29 replies to this topic

[mycatmarti]

Last week over 2 nights I discovered that the Orion Nebula looked much more defined with my 6 inch DOB than with my 10 inch. I'm assuming it's because of light from my the houses of my neighbors. Smaller aperture allows less extraneous light in. I can see more deep sky stars with the 10 inch.

[JoshUrban]

Perhaps the sky was different, too?  Seeing, darkness, and transparency would make a big difference.  

 

  While I find that more = more in terms of mirror size, certain views, magnifications, etc frame things better.  M31 looks best in binoculars in my opinion!

[Steve Cox]

Collimation makes a big difference as your 10” is at f4.7 and your 6” is at f8. Make sure your 10” is collimated very well. Also, the 10” is going to take quite a bit longer to acclimate for best viewing compared to the 6”. 

[John Miele]

Were you at the same power? A lot of times, the "sharper" view in the smaller scope is an illusion because the power is lower. If you increase the mag of the smaller scope to match the larger scope (or vice versa - lower the mag in the larger scope to match smaller scope)  the larger scope view is better unless it has inherently inferior optical quality. Of course both scopes should be thermally acclimated and well collimated before any meaningful comparison can be made.

 

cs...John

[mycatmarti]

I used the same eyepiece in both scopes. A Baider 7-24 zoom.

[Neanderthal]

I'll take a stab at this, lol. I'm still a newb but here's my 2 cents:

 

1. Just variations in Transparency.

 

2. Using a Bresser 6" Dob with a 7mm EP / I get a 0.88mm exit pupil @ 171x

 

    Using a Apertura AD10 with a 7mm EP / I get a 1.42mm exit pupil @ 179x

 

Perhaps the difference in exit pupil affected your ability to resolve? That's a considerable difference in exit pupil.

 

In the time it took for me to look this up and write it, my 2 cents is now only worth 1.9 cents.  

Edited by Neanderthal, 28 November 2022 - 10:18 AM.

[bphaneuf]

I would agree with the above comments, and would underscore the difference in transparency.  Not only can it vary widely from night to night, but also from minute to minute.  If you stay on a target for an extended period of time it's not unusual to see changes in transparency during the session.  So you may not be comparing apples with apples, and the difference in aperture may not be the deciding factor.

-b

[mycatmarti]

This all makes sense. I also re-collimated the 10 inch DOB. Now I wait for some more clear nights.

[Redbetter]

Assuming a good mirror, the larger I go in aperture, the more detailed the structure that is seen within Orion.  The central portion of Orion is rather bright, enough that I can still see the nebula in the morning with a 60mm scope in a red zone when the sky is turning light blue before sunrise.  

[mycatmarti]

I could see the stars in the nebula very clearly with both scopes. It was the cloudy part that was more defined with the 6 inch.

[Tony Flanders]

I used the same eyepiece in both scopes. A Baider 7-24 zoom.

A zoom is only "the same eyepiece" if you use it at the same focal-length setting. Did you use a single focal length or try the entire range from 7 to 21? If the former, what focal length did you use? If the latter, how did the nebula change as you zoomed in each of the scopes?

I assume that your 6-inch Dob is f/8, while the 10-incher is likely around f/4.8. That means that if you use eyepieces with identical focal length in both scopes, the exit pupil will be 8/4.8 ~= 1.7 times bigger in the 10-incher, yielding a nebula with much higher surface brightness -- possibly enough to overwhelm your eye. To achieve comparable magnifications in both scopes, you should set the zoom to 4.8/8 = 0.6 times the focal length in the 10-incher. For instance if you set the zoom to 20 mm in the 6-incher, you would get a comparable magnification (relative to aperture) at 20 * 0.6 = 12 mm in the 10-incher.

It's conceivable that your explanation about the 10-inch being more affected by ambient light is part of the explanation. But if so, that's certainly not due to increased aperture. Bigger apertures collect more light from the nebula as well as more light pollution, leaving the ratio unchanged. But it is possible that for some reason the 6-incher is better baffled against light shining into the tube at an angle -- for instance if the secondary mirror is more deeply buried in the 6-incher's tube than in the 10-incher's tube.

Even so, I find it hard to imagine that this would overwhelm the huge difference in the nebula's brightness between the two scopes.

Also, what precisely to you mean by "more defined?" Did you see more detail in the smaller scope, or was this a purely subjective impression?

[mycatmarti]

I will compare things with your suggested magnification. The 6 inch is a deluxe Orion xt6 which does have a special coating in the the tube. My 10 inch is the Apertura Ad10. I'm not sure it has any additional coatings.

[Spartinix]

If you mean in particular the detail in the mid- to outer parts of the nebulosity was more defined, try to match the framing; perhaps you see more detail because there is more sky background surrounding the object in the 6" (so the perception of contrast and instictively using averted vision is causing you to see more detail).

A lower magnification or an eyepiece with a greater apparent field might do the trick with the 10"...

Edited by Spartinix, 29 November 2022 - 08:17 AM.

[mycatmarti]

I think so too. There are supposed to be clear nights here in Thursday. I will try the suggestions and the collimation.

[mycatmarti]

I may try my 40mm televue. And my Astrotech wide eyepiece.

[azure1961p]

My view of 42, 43 lacked any real detail but had that wonderful span across the for. Pete

[Neanderthal]

I may try my 40mm televue. And my Astrotech wide eyepiece.

My best overall view of M42 is with the 12.5mm / 76°. More magnification gets me into the trapezium. A 40mm sounds way too low, but go for it. 

Edited by Neanderthal, 29 November 2022 - 09:47 PM.

[bphaneuf]

Seeing was good enough last night, despite prediction to the contrary, to use up to 762x.  E and F were in view and the nebulosity was epic.  Like being in the nebula flying through clouds.  Good or better seeing and high mag for details, and low-mag, wide view to take in as much of the expansive nebula as possible.  I think the answer to which is better is "yes".

-b

[havasman]

If DSO observations are better using a 6" mass market Dob than they are using a 10" mass market Dob and all complementary factors are equal (as they seldom are) then the 10" is compromised or broken. If they are both capable of their best performance then complementary factors need sorting and, if possible, optimizing. Though a 6" mass market Dob can be an effective and powerful observing tool, it should always be outperformed by an equivalent 10" aperture.

[Redbetter]

I will compare things with your suggested magnification. The 6 inch is a deluxe Orion xt6 which does have a special coating in the the tube. My 10 inch is the Apertura Ad10. I'm not sure it has any additional coatings.

Highly unlikely.  From what I see both use the standard aluminum and silicon dioxide coatings.  The AD10 is the same as the Z10 we have.  No problems with the coating or figure in our Z10. 

 

I don't know what the problem is that you are seeing, but novice "operator error" seems the most likely.  Collimation, cooling, etc.  A  10" will tend to take somewhat longer to cool, although not that long assuming you are using the supplied fan...you are using the fan, right?  

 

A fellow observer has a 10" Orion Dob and it put up really poor mushy images, including for DSO's.  It lacked a fan, which he has since rectified, and it is now putting up decent images.  It was strange, because when I checked it a few times when it was performing poorly, it should have had plenty of time to cool in what was mild conditions.  I even used my collimation tools on it.  

 

I have seen 6" Orion Dobs put up poor images for similar reasons:  hot scope brought out from hot storage for public viewing.  I could collimate it, but I couldn't do anything about the lack of forethought by the organizer of the program.  

[mycatmarti]

I think collimination and magnification has a lot to do with it. I keep the 10 inch in the unheated mudroom. I do use the fan. It may also have been a bad night for seeing.

Edited by mycatmarti, 30 November 2022 - 07:35 PM.

[mycatmarti]

I took my 10 inch DOB out tonight. After re doing collimation. Things improved. The nebula looked very good, especially with my 18.2 and 7mm Delites. I was also able to see some detail on Mars which was pretty cool.

[Roman M]

My first telescope was the Sky Watcher 120/1000mm refractor.
As soon as I bought the Sky Watcher 250/1200 mm reflector, I put them side by side and pointed them at the M 42.
First I looked through the refractor. The nebula was as I used to see it. Everything is nice enough, but nothing new.
But when I looked at 250mm, I was shocked... It was like a completely different nebula!
The difference in views was huge.
At that moment, I realized what the difference in aperture ratio was.

[Dave Mitsky]

A smaller telescope is not somehow less affected by light pollution than a larger one.

 

All telescopes are equally impaired by light pollution compared to what they would show under a dark sky. A large telescope's advantages — superior light grasp and resolution — are the same under bright skies and dark ones.
 

https://skyandtelesc...elescope-myths/

 

[TayM57]

A smaller telescope is not somehow less affected by light pollution than a larger one.

 

All telescopes are equally impaired by light pollution compared to what they would show under a dark sky. A large telescope's advantages — superior light grasp and resolution — are the same under bright skies and dark ones.
 

https://skyandtelesc...elescope-myths/

 

I agree. OP likely experienced variation in conditions from night to night, which likely accounts for the difference in views he saw.

 

 

Some nights, M42 looks great. Some nights, it's terrible. Really condition dependent.