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Soft spot in centre of FOV with SCT.


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We had great seeing last night and I was using the 14 inch Meade SCT on Jupiter. It became clear that, right on axis, the view was inferior to that just off axis. There was a very slightly darker, softer central spot. I was using a Moonlite focuser which has a curiously long adapter, putting the star diagonal a long way back. I wondered if this was causing me to see the shadow of the secondary.

I do have a regular Meade 2 inch back to try but I wondered if this was a known issue?

Olly

Edited by ollypenrice
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Olly,

Can't say I ever noticed it on the 12 inch with the Moonlite focuser made for the reducer hich I used for about 3 years or more. Can't double check it for you as someone is coming to buy it this morning. I would give that idea with the SC 2 inch back a try, the Moonlite is somewhat longer than say the Baader which I also had once.

Alan

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I doubt it.

Outgoing rays are collimated and you should see the same image regardless of the distance.

We can't see shadow of the secondary if exit pupil is small enough (we can't even if it is larger, but we can see other issues related to this).

This can be from smudge on the eye lens of the eyepiece - accidental touch that left some gunk on it? That will cause some light scatter and softness.

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3 hours ago, vlaiv said:

I doubt it.

Outgoing rays are collimated and you should see the same image regardless of the distance.

We can't see shadow of the secondary if exit pupil is small enough (we can't even if it is larger, but we can see other issues related to this).

This can be from smudge on the eye lens of the eyepiece - accidental touch that left some gunk on it? That will cause some light scatter and softness.

I tried three eyepieces, all recently cleaned.  Initially I wondered if the Ethos I started out with might have too much lens 'complexity' for the planets so I tried a simpler Meade UWA and then an even simpler orthoscopic, all of similar FL. The soft spot was the same in all cases but the Ethos gave more visual room to avoid it.

Maybe it's my eyes.

Olly

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1 minute ago, ollypenrice said:

I tried three eyepieces, all recently cleaned.  Initially I wondered if the Ethos I started out with might have too much lens 'complexity' for the planets so I tried a simpler Meade UWA and then an even simpler orthoscopic, all of similar FL. The soft spot was the same in all cases but the Ethos gave more visual room to avoid it.

Maybe it's my eyes.

Olly

Did you use barlow?

I experienced once very strange effect when using a barlow. There was mushy / soft spot in the center of the view with sort of "rays" emanating from it - much like those "electricity/lightning" generating balls.

image.png.26dd0d744a0bc9481c75f3f0b63df00d.png

It was definitively due to that barlow as I've seen it in different scopes with different eyepieces. To this day I haven't figured out what might have been the cause of it.

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All obstructed optical telescopes have a tendency to "soften" at the centre hence the permanent obstruction size debate.  It's not usually noticeable unless too lower a magnification is used but it's always there.  As we all get older, characteristics of our eyes start to play a part.     🙂

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2 minutes ago, Peter Drew said:

All obstructed optical telescopes have a tendency to "soften" at the centre hence the permanent obstruction size debate.

Can you explain mechanism for this?

I understand how all obstructed telescopes soften image all over the FOV - it has to do with MTF of telescope - but I'm failing to see why would center of the field be impacted differently?

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Only physiologically  🙂.   I have always noticed that when the eye is withdrawn from the exit pupil position of an obstructed telescope the obstruction is obvious, as the eye is moved nearer to the exit pupil the apparent obstruction increases to the point where it fills the field and seems to disappear although it is still there slightly reducing the famous contrast.  Maybe my eyes (81) are giving the effect of central softness?  I do find these days that looking a little off centre gives me the best view of planetary detail. 

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28 minutes ago, Peter Drew said:

Only physiologically  🙂.   I have always noticed that when the eye is withdrawn from the exit pupil position of an obstructed telescope the obstruction is obvious, as the eye is moved nearer to the exit pupil the apparent obstruction increases to the point where it fills the field and seems to disappear although it is still there slightly reducing the famous contrast.  Maybe my eyes (81) are giving the effect of central softness?  I do find these days that looking a little off centre gives me the best view of planetary detail. 

It sounds like we are seeing the same thing, Peter. I'll stop worrying about it!

Olly

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1 hour ago, vlaiv said:

Did you use barlow?

I experienced once very strange effect when using a barlow. There was mushy / soft spot in the center of the view with sort of "rays" emanating from it - much like those "electricity/lightning" generating balls.

image.png.26dd0d744a0bc9481c75f3f0b63df00d.png

It was definitively due to that barlow as I've seen it in different scopes with different eyepieces. To this day I haven't figured out what might have been the cause of it.

No, the last thing I'd want to use with a 3.5 metre focal length is a Barlow! :grin:

That's a stunning picture...

OPlly

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