Old 11-07-2019, 11:11 AM
Cliff (Clifford)
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SCT's black spot ?

Hi all,

Well finally got my 6" Meade ACF SCT (after original mount deemed irreparable) on a manual AZ-EL mount using rings and a dovetail, added quite a bit of weight too.

Now I know about the central mirror but notice a pronounced spot in the middle when using low power eyepieces in particular. I am guessing at night not so noticeable but I have it set up watching some birds just for testing and getting used to focussing etc.. No scale reference like my refractor has.

So all you SCT users is this totally normal like the Dob hole I see so much written about ?

Any feedback welcome.
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Old 11-07-2019, 12:31 PM
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multiweb (Marc)
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Depending on the eyepiece type and magnification used I'd say you'd have to move your pupil back or forward to the right distance to avoid seeing the central obstruction. TBH although I'm aware of it I've never noticed any difference whether it's a newt or SCT.
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Old 11-07-2019, 12:56 PM
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mental4astro (Alexander)

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Cliff, this dark spot is seen in two circumstances in all scopes with a central obstruction:

1, when inside and outside focus it is seen in the centre of all out of focus stars.

How well concentric this dark spot is to the outside of the "doughnut" tells you how well collimated your optics are.

2, when using very low power under light polluted skies.

When I use my Newts (dobs) from home I see the shadow when I use a 30mm eyepiece. It is like a dark ghostly round spot that moves about as you move your head. In my SCT and Mak, I only see really see this shadow when I use a 50mm eyepiece. When you go bush with your scope this shadow isn't seen, only under light polluted skies.

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Old 11-07-2019, 04:41 PM
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At night time you pupil dilates more so the percent of central obstruction to pupil diameter is less. E.g. For a particular eyepiece perhaps 2mm of 6mm is obstructed (33% by diameter). At daytime the pupil is smaller say 3mm. At 3mm you have the same 2mm central obstruction for the same eyepiece, so (66% by diameter).

The effect is more noticeable at low power. I should really be talking about area not diameter but the ratio is the same and is easier to follow.
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Old 11-07-2019, 07:04 PM
Cliff (Clifford)
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Thanks for all the Replies

Hi astro744 , mental4astro, multiweb

Thank you for all your suggestions, I had an idea but having never really used a SCT before wasn't sure.

The spot seems very central so looks like Collimation is pretty good.

I hadn't thought about the Pupil dilation at night but that would have a large effect.

I certainly have light pollution , a Bortle 6 according to the Clear Skies App.

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Old 11-07-2019, 08:05 PM
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Originally Posted by Cliff View Post
The spot seems very central so looks like Collimation is pretty good.
Er... no not true. You need to learn how to do a star test, ie a star approx mag 2, near the zenith, at high magnification (at least 200X) and defocus just a tad. Centre this star in the field of view (this is crucial).

In the centre of the black blob (ie the shadow of the secondary mirror) you should see a bright dot, which is called the "Poisson spot". This is a product of diffraction around the secondary mirror and its useful because it marks the optical axis precisely and isn't affected by changes to the secondary mirror alignment. When the defocussed image rings are concentric with the Poisson spot the scope is collimated.

certainly have light pollution , a Bortle 6
Moon and planets will be fine, also double stars...
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