What is this

[Dave scutt]

Hi all.

While doing a star test I noticed a circle on the defocused star , I change ep's as I thought I had a mark on it but still the same. 

I never noticed it before any ideas .

Dave

[DaveL59]

The central dot or the wee one to the edge of it?

First would be the secondary, other maybe internal reflection somewhere?

[Dave scutt]

6 minutes ago, DaveL59 said:

The central dot or the wee one to the edge of it?

First would be the secondary, other maybe internal reflection somewhere?

[wulfrun]

Another star? Looks like it could be. What star did you use?

[Dave scutt]

7 hours ago, wulfrun said:

Another star? Looks like it could be. What star did you use?

I used Arcturus as it was the first star out

[Captain Scarlet]

I reckon it’s a piece of dust somewhere near the eyepiece focal plane, ie sitting on one of the eyepiece lenses

[Dave scutt]

9 minutes ago, Captain Scarlet said:

I reckon it’s a piece of dust somewhere near the eyepiece focal plane, ie sitting on one of the eyepiece lenses

I tried different ep's and still the same.

When the star drifted across the fov the dot stayed still 

[dweller25]

Do you mean it stayed still relative to the central black circle or it stayed still relative to the centre of the eyepiece ?

[dweller25]

It looks like a perfectly in focus star with an airy disk around it.

Was anything in your line if site when viewing ?
Any unusual obstructions within the OTA ?
Any big blemishes/pollen/dirt on the optics ?

Just throwing ideas around, have not seen this before……
 

[Dave scutt]

It stayed still in the centre of the ep.

I had a clear view no obstruction. 

Mirror is extremely clean only just clean it 

[dweller25]

It looks like your seeing was not great, I would be tempted to check on another night with hopefully better seeing, when the scope is well cooled and the collimation has been checked.

[Captain Scarlet]

Odd. Perhaps you have a spider-web with something attached to it in the eyepiece tube? How does it change for different degrees of in- or out-focus?

[Peter Drew]

It's a very disturbed extra focal image, this condition can produce all sorts of artefacts.  Were there any anomalies at best focus?    🙂

[Dave scutt]

1 minute ago, Peter Drew said:

It's a very disturbed extra focal image, this condition can produce all sorts of artefacts.  Were there any anomalies at best focus?    🙂

No anomalies at best focus just a sharp star.

I only got the scope out to do a collinmation check as trying to collimate it all day, wish I kept the scope out longer to help out with my question

[Dave scutt]

10 minutes ago, Peter Drew said:

It's a very disturbed extra focal image, this condition can produce all sorts of artefacts.  Were there any anomalies at best focus?    🙂

What causes a disturbed extra focal image

[dweller25]

4 hours ago, Dave scutt said:

What causes a disturbed extra focal image

Insufficient cool down and or poor seeing and or observing on or near heat sources.

[vlaiv]

I don't think it is duplicate of the star in any shape or form ("disturbed extra focal image") - as it stays in center when star drifts.