33 replies to this topic

[REC]

Sorry if this has been asked before, but I don't remember when.

 

I have a AD-10 f/1250, f/5 Dob that I use for wider views of the sky. I also have a Meade LS-8 f/2000mm f2000mm f/10 computerized scope for when I can't find stuff with the Dob....lol

 

I plan on using them both together sometime when a friend comes over to join me, plus help me lift them with my 70 yr. old back. So I was wondering what an object would look like side by side.

 

So, I'll pick M13 for this example. I put in a 20mm eyepiece in the SCT for 100x and a 13mm in the Dob for 96x. Will I see a noticeable difference at the eyepiece with only 2" of aperture?

 

Same question on a planet, say Jupiter?  Maybe nothing on the moon as it's so bright to start with.

 

Thanks,

 

Bob

[junomike]

Obviously not to scale as the aperture difference greater but IMO using the M13 chart I see the 8" as the 12.5" and the 10" as the 18".

10" is where DSO's "light up" however Planetary difference isn't as great for me.  Still much better in the larger OTA, just not as drastic (for me at least).

[Augustus]

Really depends on the optical quality for Jupiter. I've seen some good Aperturas and a few lemons. I've seen a lot of mediocre Meade SCTs and a few really good ones. My money would be on the Apertura.

 

The Apertura will do better on globs unless the mirror is poor quality, which seems unlikely.

[JGass]

It would be unwise, IMO, to assume that both SCT and dob are well collimated.  Odds are that one or both would need some adjustment.  Poor collimation in one scope could skew the goodness of the image in favor of the other.  And on this subject, mis-collimation of the SCT might degrade the sharpness of the image faster than a similar degree of mis-collimation of the newtonian dob.

[lsfinn]

All other things - e.g., optical quality and cleanliness, collimation - being equal, it comes down to aperture and field of view. The dob has a larger aperture and smaller central obstruction than the SCT, corresponding to greater light-gathering power and greater resolution. I would expect better views from the dob than the SCT. How much better? Certainly noticeable as a brighter image and perhaps noticeable as greater resolution toward the center of M13.

 

Of course, "all other things" are rarely equal. The difference we're talking about could easily be overcome differences in collimation, temperature equilibration, cleanliness, or optics (e.g., is the dob's mirror aluminized or silvered?) of the dob and the sct.

[havasman]

I too certainly expect the 10" Newt will outperform the 8" SCT - fewer mirrors in the optical path, easier spot-on collimation, smaller CO, etc. Plus, in my experience globs are more benefited by > aperture than other objects.

 

But you will know for sure after a night's observing. Please clue us in on your findings!

[Astro-Master]

About 20 years ago I was thinking of downsizing from a 12.5" Starliner reflector to a smaller scope because of back pain.  I made aperture masks of 10" and 8" to see what I would like for my next scope.

 

Their was a difference between the 12.5" and the 10", but most DSO's still looked good in the 10".  When I went from 10" to 8" there was a big difference in all DSO's.  The biggest difference was on Globular Clusters, which are one of my favorite DSO's.

 

I ended up buying a Meade 10" XL 200 that I used for 16 years till I bought a used 18" Obsession Classic F 4.5 which is my main telescope today.

 

To make a long story short, all things being equal. the 10" should do a better job on M13, but I would bump the power up to 150x or more with a wide field eyepiece for a better comparison between the 8" and the 10", if the seeing will allow.

Edited by Astro-Master, 21 May 2019 - 03:38 PM.

[TOMDEY]

If they're both decent scopes... the Dob should win... but not by a lot. Most of what you will probably be noticing is the difference in quality.    Tom

[spaceoddity]

It would be unwise, IMO, to assume that both SCT and dob are well collimated.  Odds are that one or both would need some adjustment.  Poor collimation in one scope could skew the goodness of the image in favor of the other.  And on this subject, mis-collimation of the SCT might degrade the sharpness of the image faster than a similar degree of mis-collimation of the newtonian dob.

Thermals are another issue that will affect the view and both issues are easier to address with the dob. Dew is another as the corrector plate on an SCT is a dew magnet.

[REC]

All other things - e.g., optical quality and cleanliness, collimation - being equal, it comes down to aperture and field of view. The dob has a larger aperture and smaller central obstruction than the SCT, corresponding to greater light-gathering power and greater resolution. I would expect better views from the dob than the SCT. How much better? Certainly noticeable as a brighter image and perhaps noticeable as greater resolution toward the center of M13.

 

Of course, "all other things" are rarely equal. The difference we're talking about could easily be overcome differences in collimation, temperature equilibration, cleanliness, or optics (e.g., is the dob's mirror aluminized or silvered?) of the dob and the sct.

Good point on the collimation and thermal conditions to keep in mind. Thanks!

[Eddgie]

If the mirror is of equal quality in both, the Newtonian should show noticeably better planetary contrast.  Not only is it larger, but it has a much smaller obstruction, so the contrast should be considerably better. Seeing on most nights will be the primary limit for the 10" dob, but having used both types/apertures, I would use the Newt for planets except for one thing.  It does not track.  If you and your friend are going back and forth, it may be better to use the SCT for planets simply because if your friend is not used to pushing the dob, he might find it a bit frustrating.  Maybe not,

 

Newtonian will also have the edge on Globulars.  

 

But why not have fun with it and take turns comparing one and the other.

 

Whatever you use, hope you both have fun!

[ausastronomer]

Sorry if this has been asked before, but I don't remember when.

 

I have a AD-10 f/1250, f/5 Dob that I use for wider views of the sky. I also have a Meade LS-8 f/2000mm f2000mm f/10 computerized scope for when I can't find stuff with the Dob....lol

 

I plan on using them both together sometime when a friend comes over to join me, plus help me lift them with my 70 yr. old back. So I was wondering what an object would look like side by side.

 

So, I'll pick M13 for this example. I put in a 20mm eyepiece in the SCT for 100x and a 13mm in the Dob for 96x. Will I see a noticeable difference at the eyepiece with only 2" of aperture?

 

Same question on a planet, say Jupiter?  Maybe nothing on the moon as it's so bright to start with.

 

Thanks,

 

Bob

Hi Bob,

 

Before you can compare them you need to be comparing apples with apples, to do that you must make a number of assumptions.

 

1) Both scopes are of equal optical quality.   (ie lets assume diffraction limited, which is reasonable).

2) Both scopes are properly collimated

3) Both scopes are properly cooled.

 

Given the above 3 prevailing conditions there isn't anything the 8" SCT will do optically better than the 10" dob.

 

The laws of physics see to it that this is so. Each optical surface introduces aberrations and light loss, albeit small in each case. The SCT has a lot more reflective and refractive surfaces than a newtonian

 

SCT = Corrector Plate, Primary Mirror, Secondary mirror, Star Diagonal.  4  surfaces

Newtonian = Primary Mirror and Secondary mirror only = 2 surfaces

 

The 8" SCT will also have a much larger secondary than the 10" Newtonian (about 35% compared to 25% in the 10% dob).  The larger secondary causes additional diffraction and affects the MTF curves.  This only applies to visual not imaging.  A premium 10" Newtonian will have a further smaller secondary than a 10" mass produced dob.  Usually a premium 10"/ ~F5 Newtonian will use a 1.83" secondary for an 18% CO whereas the 10"/F5 Asian made dobs use about a 63mm secondary.

 

The 10" dob will give much better wide field low power views.  You can still wind the power up with a 10" dob by using shorter focal length eyepieces but you can't widen the FOV and reduce the power in a SCT beyond a certain point due to the long focal length and slow F-ratio.

 

While I have only ever owned newtonians and a couple  of refractors over 47 years as a visual observer, I have spent a lot of time looking through colleagues SCT's at every type of target available.  Given equal aperture I am yet to come across any SCT that can come remotely close to the views through a decent well set up and tuned newtonian.  Giving 2" of aperture away is like taking a knife to a gun fight.  The SCT will have  a greater "depth of focus" but at F5 the depth of focus is reasonable on the Newtonian. The SCT will have a far greater tendency to "dew up" than the Newtonian.  Corrector plates are renowned dew magnets.

 

Cheers

[Tony Flanders]

So, I'll pick M13 for this example. I put in a 20mm eyepiece in the (8-inch) SCT for 100x and a 13mm in the (10-inch) Dob for 96x. Will I see a noticeable difference at the eyepiece with only 2" of aperture?

Yes, you should see significantly more stars in the 10-inch Dob than in the 8-inch SCT if you compare the views carefully side by side.

But this isn't an altogether fair comparison. To take full advantage of the extra aperture of the Dob, you should increase the magnification proportionally, using 100X in the 8-incher and 125X in the 10-incher. At these fairly big exit pupils, magnification is more important than aperture in determining how many stars you can see.

For instance, the 8-incher at 200X would resolve M13 vastly better than the 10-incher at 125X.

[Darren Drake]

All else being equal the 10 inch will show about a 56% brighter image.  That is fairly significant.   Put another way the aperture of the 8 inch lost to the 10 inch is equal to the light grasp of a 6 inch.  So the light of an 8 plus a 6 would equal that of a 10..

[REC]

I remember seeing a chart on how an object may look using different. I believe it was on our host, Astronomics site, that showed what M13 would look like in a 4" 6" 8", 10 and 12" scope. I believe the picture of M13 started to resolve the cluster with an 8" scope. I just looked at their website and could not find the article? Too bad, the reference chart was really a nice way to see the difference.

[Darren Drake]

M13 starts to resolve well into stars at about 4 inches..

[Migwan]

My 2080 resolves many stars in M13.  My 1100 resolves even more, especially down there in the center.  Your dob will at least as well as the 1100.  Probably a smidgen or two better for as mentioned above, there is considerably less manipulation of the light and a smaller central obstruction.  

 

I reckon you'll take care of any dew problems or collimation issues with either scope.   

 

jd

[Procyon]

I see almost all objects brigther or more resolved with my CPC 1100 then with a previous regular 10" Dob. Could be my new location though. Definitely much brighter than a previous 6 or 8" SCT as well.

Edited by Procyon, 22 May 2019 - 10:18 PM.

[ausastronomer]

I remember seeing a chart on how an object may look using different. I believe it was on our host, Astronomics site, that showed what M13 would look like in a 4" 6" 8", 10 and 12" scope. I believe the picture of M13 started to resolve the cluster with an 8" scope. I just looked at their website and could not find the article? Too bad, the reference chart was really a nice way to see the difference.

 

There is one on the Obsession Telescopes website but that relates to larger apertures

 

 https://www.obsessiontelescopes.com/

 

There is no simulation for a 10" telescop,e but you can use your imagination and put it 1/2 way between an 8" and the 12.5" which still provides a significant gain over an 8" telescope

 

Cheers

[Tony Flanders]

M13 starts to resolve well into stars at about 4 inches..

That depends on how much light pollution you have to fight. M13 is glorious through a 4-inch scope under dark skies. You'd need at least an 8-inch scope -- and more magnification -- to achieve equal resolution from a typical suburb.

[Procyon]

That depends on how much light pollution you have to fight. M13 is glorious through a 4-inch scope under dark skies. You'd need at least an 8-inch scope -- and more magnification -- to achieve equal resolution from a typical suburb.

So true, with a 6" sct, bortle 8, could barely make out Globs, from bortle 4-5 it showed them much better. With an 8" at bortle 8 they showed up much better.

[RazvanUnderStars]

I have a 10" f/4.7 dob and an 8" SCT (f/10 or with a reducer, f/6.3, both collimated. The views in the dob are clearly brighter and with more detail, I prefer them any time. I use the SCT for EAA only these days.

[REC]

There is one on the Obsession Telescopes website but that relates to larger apertures

 

 https://www.obsessiontelescopes.com/

 

There is no simulation for a 10" telescop,e but you can use your imagination and put it 1/2 way between an 8" and the 12.5" which still provides a significant gain over an 8" telescope

 

Cheers

That's a good one too, thanks!

[REC]

That depends on how much light pollution you have to fight. M13 is glorious through a 4-inch scope under dark skies. You'd need at least an 8-inch scope -- and more magnification -- to achieve equal resolution from a typical suburb.

I think that's it, my suburban sky. No way can I resolve stars in M13 in my 4" C102. Maybe in my ETX-125 at 150x with averted vision as I recall.

[REC]

Thanks for all your reply's. I have a pretty good cross section on this subject now! Now it's time to out and observe if I ever get a clear night??? We have a hot muggy heat wave upon us for the next 10 days