Focus in Centre of FOV question

[mikeyjames]

Hi all,
Last night when I was looking at Venus I noticed that the view was very crisp everywhere in the FOV except the very centre. Then when I attached my 5x Powermate it really exacerbated the issue. There seemed to be a faint outline of the donut in the centre of the FOV and whenever Venus was within the donut it was out of focus but anywhere out of the donut it looked nice and crisp.

Then a little later on with Saturn same issue but not as bad.

I had checked collimation earlier in the evening and it seemed spot on.

Anyone know what could be causing this?

Thanks
Mick

[The_bluester]

Any chance of condensation on one element or other of the eyepiece? Or dried condensation which has left a residue?

I noticed over the weekend that as soon as I put in my X2 Barlow everything went milky as though it was fogged up. It was not but there must be residue from one of the many times it has been. I have noticed it before but I don't use it very often so I keep neglecting to clean it.

[AstralTraveller]

Hmmm ... tricky. I assume you tried to tweek the focus but just couldn't get it right in the centre. [At f/5 the curvature of the scope's focal plan means that with many eps the best focus in the centre is different to the best focus at the edge of the fov.] Otherwise, do you have a smudge on the eye lens of your ep? With many short fl eps it's easy to touch the eye lens with your eyeball without noticing. I have that problem with my 5mm ortho but not with the 7mm. And plossls have even shorter eye relief than orthos.

[mental4astro]

Mick, what was the eyepiece you were using? Or was this problem present with all your eyepieces?

Were you able to adjust the focus while Venus was in the centre of the FOV and get a good image?

Without knowing the eyepiece design, it is impossible to offer any help. There are some eyepiece designs that are a very poor optical match with Newtonians. Nothing wrong with the eyepiece or scope, just that the eyepiece would most likely be designed to be an optical match with refractors, SCT's and Maks. There is a big difference between these three and Newtonians. Use the wrong EP in the wrong scope design, and the image quality can be terrible. Put the same EP in the appropriate scope design, and the image is brilliant.

Barlow lenses will also typically exacerbate an optical mismatch. Barlow lenses (by what ever name you want to give them, the term is used generically here), are not optically neutral. They are an active member of the optical train, so by definition WILL influence the way light is transmitted and redirected, and will have an affect on the way light is dealt with by the eyepiece.

Alex.

[mikeyjames]

Hi Paul,
There was definitely quite a bit of condensation around as once the sun went down the temp really dropped. I will keep an eye out for it next time.

Thanks
Mick

[mikeyjames]

Hi David,
I will check out the eyepieces. I hadn't thought of that.

Thanks
Mick

[mikeyjames]

Hi Alex,
I was using either my 40mm GSO Super Plossl or my 32mm Celestron Omni Plossl. I will take a better note this evening and report back.

When in the centre I could still adjust focus but it was never good.

Thanks
Mick

[bojan]

Because you were using low power eyepiece, it could have been an issue with too large exit pupil - if larger than your own eye iris, you could perceive effects like described.
Also it could be the eye relief issue.
Basically, what you saw was the diagonal shadow over image of Venus...
Next time try to move further or closer to eyepiece - the telescope exit pupil should coincide with the iris of your eye, both in size and distance from eyepiece

What is the f number of your mirror?

[mikeyjames]

Hi Bojan
Thanks for the info. I did notice that I can really have a good look around inside the 40mm eyepiece.

The mirror is an F5

Thanks
Mick

[bojan]

That means the power was 25x and your exit pupil was 200/25 = 8mm... not too far from well adapted eye... but the glare of Venus and the fact that it was not very dark when you observed could have caused your iris to contract and block the light from the mirror edges.. however the powermate should have fixed that.

Perhaps it was the dew drop after all...

[mental4astro]

Sorry to be a fuss pot, but I'm curious about your 40mm eyepiece. Being a plossl, it should work well with your Newt. Despite the large exit pupil, even from urban skies the most you should have noticed is the shadow of the secondary mirror. Maybe a chance then of an on axis diffraction pattern, but I have my doubts. I'm happy to here other people's thoughtson this as i just don't recall ever having seen such an aberration in my Newts while even using a 55mm plossl. Being an optical match, plossl with Newts (slower ratios more so), I am surprised at the aberration you've seen.

Mike, are you positive that the 40mm IS a plossl? Just that I am most surprised and intrigued by this aberration you've seen .

Not discouting the possibility of dew, external or intrenal condensation, but even that seems like a curious artifact to see, an on axis de-focusing with a plossl, even at f/5.

[Stonius]

Could be blindingly obvious, but I find I have to be aware of my breathing when viewing through the eyepiece on a cold night. If you breathe out in the wrong direction / too close to the eyepiece it fogs it just like breathing on a mirror.

Just a thought...

Markus

[mikeyjames]

Hi all,
As an update. I think a couple of things were going on.

1) I wasn't getting close enough to the eyepiece. I noticed last night when I went in a bit closer the obstruction disappeared.

2) until last night I had no idea how badly a stray light can affect the view. a neighbour turned a backyard light on and all of a sudden I was looking at all sorts of weird things.

Also, on the night in question there was a big temp drop as soon as the sun went down. I remember picking up my compass and the underside was covered in water droplets.

Cheers
Mick