12 replies to this topic

[azure1961p]

Strictly planets. Your thoughts. Pete

[gstrumol]

My gut reaction:

 

6" Newt isn't in the running.

 

Both have about the same obstruction size, yes? The Mak has cool down issues (but you could insulate it to mitigate that somewhat). Greater FL for high magnification (but you could pop in a Barlow in the Newt to mitigate that somewhat too).

 

So we have 8" vs 7".

 

I'd go with the 8".

[MrRoberts]

Mak.....hands down 

[vtornado]

Newts can be tinkered with.  you can change out the mirrors if want to get premium optics.

Mak you buy it and it is yours, cross your fingers you get a good one.

 

For overall view, I would go aperture of the 8 inch.

 

the mak would be small enough to put it on a tracking mount, which is nice for planets.

Mounting an 8 inch f/8 newt would require a huge mount. (if eq).

I suppose you could get a tracking platform for the newt as another options,

 

Much might depend upon your seeing. In my skies a 10 inch dob is seeing or cloud limited 363 nights a year.

[ngc7319_20]

The effective "unobstructed" resolution for an obstructed scope is roughly equivalent to the aperture minus the obstruction diameters (looking at MTF plots provides the evidence).  So a 6" F/8 will be something like 5" assuming a 1" secondary.  The Mak will also be something like 5" assuming a 2" secondary.  And the 8" will be something like 6.5" assuming a 1.5" secondary.  So the 8" wins on resolution.  This is born out by my own experience. The 8" will also have more glass to cool, but with suitable fans, etc., this can be overcome.  An old school 8" newt on an equatorial mount (think Cave, Criterion) will be hard to beat for planetary views.

 

I consider tracking as "required" for planetary.  Otherwise you waste too much time resetting the scope and waiting for vibration to die down.  Also the image will be optimal only at the field center -- off-axis there will be coma, eyepiece aberrations, etc.  With tracking you can concentrate on the view, and grasp moments of best seeing.

 

If you added a requirement for portability, ease of use, etc. -- then the 6" and 7" might be preferable.

Edited by ngc7319_20, 22 March 2023 - 12:02 PM.

[RichD]

All other things being equal, i'd say the 8" would show the better image due to larger aperture but i'd rather use the mak personally. Much easier to mount, easier to use, no collimation, less hassle.

Unless its a zambuto mirror in the newt, the mak would not be far behind the 8" at all.

[NinePlanets]

An 8" F/8 is a sweet spot for Newtonians. I absolutely love mine.

The normal (not fast) focal ratio reduces the off-axis aberration and secondary obstruction a lot. 64 inches of focal length makes high magnification easy.

[Kevin Barker]

I am goin to assume planets are the target.

My thoughts are the 8" f8 Newt will have a small CO and be a winner.

The 7 inch mak is a Mak Newt will be second, if a mak cass perhaps be close to what a 6 inch f-8 can do.

[maroubra_boy]

In a purely observing vein, and assuming good optics in all three, the 8" Newt will be King.  At f/8 the secondary will be very small, to all purposes inconsequential.

 

HOWEVER, an 8" f/8 OTA is not a trivial beast.  It will be BIG.  When considering ANY scope integral to this must be the way it is to be mounted otherwise you are making a half cooked decision.  How to store the scope & mount is another aspect too that needs consideration at the same time.  

 

At f/8, this is not an off the shelf mass production mirror, so it will mean a custom piece.  A dob is an obvious solution, but it has the challenge of being motorised for tracking purposes.  Not essential but very convenient.  And if you do go down the non-motorised way you need to make sure that you design the OTA and mount so it has a smooth and easy action at high magnification.  An 8" f/8 Newt on an eq mount is just asking for hurt...

 

A 7" Mak presents obvious mount simplification.  No customising, everything straight off the shelf, alt az or eq.

 

Since you are considering exotica such as an 8" f/8 Newt, I'll suggest to also consider other exotica such as an 8" to 10" Mak.   Intes being the most obvious and more readily available.  Now you are getting into the same aperture league and quality of optics with the same off the shelf mount solutions.

 

My own planet killer is a 9" Santel Mak.  I've been considering an f/8 Newt as well, but the ease of use of this beast just leaves the f/8 Newt as a happy dream...  a couple of 8" & 10" f/7 to f/8 mirrors had popped up for sale in the classifieds recently too that had me seriously thinking about this.  But this Mak has shown me the Enke Division along with Vallis Marineris in such detail that it resembled the magnified image of a vinyl record groove.  Cooling of this big Mak is no issue as I use an insulating wrap & I can rip as high magnification as seeing conditions allow, not what a cooling scope will allow.  A Newt will always have a cooling period.

 

I've looked at threads that you have started (important to do so I can see where your interests lie so I can best phrase my own post here).  I have seen that you are big into sketching just like am.  And just like you, it isn't just about the scope but you also understand the ease of use factor while at the eyepiece and the importance of being comfortable while holding a pencil.  With sketching in mind, my own personal preference is with a Mak.  Pop it on a suitable alt az mount and you are ready for a comfortable night's sketching at high magnification.  My own 9" Mak I put on a single arm converted CPC 1100 mount - this rig is very stable and kills vibrations significantly faster than my friend's 10" Mak on an AZEQ6 mount.

 

I also have a 17.5" dob.  I do sketch with it too.  It has the same focal length that an 8" f/8 Newt would have.  I designed and built the scope myself and its action is very easy and smooth and entirely manual push-pull.  Do I use it to sketch the Moon and planets?  No, I use it just to sketch DSO's.  Because it doesn't have tracking, needing to nudge, nudge, nudge it constantly at 400X to sketch the Moon or planets it would become very boring very quickly.  With the f/8 Newt I have been thinking about, I have also been thinking about how to motorise such a custom dob - not such an easy proposition either.

 

I hope my own experience helps shed some light on to your thinking.

 

Alex.

Attached Thumbnails

• 
• 

Edited by maroubra_boy, 22 March 2023 - 04:41 PM.

[CHASLX200]

Strictly planets. Your thoughts. Pete

Be close. But i would give the win for the 8" F/8 Newt if built right.   Had around 10 of them over the years and all were great. No Paracorr needed and collimation you can eye ball.

Edited by CHASLX200, 22 March 2023 - 06:45 PM.

[N-1]

8" Newt. If "strictly planets" also includes imaging UV/IR, even an 8" Mak wouldn't cut it for me.

[jgraham]

What Alex said.

 

I have a couple of 7” Maks and I have built and used several classic f/8 and f/10 Newtonians. Without a doubt a well built long focus Newtonian is a thing of beauty and simplicity, but they may not be the most comfortable scope to use. However, a well designed Newtonian can be very effective. My all-time favorite planetary scope was a class 12.5” f/10 Newtonian. We had a large platform ladder with a chair positioned so that you could sit comfortably at the eyepiece. It was an absolute joy to use. However, most of my personal Newtonians aren’t as nice. In contrast, my Meade Mak 7 is on an LX90GPS mount which is very compact and super comfortable to use when paired with my Starbound chair. The Mak 7 has superb optics and is my most often used planetary scope. Having said that, I’m working on building an 8” f/8 that can be used as either a Newtonian or as an unobstructed, corrected, tilted optics reflector (an Ed Jones Chief telescope).

[dweller25]

The 8” F8 should have a performance advantage visually.

 

The 7” Mak will be more comfortable to use and easier to mount.

 

Good choices