Resolving Doubles

[acercon3]

Last night we had the first clear night for sometime so decided to get my 8SE out early at 1600 hrs in order to cool down . The plan was to have a look at Mars and Uranus and later on when Orion was up to have a look around here and try to resolve doubles in Alnitak , Mintaka and Rigel. 
I’ve only been observing since last January with bins and since July with my 8SE so am not hugely experienced. I’ve done loads of research etc and think that I’ve understood the principals but have had no practical experience gained from interaction with a fellow enthusiast.

My eyepieces consist of the Meade Plossl kit which includes the 2x Barlow and the 25 mm Celestron ep that came with my scope.

Having tried to unsuccessfully resolve the aforementioned doubles previously I decided to check my collimation just to be sure that all was in order. This exercise showed all to be in order.

Mars appeared pretty much as I expected although rather smaller than we were seeing in recent months. Over the piece my views of Mars have been a bit disappointing , I think that I have only had one fleeting glimpse of the SPC over the past few months. On to Uranus which has also disappointed. I have never been able to resolve it to a small disc. Rather a blueish tinged source of light which could not be distinguished as a planet. Mags used were 160 x & 220 x.

The exercise to resolve the aforementioned doubles resulted in each of these stars being brought to a point of bright light under various mags. None of which showed any separation .

From my limited experience I thought that the conditions last night were pretty good and that prior to setting up was hopeful that some clear and crisp views of my targets may be had. BTW I live rurally and have Bortle 4 skies.

I’m in my 60 s so eyesight is not what it once was , may this be a factor in why I am unable to get a resolution on these doubles ?

 

 

 

 

 

 

 

 

[John]

Two things spring to my mind reading your post: cooling and collimation. If you got the scope out at 16:00 hrs then I assume that it had cooled for a decent time before you observed ?

Did you star test to check the collimation and, if so, how did the star test look ?

The intra and extra images of a star such as Polaris show much about the state of cooling and collimation of a scope. Plus, it's another double star of course !

Your 8 inch SCT should be resolving the double stars you mention, some pretty easily. Mars is whisking away from us now and it's disk is now less than 9 arc seconds but should still be visible as a disk with vague dark markings just about visible. Uranus should show as a grey-green disk, although small, but plainly a disk, at around 200x magnification.

I expect you have seen this web page ?:

http://www.astrophoto.fr/collim.html

 

 

[acercon3]

17 minutes ago, John said:

Two things spring to my mind reading your post: cooling and collimation. If you got the scope out at 16:00 hrs then I assume that it had cooled for a decent time before you observed ?

Did you star test to check the collimation and, if so, how did the star test look ?

The intra and extra images of a star such as Polaris show much about the state of cooling and collimation of a scope. Plus, it's another double star of course !

Your 8 inch SCT should be resolving the double stars you mention, some pretty easily. Mars is whisking away from us now and it's disk is now less than 9 arc seconds but should still be visible as a disk with vague dark markings just about visible. Uranus should show as a grey-green disk, although small, but plainly a disk, at around 200x magnification.

I expect you have seen this web page ?:

http://www.astrophoto.fr/collim.html

 

 

 

[acercon3]

Hi John

Thanks for getting back to me .

iI popped the scope out ar 1600 and started off with Mars and Uranus around 1830.  At around this time I decided to check the collocation and think this was done on either Pollux or Aldebaran ( my alignment stars). The test showed a perfectly centered obstruction , no skewing.

I’ll check out the link you provided.

thanks

Alan

[acercon3]

A further bit of info ......the scope was out until 2200 ish , I attempted the doubles between 9 and 10 so cool down should not have been an issue.

[Nik271]

Of the three doubles (Alnitak, Rigel and Mintaka) you mentioned by far the easiest is Mintaka, its companion is almost 1 arcminute away from the primary and should be visible in almost any telescope. Could it be you had dew on the corrector plate of the SCT?  

 

[John]

When you star tested, were you getting a reasonably well defined airy disk and diffraction ring / rings when at sharp focus ?

Or as the star image quite "hairy" ?

I have observed with an 8 inch SCT that was struggling to split Epsilon Lyrae (the "double double") a while back but that did turn out to be out of collimation when we star tested it.

[Mr Spock]

Collimation with SCTs is critical. Even slightly out can render the image fuzzy. A good star test with a clean airy disc and diffraction rings is the only way to get them spot on.

[acercon3]

Nik271,John,Mr Spock

I did not check for dew on the corrector plate but did have my dew shield in place. Perhaps I should make a mental note to check this in future.

That was my first attempt at a star test so I may be a bit woolly on exactly what I was seeing other than reporting that the obstruction was centred.

I have checked out the link provided by John and suspect that I need to revisit the star test procedure and subsequent steps .

I bought the 8SE in July and have never run this type of test or collimated before so strongly suspect that this is where I need concentrate my efforts.

Many thanks for your input.

[Spile]

The star test is my first port of call for collimatrion, If it checks out I know any issues will be down to another factor.

I got a lot of enjoyment from Castor (ice white) and Almach (yellow and blue) last week using a 8" reflector.

[lunator]

Hi Alan

After you have checked the collimation of your scope I recommend looking at theses 2 doubles.

Eta Cassiopeiae and Castor (Alph Gem)

 They should be easily split in your scope.

Eta Cass is 3.5/7.5 magnitude pair with a sepaeration of 13.5"

Castor is 1.6/3 magnitude pair with a seperation of 5.2"

let us know how these go.

cheers

Ian

[Ricochet]

6 hours ago, acercon3 said:

That was my first attempt at a star test so I may be a bit woolly on exactly what I was seeing other than reporting that the obstruction was centred

If you can see the obstruction then you have defocused way too much and are not doing a star test. You need to have a high power eyepiece, equal or shorter than the telescope's focal ratio, and to have the star perfectly centred on the field of view. At this magnification at least the first diffraction ring should be visible when the star is in focus, you then want to slightly defocus to make the rings more visible. You should see a bright disk with a series of concentric rings around it. The concentricity of the rings shows the collimation, but beware that any star not quite centred will not show concentric rings and so using an off centre star to collimate could make your collimation worse. 

[acercon3]

Hi All

I had an aborted attempt at a star test and collimation last night. It was clear and very frosty but 8SE was put outdoors nice and early in order to achieve thermal equilibrium . When I did go out I did my usual 2 star align etc and then selected my star upon which to test. At this point I removed my dew shield in anticipation of needing access to screws and the front of the corrector plate in order to ascertain orientation using a screwdriver. The corrector plate quickly fogged over rendering the exercise a failure.

Anyhow it looks as though it will be clear agin to its albeit at least as frosty.

The plan will be to not remove the dew shield and if necessary access the screws separately from simultaneous viewing. I plan to start with my 18 mm and 2 x Barlow giving 225 x moving onto 13 mm 312x & 9 mm 451 x

 

I have also seen Artificial Stars for collimating , are these effective ?

[skyhog]

I've always managed pretty good collimation using the stars. One other thing to consider, and it's the only way I use now is to insert a webcam and see the results in real time as you tweek. An absolute godsend. 

[acercon3]

I eventually managed to get around to checking the collimation on my 8SE. I purchased an artificial star and some Bob’s Knobs which seems to make things a bit easier. The check and adjustments were made indoors. We have a long central hallway and rooms at either end so was able to find the necessary distance needed to carry out the procedure . Not having external distractions such as wind, fogging , tracking etc made things much easier for my first real attempt at collimation.

it was pretty obvious that things were improving as I moved thru the adjustment process using the ever stronger eyepieces.

Last night was my first outing with the 8SE so was keen to see if I could split some of the doubles that evaded me previously.

it was suggested earlier in the thread that I try Castor , this was easily split . Mintaka was very easy also. I failed with Alnitak and Rigel and could not see E or F in the Trapezium. Perhaps I need to perform a star test next time out in order to fine tune things. In any event my instrument is performing better than it was previously so thanks to all who have assisted.

 

 

[Nik271]

I'm glad to hear that your 8SE is performing much better. 

Alnitak is actually a tricky double: the companion is only 2.2'' from the primary and  dimmer, so you would need excellent seeing to spot it the first time. Once you know where to look it becomes easier.

Rigel and the Trapezium F and G are also highly seeing dependent. I would say Rigel is much easier than either F and G or even Alnitak, look for a much dimmer bluish companion south of the primary. 

Have fun with these doubles!

Nik

[acercon3]

51 minutes ago, Nik271 said:

I'm glad to hear that your 8SE is performing much better. 

Alnitak is actually a tricky double: the companion is only 2.2'' from the primary and  dimmer, so you would need excellent seeing to spot it the first time. Once you know where to look it becomes easier.

Rigel and the Trapezium F and G are also highly seeing dependent. I would say Rigel is much easier than either F and G or even Alnitak, look for a much dimmer bluish companion south of the primary. 

Have fun with these doubles!

Nik

Hi Nik

When viewing Alnitak I thought that I could see a very dim object to the west of it, barely visible . Could this have been what I was searching for ?
 

Alan

[John]

57 minutes ago, acercon3 said:

Hi Nik

When viewing Alnitak I thought that I could see a very dim object to the west of it, barely visible . Could this have been what I was searching for ?
 

Alan

This is quite a good representation of Alnitak through a 101mm refractor

It can be quite a tough one though if the seeing is unsteady, scope not quite cooled, etc ,etc.

Edited February 2, 2021 by John

[acercon3]

Hi John

 

That looks rather like it. The barely discernible star was well separated but I did not have a A and B resolved.