My small frac beat my dob :-/

[Size9Hex]

TL;DR. I recently resolved a double star in my 72mm frac, while my 10" dob alongside didn’t manage it. I’m flummoxed...  ?

 

A bit more...

A while ago, I had the 72ED out alongside the 250PX and was delighted to resolve a very close double (saw it as an elongated star, rather than a clean split). Should have made better notes at the time, but I recall it being just inside the Dawes limit, so maybe 1.5" separation, mag 6.5 matching stars or similar. I chose it specifically as a favourable double for testing the resolving power of the new 72mm. In the 10" however, I couldn’t detect it. Just a sparkly mish-mash of light.

In the interest of balance, very shortly afterwards on the same evening, I split an uneven double in the 10" but failed in the 72mm. The 10" showed by far the best view of Jupiter too. So 2-1 to the dob, but I’m struggling to see how a 72mm could sneak even one goal past a 10"? I would have liked to repeat the test, but midsummer came along!

Worth adding perhaps that...

... The collimation of the 10" looks ok to my eye. Through the Cheshire, a little black dot more or less on centre in the doughnut shaped sticker on the primary.

... Seeing was average in the dob (and excellent in the 72mm as seems to always be the case!).

... The dob (solid tube) was outside for a good hour on the night in question pre-observing, with I guess a 5 degree starting difference, although no doubt the night was cooling further as the scope tried to catch up.

Has anyone else got similar stories? Is there something wrong (or perhaps just not optimised) with my dob?

Partly, I’m wondering what my next astro purchase might be. If a 72ED is this good, then a large ED frac must be awesome? Cough. 120ED, cough. Excuse me. ? Or should I be thinking of what (if anything) is holding back the dob from its potential... a cooling fan to control the thermal issues, a decent micro focuser to absolutely nail the focus, or something else? Not that I’m in any rush to spend a fortune on a big frac, but if the answer is a cheap cooling fan I’d snap it up! 

[Stu]

It doesn't surprise me to be honest, I have always had more luck, and pleasure splitting doubles with refractors.

[25585]

What focal length & ratio is your dob?

[Size9Hex]

4 minutes ago, 25585 said:

What focal length & ratio is your dob?

1200mm f/4.7, although the frac is only a bit slower at 420mm f/5.8

[cloudsweeper]

With a largish aperture and a close, fairly matched pair, would the enhanced brightness not make it harder to split the components?  Just a thought - I'd be glad to have it confirmed or rejected!

Doug.

[Size9Hex]

9 minutes ago, Stu said:

It doesn't surprise me to be honest, I have always had more luck, and pleasure splitting doubles with refractors.

Really interesting! I hadn’t expected this, especially with the big size difference, although my experience with different scopes is pretty limited. Maybe I should shelve the ideas of modding the dob and start saving! ? Although for now I am really enjoying seeing what the little frac can offer. ?

[John]

In theory the larger aperture will be able to resolve much closer pairs than the much smaller one. In practice the tight star images produced by a good refractor make splitting close pairs a more precise and definite business wheras the less tidy star image that the obstructed aperture procuces can mask the split.

 

[Size9Hex]

8 minutes ago, cloudsweeper said:

With a largish aperture and a close, fairly matched pair, would the enhanced brightness not make it harder to split the components?  Just a thought - I'd be glad to have it confirmed or rejected!

Doug.

It’s a good thought. You’ve got me wondering. I’m asleep before the stars appear at this time of year, but aiming to compare a few more in the two scopes when darker skies return.

[niallk]

Say it aint sooo....!!!

I'll have to look through a frac someday

Seeing + cooling does need to be good for a big mirror to deliver its best.

[Ben the Ignorant]

18 minutes ago, Size9Hex said:

Has anyone else got similar stories? Is there something wrong (or perhaps just not optimised) with my dob?

Yes to the first question, and no to the second. If you look into observational astronomy history these stories as as old as the first large quality refractors and reflectors, say the mid-ninetieth century. Their optical tolerances would match but the refractor users looking into big reflectors wondered what happened to their pristine Airy disks. I suppose they were the first to call them hairy disks.

Aperture rules, big dobs sell (I've got one) but apos of necessarily smaller aperture sell, too (I've got two).

[Stu]

I know from over the years that even refractors as small as 60mm will split the Double Double and Izar for instance quite easily. Often when I have viewed these with larger newts they take more power to split and just seem much messier! Targets like Rigel or Polaris often seem to challenge many that I see reporting on the forum, but a good frac shows the tiny secondary stars beautifully.

[Size9Hex]

9 minutes ago, John said:

In theory the larger aperture will be able to resolve much closer pairs than the much smaller one. In practice the tight star images produced by a good refractor make splitting close pairs a more precise and definite business wheras the less tidy star image that the obstructed aperture procuces can mask the split.

 

That really matches what I saw. A noisy mish mash of light in the dob. I guess I’m still surprised. The theory would seem to so heavily favour a scope 3 times the size of another one, but the proof is in the eyepiece I suppose!

[Size9Hex]

6 minutes ago, niallk said:

Say it aint sooo....!!!

I'll have to look through a frac someday

Seeing + cooling does need to be good for a big mirror to deliver its best.

He he. It was a sample size of 2, so I’m not going to make any generalisation! ?

Most of the side by side views I’ve had have heavily favoured the dob for detail, but there’s something about the frac... it sure is a pretty view! ?

Interested in whether you use any active cooling in the you dob?

[Victor Boesen]

ohh no, don't make me fall for a small apo again?

[Size9Hex]

11 minutes ago, Stu said:

I know from over the years that even refractors as small as 60mm will split the Double Double and Izar for instance quite easily. Often when I have viewed these with larger newts they take more power to split and just seem much messier! Targets like Rigel or Polaris often seem to challenge many that I see reporting on the forum, but a good frac shows the tiny secondary stars beautifully.

I’ve tried the double double for giggles in my 50mm masked dob at f/24. I’m unsure whether this supports the dob or the frac side of the discussion though!

Interesting comments though. Matches my experience, particularly with Rigel which always seems harder than it feels it should be.

[Stu]

5 minutes ago, Size9Hex said:

I’ve tried the double double for giggles in my 50mm masked dob at f/24

Did you manage it?

[Size9Hex]

5 minutes ago, Victor Boesen said:

ohh no, don't make me fall for a small apo again?

The small apo sells itself! ? . If I could only pick one though, the brain would overrule the heart; The 250px (I see you have one too) brings an awful lot more of the night sky into reach.

[cloudsweeper]

Interesting topic, and all valid points about seeing, cooling, collimation, resolution and obstructions/contrast.  The point about a frac needing less mag makes sense because of the sharper, cleaner image. 

Thinking a bit more about what I offered before, a large aperture would make a matched pair rather bright - much more so than a smaller aperture because point-source brightness is a function of the square of the aperture.  So, being close, would not the two images appear to "blend" a bit?

Doug.

 

[Stu]

17 minutes ago, Size9Hex said:

it sure is a pretty view! ?

That's alot of it I think. I do the hobby for pleasure and relaxation.  When I view through a refractor I just feel that the views are presented in a way that helps my relaxation and enjoyment. Now,  that counts for doubles, treble, planets and the moon,  plus open clusters. For DSOs such a galaxies,  globs etc then these benefits are over taken by the benefits of aperture and greater resolution. That's how I see it anyway

[Victor Boesen]

1 minute ago, Size9Hex said:

The small apo sells itself! ? . If I could only pick one though, the brain would overrule the heart; The 250px (I see you have one too) brings an awful lot more of the night sky into reach.

I also have the 250px, and it has provided me with beautiful views of Jupiter last year, planetary nebula and other night sky objects, and yet I have only observed with it from my bortle 8 skies which will hopefully change this summer. The only criticism I have is the size and weight of it. This is also why I mostly bring my helios 15X70 on my manfrotto when travelling to my grandparents' house(bortle 4) 

[Size9Hex]

8 minutes ago, Stu said:

Did you manage it?

It was last year, and you’ve got me wanting to double check. I was trying it partly to see what a much reduced aperture might be like, prior to possibly buying a small frac. If I recall correctly the easier pair was detectable while the other... I couldn’t swear to it. The skies are unexpectedly clear, so I’ll have a look later if I can stay up!

[Timebandit]

 

 

It does not surprise me, as Fracs rule on double stars IMO ,and light buckets are the king on fainter DSO. 

Horses for courses, everyone should have a frac in the kit ?

[Size9Hex]

13 minutes ago, Stu said:

That's alot of it I think. I do the hobby for pleasure and relaxation.  When I view through a refractor I just feel that the views are presented in a way that helps my relaxation and enjoyment. Now,  that counts for doubles, treble, planets and the moon,  plus open clusters. For DSOs such a galaxies,  globs etc then these benefits are over taken by the benefits of aperture and greater resolution. That's how I see it anyway

? Well said! Brings to mind two sessions on Saturn. Last year in the 10 inch, faffing with the focuser and seeing conditions and just not enjoying Saturn as much as many folks seem too. This year in the frac, a totally different and much more relaxing experience.

[John]

56 minutes ago, Size9Hex said:

That really matches what I saw. A noisy mish mash of light in the dob. I guess I’m still surprised. The theory would seem to so heavily favour a scope 3 times the size of another one, but the proof is in the eyepiece I suppose!

Thats why we often end up with more than one telescope. They all have their strong points and not so strong points

Try comparing the view of Messier 13 between the 2 scopes and the 10" dob will be massively more impressive than the 72mm refractor.

[celestron8g8]

Even tho the dob is bigger is it a fair comparison when one is half mirror while the other is all glass ? Seems to me there should be some default in comparisons .