Splitting the Trapezium

[Littleguy80]

Last night's session was only about an hour or so before bed from my back garden. Not the type of session I would normally write up. It was around 11:45 PM by the time I got outside. It has been patchy cloud for most of the evening and I had a bunch of stuff to do around the house too. On going outside the first thing that really caught my eye was the fact Sirius wasn't twinkling quite as much as normal. This gave me hope that the seeing would be steady. I took a look at M42, and the 4 main stars of the Trapezium were nice and sharp within the green nebulosity. I put the 6mm BGO into the focuser, giving 200x, and settled on the Trapezium. Straight away I could see the E star. Not my first observation of this star but I certainly don't get it every time. No sign of the F star but the view was nice and steady. I had two passes of letting the trap pass through the eyepiece, adjusting focus to try and draw the elusive F star out. On the third pass, the E star stood out well and finally the F star also showed itself! I've regularly referenced failed attempts to see the E and F stars in my observing reports going back to last autumn when I using a 130mm reflector. It's a small personal victory! I enjoyed a few more passes to confirm the observation. By the end, I could hold the F star comfortably in direct vision.

Following the successful split of the Trapezium, I went onto try for Sirius' companion, the pup star, but no joy. I think some more research is required to figure out the best approach for this split. Sigma Orionis was a nice easy split with all 4 stars easily seen. I added an OIII filter and observed NGC 2174, the Monkey Head Nebula. I didn't see it as a Monkey head but could comfortably trace the edge of this nebula. I next spent a good amount of time trying to pick out the Flame nebula, NGC 2024. I tried various magnifications and putting the Flame's location at different points in the field of view. A couple of times, I felt like I was detecting faint nebulosity but nothing that I would count as a conclusive observation. The Rosette Nebula, NGC 2238, was quite the opposite. Thick nebulosity was easily seen with sections of fainter nebulosity detected around the central cluster. A quick look a the fun Christmas Tree cluster, NGC 2264, following by the Pleiades, M45. For the finale, I tracked down Hind's Crimson star, a stunningly red star which seems to just pop out when it catches your eye. Very enjoyable short session with another observing goal ticked off the list.

[mapstar]

Ah the joys of splitting the trapezium, great report which brings back memories of my 200pds skywatcher.

There has been some nice nights of late and glad someone is making the most of it.

The pup star is a difficult one and a very difficult split. Low in the atmosphere will make the task even harder as it shimmies that much. 

Keep at it, star splitting is something you can do from light polluted skies, so anytime really. 

La Superba is another nice carbon star often labelled as Y CVn (canes Venatici) on most star charts. Take a look.

[John]

Nice report Neil - the Orion season is with us again !

I've spotted the E & F stars regularly with my ED120 refractor but much more occasionally with my 100/102mm refractors.

Splitting Sirius remains the preserve of my 12" dob and the 130mm triplet refractor. I've not managed it with the ED120 as yet.

Antares is another challenging split, if you can get a scope on it.

 

[estwing]

Good read Neil...flame I saw best at the SWSP few years ago...never really bothered too much hunting inside the trap, clear skies. 

[cloudsweeper]

Another good read, Neil.  I got the E & F stars easily with the Dob a few weeks ago - I was using x144 and up they popped with DV.  Since they are faint, the transparency must've been good that morning!

Doug.

[Littleguy80]

26 minutes ago, cloudsweeper said:

Another good read, Neil.  I got the E & F stars easily with the Dob a few weeks ago - I was using x144 and up they popped with DV.  Since they are faint, the transparency must've been good that morning!

Doug.

Thanks, Doug. I remember reading your report on seeing the E & F stars and thinking I must be missing a trick! As you say it's probably just down to having the rights conditions on the night. I also think that I sometimes talk myself out of seeing things, especially if I don't see it in the first minute or so of trying. I should know better from my own experiences really! The F star go easier and easier to see the more I looked.

[wookie1965]

I've only ever seen the E and F stars once in my first scope a 6" reflector on a cold frosty night never seen it since. Although I have not had the chance with my refractors yet as have not had the weather low cloud here last night and chucking it down now. No clear nights for the forciable future I've not been out in weeks.

So nice to read reports from other people when you cannot get out.

Thank you for writing it.

[Littleguy80]

11 hours ago, John said:

Nice report Neil - the Orion season is with us again !

I've spotted the E & F stars regularly with my ED120 refractor but much more occasionally with my 100/102mm refractors.

Splitting Sirius remains the preserve of my 12" dob and the 130mm triplet refractor. I've not managed it with the ED120 as yet.

Antares is another challenging split, if you can get a scope on it.

 

Thanks John. Orion is a fab constellation. So much to see

I have very little experience with refractors. My impression, from reports on here, is that they're a better scope for splitting stars with as the stars are sharper than with a reflector? 

I'll definitely give Antares a go when it comes back around. I didn't even know it was a double star until now!

[John]

5 hours ago, Littleguy80 said:

I have very little experience with refractors. My impression, from reports on here, is that they're a better scope for splitting stars with as the stars are sharper than with a reflector? 

 

The larger aperture of the reflector will enable it to "out resolve" a smaller refractor but refractors seem to produce a cleaner star image (no diffraction from obstructions in the light path) which, for me, is why I tend to prefer double star splitting with a refractor. For really challenging splits such as Sirius I've found the 12" aperture does the job though.

 

 

[chiltonstar]

13 hours ago, Littleguy80 said:

 

I have very little experience with refractors. My impression, from reports on here, is that they're a better scope for splitting stars with as the stars are sharper than with a reflector? 

 

The point about refractors is that they have no central obstruction, which makes the view "cleaner" and with better contrast. The greater the % obstruction, the more contrast is lost. Newtonians (dobs), SCTs and Maks all have this problem to a greater or lesser extent.

This computer simulation shows the effect well, illustrated using a 150mm f7.5 refractor or 150mm f7.5 newtonian on the same double star pair:-

With an "obstructed" scope, more light goes in to the first diffraction ring than with a refractor (although it does mean the central Airy disk is slightly smaller and less intense).

Chris

[alan potts]

In the past I have found Sirius not to difficult to split but it does get 11 degrees higher here than in the UK, I played a game for a long time by trying to split the star before it got above the height it appears in the Uk, must say that makes a difference, only once spliting the Pup in upteen trys and that was with the 18 inch. Stars E and F I have always found fairly easy, be it with the 115mm APO or the 190mm M/N, both give excellent images, but again being where I am must make a difference.

Alan 

[Littleguy80]

9 hours ago, chiltonstar said:

The point about refractors is that they have no central obstruction, which makes the view "cleaner" and with better contrast. The greater the % obstruction, the more contrast is lost. Newtonians (dobs), SCTs and Maks all have this problem to a greater or lesser extent.

This computer simulation shows the effect well, illustrated using a 150mm f7.5 refractor or 150mm f7.5 newtonian on the same double star pair:-

With an "obstructed" scope, more light goes in to the first diffraction ring than with a refractor (although it does mean the central Airy disk is slightly smaller and less intense).

Chris

That's a great explanation. Thanks Chris!

[Littleguy80]

1 hour ago, alan potts said:

In the past I have found Sirius not to difficult to split but it does get 11 degrees higher here than in the UK, I played a game for a long time by trying to split the star before it got above the height it appears in the Uk, must say that makes a difference, only once spliting the Pup in upteen trys and that was with the 18 inch. Stars E and F I have always found fairly easy, be it with the 115mm APO or the 190mm M/N, both give excellent images, but again being where I am must make a difference.

Alan 

Eeeek the pup star might be out of reach of my 10" dob then.  I won't give up though

[John]

23 minutes ago, Littleguy80 said:

Eeeek the pup star might be out of reach of my 10" dob then.  I won't give up though

I've done it with my 130mm refractor from the UK.

The gap between the stars is around 10 arc seconds currently so not too tight but it's combatting the glare from Sirius A to allow the much dimmer "Pup" to glimmer through that is the challenge. I find that ortho eyepieces help and around 200x - 250x magnification.

Steady seeing conditions help too.

[Littleguy80]

1 minute ago, John said:

I've done it with my 130mm refractor from the UK.

The gap between the stars is around 10 arc seconds currently so not too tight but it's combatting the glare from Sirius A to allow the much dimmer "Pup" to glimmer through that is the challenge. I find that ortho eyepieces help and around 200x - 250x magnification.

Steady seeing conditions help too.

Awesome. Thanks John. I’ll give it a go with the 5mm and 6mm BGO then.

[Owmuchonomy]

18 hours ago, John said:

The larger aperture of the reflector will enable it to "out resolve" a smaller refractor but refractors seem to produce a cleaner star image (no diffraction from obstructions in the light path) which, for me, is why I tend to prefer double star splitting with a refractor. For really challenging splits such as Sirius I've found the 12" aperture does the job though.

 

 

The ED150 side by side with my 9.25 SCT (collimated) definitely produces an easier visual experience on the A-F Trapezium stars (and others).

[chiltonstar]

3 hours ago, John said:

I've done it with my 130mm refractor from the UK.

The gap between the stars is around 10 arc seconds currently so not too tight but it's combatting the glare from Sirius A to allow the much dimmer "Pup" to glimmer through that is the challenge. I find that ortho eyepieces help and around 200x - 250x magnification.

Steady seeing conditions help too.

My ED80 has managed it on two occasions.

Chris

[alan potts]

4 hours ago, Littleguy80 said:

Eeeek the pup star might be out of reach of my 10" dob then.  I won't give up though

No I am sure it has been done with smaller scopes than that, I find morning a better time after the air has had time to settle. I have done it with my M/N 190 and my 115mm APO but that is from a better latitiude of course. I know John has done it a few times with his 12 inch, it is about it's widest separation now so good time to try.

[Peter Drew]

15 hours ago, chiltonstar said:

The point about refractors is that they have no central obstruction, which makes the view "cleaner" and with better contrast. The greater the % obstruction, the more contrast is lost. Newtonians (dobs), SCTs and Maks all have this problem to a greater or lesser extent.

This computer simulation shows the effect well, illustrated using a 150mm f7.5 refractor or 150mm f7.5 newtonian on the same double star pair:-

With an "obstructed" scope, more light goes in to the first diffraction ring than with a refractor (although it does mean the central Airy disk is slightly smaller and less intense).

Chris

There can't be many F7.5 Newtonian reflectors with 35% obstruction?    ?

[chiltonstar]

1 hour ago, Peter Drew said:

There can't be many F7.5 Newtonian reflectors with 35% obstruction?    ?

Not typical, I agree, but I took the worst case to make the graphic obvious. 

Chris

[John]

When the central obstruction is down to around 20% or less and other things like spider vanes are dispensed with, the images that reflecting scopes deliver can get very "refractor like". I'm thinking of designs such as the maksutov-newtonian.

 

[chiltonstar]

32 minutes ago, John said:

When the central obstruction is down to around 20% or less and other things like spider vanes are dispensed with, the images that reflecting scopes deliver can get very "refractor like". I'm thinking of designs such as the maksutov-newtonian.

 

What is the obstruction with a 2 inch focuser John?  More than this surely? 

Chris

[John]

3 hours ago, chiltonstar said:

What is the obstruction with a 2 inch focuser John?  More than this surely? 

Chris

My 12" F/5.3 has a 21% obstruction (63mm minor axis secondary). That has a 2" focuser of course.

 

[chiltonstar]

1 hour ago, John said:

My 12" F/5.3 has a 21% obstruction (63mm minor axis secondary). That has a 2" focuser of course.

 

I was thinking of the Mak Newt which is only 190mm and has (presumably) a 52mm or so secondary?

Chris

[John]

14 hours ago, chiltonstar said:

I was thinking of the Mak Newt which is only 190mm and has (presumably) a 52mm or so secondary?

Chris

The Skywatcher mak-newt does not use as small a % secondary as the Russian mak-newts which I've owned.