Reflectors?

[dave1978]

Probably a really daft question but.

Do reflectors have a blind spot caused by the secondary mirror? or does the curvature of the primary get around this?

[GazOC]

No, the light loss caused by the secondary is spread across the whole image rather than being a 'black spot'.

[thing]

It's not a blind spot as such but it does obviously cut down on the amount of light getting to the mirror. Most Newt scope specs will tell you what the secondary area is percentage wise compared to the primary.

[Doc]

It will also degrade the contrast slightly.

[Rooster]

In a nutshell... you cannot see it !

[chrisdg1968]

Dave, do not worry about the secondary on a good quality reflector - since getting back into this hobby a couple of years ago I have only had Newts. (not the amphibious type!) and as long as they are collimated and left cool down properly, you will get great views, and much more aperture for your money than other types of scopes.

Chris

[jonbarr87]

I thought the secondary could cause a "blind spot" if you were at extremely low magnification...... ??

[Astronut]

I thought the secondary could cause a "blind spot" if you were at extremely low magnification...... ??

It can - if you're using a too long focal length eyepiece (low mag) that gives an exit pupil diameter quite a bit wider than your pupil diameter so that light from the edge of the light cone doesn't enter your eye, the secondary shadow can be seen floating in the centre of the field of view. It becomes more pronounced if you're not fully dark-adapted and your eye pupil is smaller. The usual advice for reflectors is to avoid using eyepieces that give an exit pupil diameter greater than 7mm (6mm if you're older). I've not seen this effect but it would be interesting to try a too low power eyepiece just to see it

[Andrew*]

It's not a blind spot as such but it does obviously cut down on the amount of light getting to the mirror. Most Newt scope specs will tell you what the secondary area is percentage wise compared to the primary.

Not quite - they never give the percentage area, they give the percentage diameter. I don't know why, because it doesn't help them or us, but that's how they do it.

The area of the secondary compared to the primary is usually negligible. For example, a 23% CO (common amongst newts) is only 5.3% of the primary's area.

Andrew

[GazOC]

The diameter is used because it gives an idea of the amount of low contrast detail lost over an unobstructed system. A percentage of area doesn't.

[Andrew*]

The diameter is used because it gives an idea of the amount of low contrast detail lost over an unobstructed system. A percentage of area doesn't.

Ahah! That sort of makes sense. Thanks Gaz :salute:

Andrew

[AstroDonut]

This was a concern of mine also, but was surprised to see a full image in my scope, i was expecting to have to look around the dull bit in the middle but thanks to the laws of physics and some clever mirror grinding i get a full image

Happy days...

Bri

:moon: