My small frac beat my dob :-/

[niallk]

1 hour ago, Size9Hex said:

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?

I don't in my 250px - but usually give it 45+ mins for cooling, sometimes longer.  They are thick enough mirrors in the 250px!

I do always use a rear fan to initially help cool my 15" (2" thick).  In fact, I'm planning to make a bracket to allow a front fan as well - just for initial cool down, not while observing.  When it is cooled, it gives what I think are pretty nice views on planets, for example. I'm not really a double star observer.  Its main job is for glaxies & planetary nebs

Hoping to attend a star party in August In deepest darkest Kerry - might get to sample a frac there!

[Peter Drew]

Take away the atmosphere and it would be a different matter.

[Stu]

1 minute ago, Peter Drew said:

Take away the atmosphere and it would be a different matter.

That's the bit I haven't quite worked out yet Peter

[Peter Drew]

The seing conditions (atmosphere) is the main reason why well collimated reflectors struggle to match smaller refractors on star images and double separation. An obstructed telescope theoretically has a better double star performance than a refractor of similar size as the Airy disc is smaller.

[Stu]

8 minutes ago, Peter Drew said:

The seing conditions (atmosphere) is the main reason why well collimated reflectors struggle to match smaller refractors on star images and double separation. An obstructed telescope theoretically has a better double star performance than a refractor of similar size as the Airy disc is smaller.

Thanks very much, but I more meant that I hadn't worked out how to get rid of the atmosphere yet

[Peter Drew]

It's been a long day.  ?

[Size9Hex]

1 hour ago, Stu said:

Did you manage it?

Just an update after a brief and welcome gap in the clouds. The double double was detectable with the 50mm aperture mask in the 250px, although it wasn’t exactly stunning to look at. The harder of the two pairs appeared as a "not quite right" elongated star with a kind of messy tail fading out a few degrees off axis from a straight line connecting the two pairs. Easiest at 130x, degrading again by 180x, but the orientation was also suspected at the initial 80x. I think knowing it was a slightly uneven double helped, although I’d forgotten the orientation which I made sure to pin down in the eyepiece before checking Sky Safari.

[Size9Hex]

1 hour ago, celestron8g8 said:

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 . 

No intention to be unfair to either scope and I hope it didn’t come across that way. I’m generally really impressed with both scopes and enjoy them both.

[iPeace]

It happens.

If the TV-85 can make the split, it will look nicer than in the VX6L. Izar, for example. Then there are those that the TV-85 can't do, like the 3-way Tegmine split. For those, I need the Newt.

Doesn't mean I won't keep trying with the frac, though. Might get lucky some time.

[celestron8g8]

4 hours ago, Size9Hex said:

No intention to be unfair to either scope and I hope it didn’t come across that way. I’m generally really impressed with both scopes and enjoy them both.

No harm meant or taken , just that when it comes to comparison of glass and mirror it’s a fact glass will give a more clear crisp image which when seperating stars would actually be easier depending tho on how well made the mirror is . In my opinion glass grind for perfection is more easy than grinding mirrors therefore the reason for glass being better quality seeing . What few stars i have done  seperation on with my C8 was much easier mainly because of aperture size using a 2x barlow and 12mm  Plossi but were easy stars to seperate . I think mainly comparison would be better comparing say a 80mm to a 120mm or larger refractor like a 130mm or 150mm and more fun . But just know alot of ppl including me would love to have your 10” reguardless of comparisons ! When it comes to DSOs’ aperture rules !!   

[jetstream]

We need a couple of good telescopes in order to match the conditions. I do love doubles through a refractor and the presentation of stars in general through them.120mm class refractors is a nice aperture IMHO and compliment a dob well.Recently a 120mm scope surprised me on M13..

[25585]

I have noticed clearer splits with my 10 inch F6 (60 inch) than my 10 inch F5 (50 inch) gives. Both resolve the same but stars in F6 have a more frac-like appearance.

 

[PeterW]

Scopes larger than around 6” will be progressively more affected by atmospheric turbulence.

https://en.m.wikipedia.org/wiki/Astronomical_seeing (r0 parameter). That’s why laser guide stars are used by professionals and (almost) no one bothers with big scopes looking at the sun. Your best comparison would be a night with no jet stream interference when the stars aren’t twinkling.

 

PEter

[Paz]

I've never yet tried an apo (it is on my to do list) but from the scopes I've used so far my maksutov gives the cleanest views of bright double stars but my vx14 has always been far ahead of my other scopes in resolving ability. I've also found there are things I can do to clean up the view of brighter objects in the vx14 but there's nothing I can do to increase the resolving power of my maksutov, other than to get a bigger one.

 

[25585]

https://www.rothervalleyoptics.co.uk/geoptik-variable-artificial-star-for-collimating.html

[Mr Spock]

My C9.25 splits 0.7” easily with space to spare. 

A 250 should do just as well providing the optics are good enough, they are collimated perfectly and cool down ans seeing conditions are good. 

Collimators aren’ t accurate enough; in my experience the only effective way of collimating is with a star test (a real star  ). Even aslight misscollimation will destroy image quality. You hear so many people complain about reflector or SCT image quality when the real culprit is collimation. 

[Ricochet]

The other issue you might have is scatter from the mirror(s). The best (and cheapest) mod I have made to my dob was a simple ring of black card to baffle the edge of my primary mirror. Scatter from that edge was instantly blocked and every star image is smaller and tighter. 

[Size9Hex]

54 minutes ago, Paz said:

I've never yet tried an apo (it is on my to do list) but from the scopes I've used so far my maksutov gives the cleanest views of bright double stars but my vx14 has always been far ahead of my other scopes in resolving ability. I've also found there are things I can do to clean up the view of brighter objects in the vx14 but there's nothing I can do to increase the resolving power of my maksutov, other than to get a bigger one.

 

Interesting comments. The VX14 (and indeed any large high quality dob) is on my to do list! If I may ask, what do you do clean up the view? This is one of the thoughts I had - whether I simply hadn’t optimised the view in the dob in some way. That said, from the response, it sounds like fracs really excel in this area.

[Moonshane]

I routinely mask my larger aperture dobs when looking at doubles and sometimes even planets and the results are usually better with star images tighter and to my eyes, better contrast and sharpness in the smaller unobstructed apertures. With planets the unmasked view is literally jaw dropping in good seeing and detail often greater and easier to see than with even a 120ED. However, you have to wait longer and be patient for good views with a larger unobstructed instrument. I should think you'd be able to create a mask for your 10" like the one shown belowat about 80-90mm and I'd expect this will provide similar views if not better on doubles than your 72Ed although the field will be far less for larger objects of course.

 

 

[Moonshane]

ps

I do agree that doubles with a bright primary and dim small secondary are much easier albeit 'messier' (see what I did?) with a large unobstructed dob.

[Size9Hex]

1 hour ago, PeterW said:

Scopes larger than around 6” will be progressively more affected by atmospheric turbulence.

https://en.m.wikipedia.org/wiki/Astronomical_seeing (r0 parameter). That’s why laser guide stars are used by professionals and (almost) no one bothers with big scopes looking at the sun. Your best comparison would be a night with no jet stream interference when the stars aren’t twinkling.

 

PEter

Thanks Peter. That looks really helpful in diagnosing what I’m seeing, although I’ll have to read in more detail later. Will definitely give it another go and see how it holds up under good seeing. The seeing was "only’ average (which round here is fairly reasonable I’d guess) on the night in question.

17 minutes ago, Mr Spock said:

My C9.25 splits 0.7” easily with space to spare. 

A 250 should do just as well providing the optics are good enough, they are collimated perfectly and cool down ans seeing conditions are good. 

Collimators aren’ t accurate enough; in my experience the only effective way of collimating is with a star test (a real star  ). Even aslight misscollimation will destroy image quality. You hear so many people complain about reflector or SCT image quality when the real culprit is collimation. 

Thanks Michael, that’s really helpful too. I generally haven’t been doing star tests (except when I know I’ve not collimated I a while) taking comfort instead in the well aligned view through the Cheshire. I’ll give this a go.

[Size9Hex]

13 minutes ago, Ricochet said:

The other issue you might have is scatter from the mirror(s). The best (and cheapest) mod I have made to my dob was a simple ring of black card to baffle the edge of my primary mirror. Scatter from that edge was instantly blocked and every star image is smaller and tighter. 

Thanks! Another one to try. How much did you baffle, and what was the original mirror size please?

[Ricochet]

29 minutes ago, Size9Hex said:

Thanks! Another one to try. How much did you baffle, and what was the original mirror size please?

The specified size of the mirror is 203mm but in reality the coated surface is wider than that, plus there is an unpolished/coated bevel beyond that. Off the top of my head I made the inside edge of the baffle something like 198 or 200mm in diameter, the mirror clips protrude a bit further than the baffle. 

I chose to mount mine on top of the mirror clips so the outer diameter was chosen to give a couple of mm clearance between the edge of the baffle and the tube. Sizing it so that it made contact with the tube wall impeded passive cooling and mandated use of a cooling fan at all times. I also glued several layers together to give the baffle the required strength. Other people have chosen to simply place their baffle on the mirror underneath the mirror clips in which case a single layer of card is all that is needed and the outer diameter is determined by the mirror clips. 

Have a look down your scope at the mirror. If you see a bright ring or partial bright ring at the edge it will probably be a good candidate for baffling. 

See the thread below for photos of the difference this can make:

 

[timwetherell]

One advantage refractors enjoy comes from the physics of refraction and reflection. If a lens and mirror are both out of thermal equilibrium, the angle of the glass at any point on the surface will be ever so slightly wrong. The resulting angular error in a reflected ray will be double that of the surface angle error, the upshot being that mirrors are much more sensitive to changing temperature environments than lenses. To a large extent it's horses for courses, if there was one "best" scope design, we'd all have it

[Paz]

5 hours ago, Size9Hex said:

Interesting comments. The VX14 (and indeed any large high quality dob) is on my to do list! If I may ask, what do you do clean up the view? This is one of the thoughts I had - whether I simply hadn’t optimised the view in the dob in some way. That said, from the response, it sounds like fracs really excel in this area.

Here's my list of "hacks" (to make them sound trendy)...

Stop down the aperture - I find this reduces the glare and takes spider vanes issues out of the equation (pic below).

Be patient with cooling the tube.

Patience viewing -I find good views can come and go more quickly.

I use a fan to break up tube currents if I can't wait for them to

I'm a bit obsessive about collimation accuracy, and to do this I agree with the star test comments above.

Nd0.9 filter - helps me at lower magnifications on bright things.

At larger exit pupils I make sure Ive got my glasses on.

Single polarising filter - ditto and also sometimes I find this helps with lower altitude targets if there's still a bit of daylight to contend with.

Equatorial platform for tracking - this may seem off topic but unpicking a close pair whilst being constantly distracted by a tracking workload doesn't help me especially when at ludicrous-magnification.

I try to avoid using the secondary mirror heater if I can get away without it - they make heat currents that don't help.