Black Dot?

[Katen]

The other day we were looking at the moon in daylight for the first time (uk thursday 5pm) with my daughters skywatcher explorer 150p and we had a black dot in the middle of our view, this dot was there whichever eyepiece she was using. We have not seen this on any of our night/dark time viewing. We have not used the scope at night since to see if its still there.

Is this usual for daytime viewing or is there something she needs to be doing? (she's currently attempting to collimate using a cheshire collimator in case that was causing the problem).

Any suggestions or help would be great, thank you.

[Tribal-Wolf]

Could be collimation but it sounds like you were not quite in focus.

[John]

What eyepiece were you using ?.

Long focal length eyepieces like 40mm can show the shadow of the secondary mirror when viewing against the moon or a light sky background.

[Katen]

Recollimation successfully completed, it was a little out bu not hugely.

With regard to focus, when we both use the scope at the same time we are constantly having to adjust the focus as she has perfect 17 year old vision and mine well lets just say is somewhat older! Consequently both of us have got pretty nifty at getting the focussing spot on for ourselves.

We were using both 40 and 25mm eyepieces, We did wonder if it was a daylight specific problem. Trouble is that it's been cloudy every night since so we haven't been able to check it out under dark conditions.

We await a clear night to check it again. Thanks for your answers.

[bombassa]

In daylight our pupils are usually small because of the light. When the exit pupil of the scope/eyepiece is larger than the eyepupil you may see the shadow of the secondary mirror.

I usually get this when using the scope in daylight or when observing a very bright moon. I guess in the latter case the light of the moon makes the pupil contract and thus smaller than the exit pupil = shadow is visible.

To be safe - thats at least my understanding )

[steppenwolf]

What useful information - I love this place!

[Macavity]

I think this is the "secondary shadow". In the centre of the reflecting telescope tube is the mirror assembly. At focal length 750mm e.g., a 25mm (Plossl) eyepiece will give you 30x magnification. The exit pupil (bright bit emanating from the eyepiece!) will have diameter: 150mm (mirror size) divided by 30 -> 5mm. At the centre of the view will be a "black spot", diameter 50mm (diameter of the secondary), divided by 30 -> 1.6mm.

Under Nighttime viewing, YOUR eye pupil will be open to (say) 5mm - Your (youthful!) daughters, ever wider! Simply, the "black spot" will be so small (relative to the exit pupil) to not interfear with the view! During DAYLIGHT however (both) your pupils will be a lot smaller. A couple of millimetres or less? The "black spot" thus correspondingly larger. So you will likely begin to see it...

Anyway, that's the potted explanation. Apologies for the ramble... I'm told I am obscure, cryptic even? LOL! But on this one, at least partly right? In essence, at LOWER powers, your scope is a "nighttime" instrument, not (necessarily) a daytime one!

[kenmyers]

I just keep on learning!

[Katen]

bombassa and macavity, thank you for your fantastic, logical and understandable explanations.

And I so agree that every time I come on here I learn something new. Thank you all.

[Moonshane]

I totally agree, it's the secondary shadow.