Minor circular shadow through EP.

[matelot]

Hi, all,

One of the eye-pieces that came with the SL 200 Newt/Dob was the SL SV 30mm. For what I know with my first scope, it focuses fine and been looking at some phenomenal sunspots this morning. (Don't get cold, either...) 😉

However, the pics (taken indoors afterwards and just looking out) will show that there's a perfectly centred shady/shadow portion which seems to match the secondary mirror. Sunspots were perfectly focused when off-centre, but ever-so-slightly dimmed/out of focus when EP-centred. The included 9mm Plossl doesn't exhibit the same.

As far as I can tell, collimation is near as spot-on as I can get it - everything centred, focus great, no double anything. It's just the very faint shadow.

Is this an artefact of the setup, this particular 30mm EP, or have I got some tweaking to do somewhere?

Pic 1 - blank sky, but imagine the Sun with bits of dust...;

Pic 2- rooftops showing faintest darker shadow between two stacks;

Pic 3 - straight down the focuser.

It's only a minor issue because I don't know what it is.

Cheers, peeps.

[wulfrun]

Pretty sure it's the shadow of the secondary and not down to bad collimation. I get a worse version using long f/l eps in my Heritage 114P (e.g. 32mm plossl), which is an f/4.4. I think it's more pronounced with "fast" scopes and longer eps. Don't see it at night but I suppose it's there still!

Edited September 16, 2022 by wulfrun

[matelot]

Thanks for that, wulfrun. Diddly-squat at night - only in bright skies does it show. I have it in mind that somehow it's the secondary. I've even had a good look at the 30mm with a loupe, but it's crystal.

[Ricochet]

It is due to the large exit pupil that a long focal length eyepiece creates, in this case 30mm/f6= 5mm. If you look at the exit pupil from a distance you will notice that in an obstructed scope it is a doughnut shape rather than a solid circle due to the secondary in the centre. For ease of calculation of we assume that the secondary has a diameter of 50mm then with the 30mm eyepiece the central shadow will be 1.25mm across. At night your pupil is dilated to maybe 5-7mm and the small dark centre is overwhelmed by the light coming from the outer part of the exit pupil. During the day your pupil shrinks to maybe 2mm, so having the central 1.25mm in shadow means that you begin to be able to see it. A phone camera tends to have a small aperture and hence be like a daytime pupil and so you can photograph this same effect. This effect is not specific to your 30mm eyepiece, any 30mm eyepiece will show exactly the same effect. The only way to reduce the effect is to use a smaller focal length eyepiece, which reduces the exit pupil, and hence the central shadow, to the point at which you no longer see the shadow. 

[matelot]

Thanks for your response, Ricochet. I'll study that until it becomes familiar.

Cheers.

[RT65CB-SWL]

Are the images taken at full aperture? - if "yes!" then an off-axis solar filter may eliminate it! - I know that some 'scope covers have two circles.. One is 'solid' to attach the smaller cap and the other/open off-axis will reduce the brightness. Some amateurs place some solar safety film over the open one with some adhesive sticky tape, on the inside.

BTW... catadioptric 'scopes tend do the same when focused on a 'bright' image/subject.