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If you had 8" Mak-Newt, what size Dob to get wow?

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#1 Dragonwatcher

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Posted 18 March 2013 - 02:55 PM

Wondering what size Dob would be required to really justify leap from high quality 8" Mak-Newt? Or somewhat equivalent question: if you had a 7" apo refractor, what size Dob to get very noticeable upgrade in views.

Parameters: visual use only; for planets versus DSOs.

Now I know this is a very subjective question as only an individual can decide what is a noticeable upgrade/wow factor. Think of it this way. You own a 8" mak-newt. You want to buy a Dob. What would you consider the minimum size to buy that would be worth the investment. I doubt it would make sense to buy a 8" Dob if you owned a 8" mak-newt. But if you were offered a 12" or 14" or 16", when would you consider the purchase a very significant step up in viewing planets? DSOs?

#2 Lane

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Posted 18 March 2013 - 03:35 PM

I see a lot of difference in my 8" and 11", many more DSOs seen, a little more detail in most DSO's, centers of globulars resolving much better and more nebulae visible.

To me that was a WOW difference, I seem to recall actually saying WOW several times as I was going from object to object during first light with my 11.

However, when I have compared my 11 to a 16 and an 18 I have never said WOW even once. The differences seem much more subtle to me, but there is simply no doubt that even more DSOs are seen and even more detail is available in most DSO's, centers of globulars resolving even better and even more nebulae are visible. But no WOW.

So I think if you go from 8 to any of the 3 you named you will be very happy with the result. I would pick the one you are most likely going to use a lot. An open frame 12 is relatively easy to move around and set up. Personally I am planning a jump up to a short FL 20" and the only reason I am doing that instead of a 24" is so I can fit it into the house or into a vehicle easier and of course keep the eyepiece low enough so I don't need a ladder.

#3 Dragonwatcher

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Posted 18 March 2013 - 03:48 PM

Lane, are you talking a 8" mn versus 11" Dob or are you comparing 8" to 11" reflectors? Is 8" of mak-newt going to be 8" of Dob/newt for DSOs? I think in terms of planets I have always heard mak-newts would be superior to similar size reflector.

#4 Lane

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Posted 18 March 2013 - 04:42 PM

I am talking SCT to SCT here, but it doesn't matter, aperture is aperture when it comes to DSO's.

But for Planets, what I have read is that contrast is very important in seeing fine detail, which means Apochromatic refractors in the 5" to 7" range tend to rule because they have near perfect optics and no central obstruction.

After that I think an 8" to 12" dobsonian with a long focal length and a small secondary mirror tend to do best but only if you have fans to get that big primary mirror at ambient temperature and keep it there during viewing.

That central obstruction has a great deal to do with seeing planetary detail, keeping it small is the main thing and that means a long focal length mirror.

There are also off-axis reflectors, Teeter makes them I think, and that means a big mirror with no central obstruction, that could be the best way to see planetary detail.

Maks often do well on planets because they have long focal lengths and small central obstructions, but not all of them are built that way. Your Mak-Newt may already be giving you the best view you can get without spending a fortune, if it has a long focal length. If it does not have a fan on the primary you could add one. I know someone that added a fan to his 8" SCT and it makes a huge difference when viewing planets.

#5 Achernar

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Posted 18 March 2013 - 06:37 PM

If you have an 8-inch, then I would go to at least a 12-inch, which does show a very noticeable advantage when the seeing is steady. Be aware though by the time you get into the 12 to 16-inch range, seeing is going to mess with you more than an 8-inch. I get relatively few nights where I can see a big difference in the views between my 10 and 15-inch.

Taras

#6 Dragonwatcher

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Posted 18 March 2013 - 07:16 PM

I think I got too clever with my question. I was trying to get at my true question through the backdoor rather than front door.

I have X amount of money. It will buy me either one excellent 8" mak-newt which I would mount in Dob fashion or one excellent Dob in the 12-14" range. I was just wondering if the m-n would give me as good or better planetary views as a 12-14" Dob and only slightly inferior DSO views - or whether aperture rules to the point that the Dob would be a better buy. So I was trying to see if a 8" m-n might give me views under the "wow" limit compared to a 10" Dob or 12" Dob or.....? I have no doubt bigger aperture gives better views generally but I just don't know how much of a sacrifice 8" in a mak-newt would be to a reasonably large Dob.

I would like to get the mak-newt but I am worried that I would miss a lot over say a 12.5 -14" Dob with an excellent mirror. Sorry for being so unclear in my intentions.

I do live in the Seattle area where seeing is usually not terrific and I only occasionally get to a darker site. I am exclusively visual.

#7 Tony~M

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Posted 18 March 2013 - 07:26 PM

Hello,

Hope you are well.

You already have an 8" UTI with a wonderful Royce mirror, a same aperture MN I don't believe will give you much over that instrument.

Personally I would go with a larger Dob with a premium mirror to complement your UTI 8". Take a look at Teeter's Telescopes, they are beautiful and well made with very good optics. You could even get one with a Royce conical mirror.

Later,
Tony

#8 azure1961p

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Posted 18 March 2013 - 08:15 PM

16" reflector.

Pete

#9 azure1961p

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Posted 18 March 2013 - 08:17 PM

Lane, are you talking a 8" mn versus 11" Dob or are you comparing 8" to 11" reflectors? Is 8" of mak-newt going to be 8" of Dob/newt for DSOs? I think in terms of planets I have always heard mak-newts would be superior to similar size reflector.


Of similar focal ratio.

Pete

#10 Jon Isaacs

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Posted 18 March 2013 - 09:00 PM

But for Planets, what I have read is that contrast is very important in seeing fine detail, which means Apochromatic refractors in the 5" to 7" range tend to rule because they have near perfect optics and no central obstruction.

After that I think an 8" to 12" dobsonian with a long focal length and a small secondary mirror tend to do best but only if you have fans to get that big primary mirror at ambient temperature and keep it there during viewing.

That central obstruction has a great deal to do with seeing planetary detail, keeping it small is the main thing and that means a long focal length mirror.


Actually... Planetary contrast is mostly a function of aperture. This is what Roland Christen, well known for his high quality apochromatic refractors in the 5-7 inch range says:

What is the Best Planetary Telescope?

"
Whatever system you choose, you might want to consider your local viewing conditions. For planetary, light pollution has zero effect, so you can observe right from your backyard in a downtown area. 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. If you can only afford a 6"or 7" instrument, don't despair that you will not see anything. I know some top planetary amateurs who regularly observe with those apertures and have seen amazing detail on the planets."

When it comes to deep space, aperture, exit pupil, maximum possible field of view are all important factors. A 12 inch F/5 Newtonian goes places a C-11 won't.

Jon

#11 Dragonwatcher

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Posted 18 March 2013 - 09:01 PM

16" reflector.

Pete

I should get a 16" Dob to get the "wow" over an 8" Dob/mak-newt or are you just suggesting I get an 16" Dob because?

Actually, won't happen. I would go 14"/14.5" max. I just don't want to hassle with size/weight of anything larger. It would never get used enough to warrant buying. My ideal Dob would be something like a Webster that is about 50 pounds.

My goal is - if I go Dob route - to get something more into ultralight range. But a Dob-mounted Wirth mak-newt under 30 lbs keeps me intrigued...

#12 Daud

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Posted 18 March 2013 - 11:04 PM

You are not considering the effect of MN cool down time in your location. Rather, consider a 14.5" F/5.6 premium mirror Dob.
An used one, of a recent provenience, will be also much cheaper than the Wirth's MN 8". It does not have to have goto, if you will like the views, you can add it later.

#13 Dragonwatcher

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Posted 19 March 2013 - 11:25 AM

Daud, I think the cool down issue with a Wirth would be very acceptable. It is not a standard mak-newt. While I might consider a 14" mirror, I would not go beyond f/5 as I want my feet on the ground. One option I have is a 14" f/4.8 quartz mirror from a well-known mirror maker & then I would have a Dob built around it by someone like Dennis Steele of Dobstuff. If I went over f/5, I would need to drop down to a 12" or 12.5" mirror I think.

#14 Lane

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Posted 19 March 2013 - 03:43 PM


But for Planets, what I have read is that contrast is very important in seeing fine detail, which means Apochromatic refractors in the 5" to 7" range tend to rule because they have near perfect optics and no central obstruction.

After that I think an 8" to 12" dobsonian with a long focal length and a small secondary mirror tend to do best but only if you have fans to get that big primary mirror at ambient temperature and keep it there during viewing.

That central obstruction has a great deal to do with seeing planetary detail, keeping it small is the main thing and that means a long focal length mirror.


Actually... Planetary contrast is mostly a function of aperture. This is what Roland Christen, well known for his high quality apochromatic refractors in the 5-7 inch range says:

What is the Best Planetary Telescope?

"
Whatever system you choose, you might want to consider your local viewing conditions. For planetary, light pollution has zero effect, so you can observe right from your backyard in a downtown area. 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. If you can only afford a 6"or 7" instrument, don't despair that you will not see anything. I know some top planetary amateurs who regularly observe with those apertures and have seen amazing detail on the planets."

When it comes to deep space, aperture, exit pupil, maximum possible field of view are all important factors. A 12 inch F/5 Newtonian goes places a C-11 won't.

Jon


That article does not say anything about contrast except it is better in a refractor.

Bigger means more light gathering and more resolution, not more contrast.

#15 siriusandthepup

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Posted 19 March 2013 - 08:17 PM

One option I have is a 14" f/4.8 quartz mirror from a well-known mirror maker


Hmmm, I really like 12.5" scopes, but you should consider grabbing that one and never looking back. Lotsa power in a relatively compact package. :jump:

#16 jpcannavo

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Posted 19 March 2013 - 09:21 PM

Hello,

Hope you are well.

You already have an 8" UTI with a wonderful Royce mirror, a same aperture MN I don't believe will give you much over that instrument.

Personally I would go with a larger Dob with a premium mirror to complement your UTI 8". Take a look at Teeter's Telescopes, they are beautiful and well made with very good optics. You could even get one with a Royce conical mirror.

Later,
Tony


Absolutely!

Somehow there is this notion that adding an extra element to the optical train of a newtonian - as in a Mak Newt - magically changes the resolution and contrast of the ON-AXIS image into that of an Apo. But is there any real optical basis for that being the case.?

With respect to the on-axis image, and from a strictly optical standpoint, the Mak Newt has 3 potential advantages:
1) Optical quality: These scopes have spherical primaries that are easier to figure. Therefore they tend to be of higher optical quality than mass produced dobs of similar aperture. But with a premium paraboloid, this advantage vanishes.
2) Central obstruction: These scopes tend to be slower, and therefore utilize smaller secondaries. But match them up against a similarly slow dob, and central obstruction is similar.
3) Lack of a spider: OK this is probably one advantage, but it is largely cosmetic. The reality is that while diffraction spikes are annoying to some, they have virtually no effect on planetary resolution and contrast.

The bottom line is that a well designed, well cooled, well baffled newtonian with premium optics will match a MakNewt of similar aperture and speed for planetary observing, and even give an Apo a run for its money as well.

Shootout!

My RV-6 (a 1970s 6" F8 classic newtonian with excellent optics) is still one of the best planetary scopes I have ever used!

As for the question of jumping up in aperture, a good rule of thumb is to at least double surface area. From an 8 inch I would at least go up to 12 inches.

Joe

#17 Jarad

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Posted 20 March 2013 - 06:51 AM

In general, bigger is better in terms of "wow" factor.

So I would say get the largest scope that meets your other criteria.

Those should include:
Cost
Eyepiece Height
Weight
Transportability

and perhaps some others depending on your personal preferences.

Like you are considering, I have settled on a 14.5" f4.3 scope as my current personal ideal size. I got it a few years ago before the super fast scopes were available. One of these years I may move up to a 20"f3 (I have been drooling over Bob's thread, and my bank account has been quaking with fear...).

Since you coming up from an 8" and seem to want to avoid ladders, anything of a similar focal length should work well for you (i.e. 16" f3.6, etc.) if it still meets you other criteria for cost, weight, transport size, etc.

Jarad






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