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dslr focus on reflector


alacant

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Hi and thanks.

I ask as that of two other reflectors I tried, one would not focus and the other focussed but was right up against the maximum inward travel; you felt that another mm or so would have been better.  Both were smaller 130mm models but both had camera threads. I was looking at small reflectors as I thought that maybe the spiky star effect would be lessened (?)

Thanks again for your time.

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I can't answer you question about focusing for that particular telescope.

But I can say that you'll get the "spiky" stars in from any reflector that uses a spider to support the secondary, regardless of size. They are caused by the light diffracting around the vanes. It's physics.

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10 hours ago, Pompey Monkey said:

you'll get the "spiky" stars in from any reflector that uses a spider to support the secondary, regardless of size. They are caused by the light diffracting around the vanes. It's physics.

Hi

Is the spike size aperture dependent? Smaller = less false spikes?

TIA

 

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1 hour ago, alacant said:

Hi

Is the spike size aperture dependent? Smaller = less false spikes?

TIA

 

Brighter star/same exposure = bigger spikes.

Fainter objects need longer (combined) exposure time, so more spikes will show on even the relatively fainter stars.

The aperture does not have much bearing on the spikes, although larger scopes can have smaller vanes in relation to their aperture so the spikes can be more "controlled".

However, refractors don't have spikes on the stars, but they do have other issues. Most notable of these is chromatic aberration where the separate colours (chroma) are brought to focus in different planes. This can be corrected, but it comes with the drawback of spending many more Euros! :(

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