the 120 is not a premium refractor, but I would wager is it 95% of one, and also I'm sure my synta dob is good, but it is no Zambuto.    I can take the dob out with one trip.  The refactor and tripod were two trips.  YMMV.

[ausastronomer]

Dave,

 

I would suggest this if looking for solar system use:

 

https://www.teeterstelescopes.com/pk

 

The issue is because it is f7, the eyepiece is going to be fairly high but these scopes are pretty well known for planet (and planetaries  ) use.

 

Good luck,

Jeff

 

There is nothing to be gained going with long slow reflectors like that in the modern day and age.  There was a lot to be said for them 30 years ago, but with the changes in mirror making and telescope design and construction and low profile focusers, a 12" / F4.5 Telescope can do anything optically that a 12"/F7 can do.   The only negatives of the F4.5 telescope are that it will show Coma, which is easily corrected with a Paracorr, it will have a shallower depth of focus and slightly tighter collimation tolerances, neither of which are issues in any way.  The MAJOR negatives of a 12"/F7 telescope are that it is the size of a telegraph pole, infinitely more difficult to store and transport; and the eyepiece quite often ends up in the clouds.  They certainly aren't a "no ladder" telescope like a 12"/F4.5.

 

The 2 major developments were the truss design telescope, which allowed the use of a narrower Upper Cage Assembly, thus reducing the intercept distance from Secondary Mirror to focal point and allowing the use of a smaller secondary for a given aperture and F-Ratio, and the modern day low profile focusers, which further reduce the Intercept distance. This enabled telescope builders to use smaller secondaries and get the Central Obstruction under 20%, with much faster Telescopes. My 10"/F5.3 SDM uses a 1.83" Quartz Secondary for a CO of 18.3%.  My 14"/F4.5 SDM uses a 2.6" Quartz secondary for an 18.6% CO and my 18"/F4.5 Obsession uses a 3.1" Secondary for a 17.2% CO. As I indicated in my previous post "#7" once the CO goes under 20% the telescope essentially performs the same as an unobstructed telescope.  There is nothing to be gained going from an 18% CO to a 15% CO. I've directly compared my own 14"/F4.5 Zambuto powered SDM with a friends Royce powered 14"/F7 Home built Truss scope and outside of the differences I mentioned previously, you can't pick them apart optically and in terms of image quality.  I know which one I'd rather use, store transport and set up !

 

Cheers

Edited by ausastronomer, 30 September 2024 - 09:42 PM.

[Sebastian_Sajaroff]

Optically speaking a 10" Dob compares to an 9" Apo, assuming both are under same optics, turbulence and mechanic conditions.
You should rather compare them from an ergonomics and budget point of view.

A 10" Dob will cost you 1000$.
A 9" Apo ? Add 2 zeroes !

If you’re in good health, a 10" Dob is relatively easy to move around, its compact shape helps a lot. You can add wheels and move it around.
A 9" Apo is very heavy and long (torque), IMHO it needs a permanent mount under a dome.

[25585]

I use a 10" F6 Dob as my main reflector. It has an aluminium tube which makes it light to handle, though you can get carbon fiber as well. 
 

https://www.teleskop...10-lambda-16693

[azure1961p]

If you can wheel out a dob you can wheel out a refractor on a tripod. 

 

Pete

[bobhen]

Living in Wisconsin, you will have very very few nights when the planets will be well positioned and the seeing will allow a 10" telescope to use all or even most of its potential resolving capability.  A 10' telescope has a resolving potential of .46 arc seconds. That's well well below one arc second. In your location, years can pass for those conditions to come together. I live in PA and my 6" apo was seeing limited on many nights. My larger scopes were even worse.  

 

Here is an interesting quote...

“How often do nights of excellent seeing occur? At the William Herschel Telescope site in the Canary Islands, even this superb viewing location  (second best in the northern Hemisphere) has many nights of relatively poor seeing: the distribution is positively skewed, and at this excellent site, a 10 inch telescope will be seeing limited on 9 out of 10 nights.”

 

My guess is that your 5" refractor is probably seeing limited on a lot of nights as well. A top of the line 5" apo in really good seeing should do 500x on Saturn and the moon and stay sharp. If you are not using powers like that and are using powers like 200x or 275x etc. on those targets, then your seeing is limiting your 5" apo.

 

Get the 10 for its light gathering capability for deep sky observing. Keep the 5" apo for the moon and planets. Or, move to Florida where they have seeing that will more regularly support a high quality and larger planetary telescope.

 

As Roland Christen says...

"The most important thing is the stability of the air above. The better your seeing i.e. steadiness of the image, the larger the instrument I would install. The farther south you live, the larger the scope that will be most effective."

 

Bob

[Dave Novoselsky]

Thank you all for your input.  After a lot of consultation here and through discussions with various long time observers who have Dobs and other reflectors in various aperture/configurations here is how it resolved:

 

As Bob correctly pointed out above, the nights that even a 6” APO is usable here in Wisconsin are limited although my particular location on the literal edge of the Western shore of Lake Michigan gives me 100miles of dark water/sky that makes for excellent planetary as they emerge on the Eastern horizon and climb up through the evening.  
 

Even so the potential usability of a 10” DOB given my condition is not going to be as good as a less pricey, lighter, easier to move and use 8” DOB given the right design for my issues (balance, restraints on lifting ability, and age).  
 

I decided on an Apertura AD 8.  I have a GSO 6” Newt, have been told by others that their mirrors in the 8” and larger apertures are generally of excellent quality and the overall quality of their products is very good considering their cost.  Reviews of the Apertura AD8 have been consistently excellent and I have used their 2speed focuser as a replacement on a number of my scopes.  The AD8 has their 2speed focuser which is an important consideration for me as most of the other 8” Dobs in this price range have lesser focusers.  

 

The AD8 design makes splitting it for movement and set up quite practical. The base is light enough that it can be moved around with its built in handle and the optical tube lighter than a 10” and with a set of their optional lifting straps well within my ability to carry that out and put onto the base ready to observe in a relatively short period of time.

 

And, it was on sale for $100 off it’s normal price at High Point.

 

So that’s what I decided and again thanks to all of you for your input and advice.

[Dave Novoselsky]

Hmmm, ordered today, shipped and picked up by FedX within two hours of ordering.  That IS impressive.

Edited by Dave Novoselsky, 01 October 2024 - 04:38 PM.

[vtornado]

Living in Wisconsin, you will have very very few nights when the planets will be well positioned and the seeing will allow a 10" telescope to use all or even most of its potential resolving capability. ---

 

It's not only about resolution for the planets, exit pupil is important too when trying to see faint detail.  I really don't like viewing at under .7mm for Jupiter/Saturn/Mars.   With a 5 inch frac that is 180x.  I live 60 miles south of Dave, and have many nights where I can go 200x or slightly above.  My largest scope is a 10 inch dob so I can get 250x with a comfortable 1mm exit pupil.    I had a 12 inch but there are very few nights where 300x works, and it took a seriously long time to cool that mirror in the great-white-north.

[CowTipton]

Thank you all for your input.  After a lot of consultation here and through discussions with various long time observers who have Dobs and other reflectors in various aperture/configurations here is how it resolved:

 

As Bob correctly pointed out above, the nights that even a 6” APO is usable here in Wisconsin are limited although my particular location on the literal edge of the Western shore of Lake Michigan gives me 100miles of dark water/sky that makes for excellent planetary as they emerge on the Eastern horizon and climb up through the evening.  
 

Even so the potential usability of a 10” DOB given my condition is not going to be as good as a less pricey, lighter, easier to move and use 8” DOB given the right design for my issues (balance, restraints on lifting ability, and age).  
 

I decided on an Apertura AD 8.  I have a GSO 6” Newt, have been told by others that their mirrors in the 8” and larger apertures are generally of excellent quality and the overall quality of their products is very good considering their cost.  Reviews of the Apertura AD8 have been consistently excellent and I have used their 2speed focuser as a replacement on a number of my scopes.  The AD8 has their 2speed focuser which is an important consideration for me as most of the other 8” Dobs in this price range have lesser focusers.  

 

The AD8 design makes splitting it for movement and set up quite practical. The base is light enough that it can be moved around with its built in handle and the optical tube lighter than a 10” and with a set of their optional lifting straps well within my ability to carry that out and put onto the base ready to observe in a relatively short period of time.

 

And, it was on sale for $100 off it’s normal price at High Point.

 

So that’s what I decided and again thanks to all of you for your input and advice.

 

It's a great scope!

I had one before buying my 14".

f/6 is easy to collimate, you won't have much coma to deal with, it comes with nice accessories too.

 

There' a great thread here:

https://www.cloudyni...-dobs/?hl= mods

 

for making some minor mods to make it even easier to use.

 

The most helpful that I found were:

-Drill a small hole into the center of the end cap and install a small knob.  This makes it very easy to remove/replace the cap.

-Replace the three secondary collimation screws with knurled end screws.  M4x35mm work perfectly.  You can get them cheap on Amazon.

-Use the Wixey/setting cirlcle combination for finding difficult to see objects.

-A shower cap makes a perfect end cap for the primary side, keeps bugs and dirt/dust out of the OTA when not in use.

-If there is a bit of slop in the U-shaped channel of the uprights that the side bearings sit in which cause the scope to shift when you make altitude adjustments, just add a little felt-side velcro strip to the bare shoulder area of the U-shaped channels.

 

You can see pics here:

https://www.cloudyni...lbum/15239-ad8/

 

 

There are many other suggestions in that Mega thread, it's worth perusing.

[Dave Novoselsky]

It's a great scope!

I had one before buying my 14".

f/6 is easy to collimate, you won't have much coma to deal with, it comes with nice accessories too.

 

There' a great thread here:

https://www.cloudyni...-dobs/?hl= mods

 

for making some minor mods to make it even easier to use.

 

The most helpful that I found were:

-Drill a small hole into the center of the end cap and install a small knob.  This makes it very easy to remove/replace the cap.

-Replace the three secondary collimation screws with knurled end screws.  M4x35mm work perfectly.  You can get them cheap on Amazon.

-Use the Wixey/setting cirlcle combination for finding difficult to see objects.

-A shower cap makes a perfect end cap for the primary side, keeps bugs and dirt/dust out of the OTA when not in use.

-If there is a bit of slop in the U-shaped channel of the uprights that the side bearings sit in which cause the scope to shift when you make altitude adjustments, just add a little felt-side velcro strip to the bare shoulder area of the U-shaped channels.

 

You can see pics here:

https://www.cloudyni...lbum/15239-ad8/

 

 

There are many other suggestions in that Mega thread, it's worth perusing.

Thank you for this very helpful input.

[Jon Isaacs]

Living in Wisconsin, you will have very very few nights when the planets will be well positioned and the seeing will allow a 10" telescope to use all or even most of its potential resolving capability.  A 10' telescope has a resolving potential of .46 arc seconds. That's well well below one arc second. In your location, years can pass for those conditions to come together. I live in PA and my 6" apo was seeing limited on many nights. My larger scopes were even worse.

 

 

The Dawes limit is 0.46 arc-seconds.  That does not apply to planetary.  Even in 2 arc-second seeing an 8 inch scope can provide better contrast and resolution of the planets.

 

Jon

[Kitfox]

Dave, there is a reason the 8" f/6 Newtonian is such a classic optical train.  Get good at collimating it and I think you will find it compares well to the best (!!!) of the 6" refractors that passed through your hands over the years.  This is a really good decision...you get to keep your feet firmly on the ground, while your head is in the stars!

Edited by Kitfox, 02 October 2024 - 12:49 PM.

[Dave Novoselsky]

Dave, there is a reason the 8" f/6 Newtonian is such a classic optical train.  Get good at collimating it and I think you will find it compares well to the best (!!!) of the 6" refractors that passed through your hands over the years.  This is a really good decision...you get to keep your feet firmly on the ground, while your head is in the stars!

According to some (many) my head is located somewhere in the other direction

Edited by Dave Novoselsky, 02 October 2024 - 01:31 PM.

[turtle86]

There is nothing to be gained going with long slow reflectors like that in the modern day and age.  There was a lot to be said for them 30 years ago, but with the changes in mirror making and telescope design and construction and low profile focusers, a 12" / F4.5 Telescope can do anything optically that a 12"/F7 can do.   The only negatives of the F4.5 telescope are that it will show Coma, which is easily corrected with a Paracorr, it will have a shallower depth of focus and slightly tighter collimation tolerances, neither of which are issues in any way.  The MAJOR negatives of a 12"/F7 telescope are that it is the size of a telegraph pole, infinitely more difficult to store and transport; and the eyepiece quite often ends up in the clouds.  They certainly aren't a "no ladder" telescope like a 12"/F4.5.

 

The 2 major developments were the truss design telescope, which allowed the use of a narrower Upper Cage Assembly, thus reducing the intercept distance from Secondary Mirror to focal point and allowing the use of a smaller secondary for a given aperture and F-Ratio, and the modern day low profile focusers, which further reduce the Intercept distance. This enabled telescope builders to use smaller secondaries and get the Central Obstruction under 20%, with much faster Telescopes. My 10"/F5.3 SDM uses a 1.83" Quartz Secondary for a CO of 18.3%.  My 14"/F4.5 SDM uses a 2.6" Quartz secondary for an 18.6% CO and my 18"/F4.5 Obsession uses a 3.1" Secondary for a 17.2% CO. As I indicated in my previous post "#7" once the CO goes under 20% the telescope essentially performs the same as an unobstructed telescope.  There is nothing to be gained going from an 18% CO to a 15% CO. I've directly compared my own 14"/F4.5 Zambuto powered SDM with a friends Royce powered 14"/F7 Home built Truss scope and outside of the differences I mentioned previously, you can't pick them apart optically and in terms of image quality.  I know which one I'd rather use, store transport and set up !

 

Cheers

 

I have a 12.5" f/4.5 Starmaster with a Zambuto primary myself, and it's a real pleasure for both deep sky and planetary work. A Paracorr is needed of course, and Rick Singmaster even designed the scope to balance with a Paracorr.  Easy to set up, easy to collimate, and easy to use.  It takes maybe 10 minutes to assemble and another five minutes to collimate with Glatter tools.  No step tool is needed for observing while standing and often can observe while seated.  On good nights, Jupiter shows lots of fine detail within the bands and GRS.  I sure can't imagine a slower Dob performing any better and think that a slower one would be more of a pain to deal with.