Trying to split Izar with 66mm

[RobertI]

Last time out I tried to split Izar with my 66mm refractor.  I’d read that it was extremely difficult with a scope of this size due to the fainter secondary lying directly in the bright diffraction ring created by it’s sibling. The best I could  manage was an slight but definite brightening of the diffraction ring in the correct position at a mag of 110x. More details here. Tonight I upped the magnification to the highest I could get, 175x. Having cooled the scope I pointed the scope at Izar. First look, the secondary was not immediately visible, just a very bright diffraction ring. Then on closer inspection, there it was, clear as day, but sitting slap bang on the diffraction ring. I’m not surprised it was so tricky. But a pleasing split. Finished off with a nice view of the double double and a very bright satellite overhead, which turned out to be NASA’s Terra satellite. 🙂

Edited May 7, 2021 by RobertI

[Stu]

Nice one Robert 👍👍

I love this double in small scopes. To me it looks like a ring with a coloured gemstone embedded in it, very beautiful with the larger airy disks you get with a smaller scope. 60mm is the smallest I’ve used to split it, a Tak FS-60C. My little 63mm Telementor makes a lovely job of it too. It’s one is always look at if it’s visible. Enjoy!

[RobertI]

16 minutes ago, Stu said:

Nice one Robert 👍👍

I love this double in small scopes. To me it looks like a ring with a coloured gemstone embedded in it, very beautiful with the larger airy disks you get with a smaller scope. 60mm is the smallest I’ve used to split it, a Tak FS-60C. My little 63mm Telementor makes a lovely job of it too. It’s one is always look at if it’s visible. Enjoy!

Thanks Stu, yes it was very much like a ring with a gemstone embedded in it, I almost used the same comparison to describe it! A lovely sight. 

[wookie1965]

10 hours ago, RobertI said:

Last time out I tried to split Izar with my 66mm refractor.  I’d read that it was extremely difficult with a scope of this size due to the fainter secondary lying directly in the bright diffraction ring created by it’s sibling. The best I could  manage was an slight but definite brightening of the diffraction ring in the correct position at a mag of 110x. More details here. Tonight I upped the magnification to the highest I could get, 175x. Having cooled the scope I pointed the scope at Izar. First look, the secondary was not immediately visible, just a very bright diffraction ring. Then on closer inspection, there it was, clear as day, but sitting slap bang on the diffraction ring. I’m not surprised it was so tricky. But a pleasing split. Finished off with a nice view of the double double and a very bright satellite overhead, which turned out to be NASA’s Terra satellite. 🙂

According to Haas Izar is showcase pair. 60-mm, 120x showing brilliant amber star with a deep blue spur on its edge; both stars are vivid in glorious colour. Smyth: "Pale orange; sea green"

[John]

I wonder if it could be done with a 50mm ?

Theoretically the 50mm limit is 2.32 arc seconds and the Izar split is 2.8. But the component stars are unequal brightness so that adds a degree of trickiness

I can easily stop my ED120 down to a 52mm F/17.3 using the aperture in the dust cap. I feel the urge to experiment on the next clear night

 

[Zermelo]

I tried and failed several times with a Bresser 70mm and stock eyepieces, even on still nights.

As soon as I traded up to a 150mm reflector+BST I could split it quite comfortably.

[RobertI]

2 hours ago, John said:

I wonder if it could be done with a 50mm ?

Theoretically the 50mm limit is 2.32 arc seconds and the Izar split is 2.8. But the component stars are unequal brightness so that adds a degree of trickiness

I can easily stop my ED120 down to a 52mm F/17.3 using the aperture in the dust cap. I feel the urge to experiment on the next clear night

 

Yes please do John, I'd love to know.

[Sunshine]

Just took a look at Izar, I expected a challenge but it was smack in my face obvious at about 120x or so, I guess it is due to the fact that I'm using a 102.  Would it be that much more difficult in lets say a 70mm? separation

is such that there is an unmistakable gap between these two.

 

[John]

6 hours ago, Sunshine said:

Just took a look at Izar, I expected a challenge but it was smack in my face obvious at about 120x or so, I guess it is due to the fact that I'm using a 102.  Would it be that much more difficult in lets say a 70mm? separation

is such that there is an unmistakable gap between these two.

 

I find it easy with 100mm / 102mm. I did it with a cheap 90mm F/11.1 achromat last week.

As the aperture shrinks the airy disks get larger and the resolving limit reduces. The uneven brightness adds quite a degree of further difficulty as well. I think I could split Izar with my old Tele Vue 70mm Ranger a couple of years ago but I can't recall definitely.

I'll try the stopped down ED120 next clear night at 52mm and see if that does it, or gets anywhere near.

Nice piece on resolution here with a lovely Damian Peach image of Izar as part of the story:

http://www.rocketmime.com/astronomy/Telescope/ResolvingPower.html

 

Edited May 9, 2021 by John

[John]

On 08/05/2021 at 13:12, RobertI said:

Yes please do John, I'd love to know.

Well I've had a go at Izar tonight with my ED120 refractor at full aperture (no problem at all at 125x) and more interestingly with the aperture stopped down to 52mm using the central aperture of the objective dust cap.

Izar was split !. The airy disks were larger and the secondary star very close to touching the primary's airy disk but there was a definite gap between the disks. The secondary star was on the inside edge of a very fine single diffraction ring created by the brighter star. The magnification used was 225x - 300x which would be a bit silly for a 52mm aperture scope but seemed to work for this target.

I was also slightly surprised to see the clear colour difference between the stars despite the small aperture.

I found this image online which nicely captures what I'm seeing although my view also includes that fine diffraction ring as well:

I was pleased as well as a little surprised by this result. I suspect that it confirms that my ED120 has a good quality objective but also that splitting Izar can be achieved with very small apertures.

While the scope was stopped down I had a go at some other doubles:

Gamma Virginis: Split

Epsilon Lyrae: One pair just about split, the closer pair touching.

Delta Cygni: Dimmer secondary star not clearly seen. Maybe just a vague hint of it ?

Iota Cassiopeia: B and C stars very dimly visible. All 3 components seemed to be split though.

Interesting experiment !

 

Edited May 10, 2021 by John

[Pixies]

8 hours ago, John said:

Well I've had a go at Izar tonight with my ED120 refractor at full aperture (no problem at all at 125x) and more interestingly with the aperture stopped down to 52mm using the central aperture of the objective dust cap.

Izar was split !. The airy disks were larger and the secondary star very close to touching the primary's airy disk but there was a definite gap between the disks. The secondary star was on the inside edge of a very fine single diffraction ring created by the brighter star. The magnification used was 225x - 300x which would be a bit silly for a 52mm aperture scope but seemed to work for this target.

I was also slightly surprised to see the clear colour difference between the stars despite the small aperture.

I found this image online which nicely captures what I'm seeing although my view also includes that fine diffraction ring as well:

I was pleased as well as a little surprised by this result. I suspect that it confirms that my ED120 has a good quality objective but also that splitting Izar can be achieved with very small apertures.

While the scope was stopped down I had a go at some other doubles:

Gamma Virginis: Split

Epsilon Lyrae: One pair just about split, the closer pair touching.

Delta Cygni: Dimmer secondary star not clearly seen. Maybe just a vague hint of it ?

Iota Cassiopeia: B and C stars very dimly visible. All 3 components seemed to be split though.

Interesting experiment !

 

@John Do you find that colours are more obvious with smaller aperture? I see colours more clearly in the ST80 compared to the dob - but that might be magnification rather then aperture, though?

[John]

1 hour ago, Pixies said:

@John Do you find that colours are more obvious with smaller aperture? I see colours more clearly in the ST80 compared to the dob - but that might be magnification rather then aperture, though?

I've not compared the views directly between my 12 inch dob and a refractor on Izar for some time but you could be right. Another experiment to try

[Voyager 3]

1 hour ago, Pixies said:

@John Do you find that colours are more obvious with smaller aperture? I see colours more clearly in the ST80 compared to the dob - but that might be magnification rather then aperture, though?

That could also be due to the refractor Vs reflector thing !

[RobertI]

11 hours ago, John said:

Izar was split !.

Very interesting! Looks like the small aperture has resulted in the diffraction ring just moving past the secondary rather than being on it? Interesting about the colours too. 

[John]

21 minutes ago, RobertI said:

Very interesting! Looks like the small aperture has resulted in the diffraction ring just moving past the secondary rather than being on it? Interesting about the colours too. 

It would be interesting to rig up some sort of variable aperture stop (like a camera lens has) and then play with the effects on resolution / airy disk / diffraction rings of various apertures on the fly.

 

[vlaiv]

3 minutes ago, John said:

It would be interesting to rig up some sort of variable aperture stop (like a camera lens has) and then play with the effects on resolution / airy disk / diffraction rings of various apertures on the fly.

 

https://iris-calculator.com/

(there is free version - but it does not export design - you can only note dimensions for later use)

https://www.instructables.com/How-to-make-a-12-leaves-Mechanical-Irirs/

https://www.instructables.com/3d-Printed-Mechanical-Iris/

etc ...

[Stu]

On 11/05/2021 at 12:25, vlaiv said:

https://iris-calculator.com/

(there is free version - but it does not export design - you can only note dimensions for later use)

https://www.instructables.com/How-to-make-a-12-leaves-Mechanical-Irirs/

https://www.instructables.com/3d-Printed-Mechanical-Iris/

etc ...

Love that Iris on instructables, very clever.