Collimation on GSO f/4 newt

[Jason D (Jason)]

Originally Posted by Shiraz

with my scope and sensor, the worst diffraction effects covered about half the field from memory (ie it was significantly compromised) - now I get some issues across much of the field, but less noticeable. Barry's scope is the same brand, so may share a similar design approach (and possibly problems).

If you do not apply the away-from-focuser secondary offset then the incoming light for the central star will shift my around 2-3mm closer to the OTA opening edge (the edge above the focuser). You should not see issues for the central area since you have 10mm margin around your primary mirror.

That's a good point re the placement of the source. Also, it is often not mentioned that your eye must be in the focal plane when using a peephole to position the secondary - for similar reasons.

In theory, the proper placement of the peephole is above the focal plane -- at the point where the reflection of the primary mirror coincides with the edge of the secondary mirror regardless of the location of the focal plane.

Jason

[Shiraz (Ray)]

Originally Posted by Jason D

If you do not apply the away-from-focuser secondary offset then the incoming light for the central star will shift my around 2-3mm closer to the OTA opening edge (the edge above the focuser). You should not see issues for the central area since you have 10mm margin around your primary mirror.

Jason

It's not that clean in practice Jason. My scope has numerous intrusions into the light column (7x casting ridges about 5mm high, 4x bolts/screw blocks for the spider and a screw to hold the safety clip, which clip also intrudes onto the secondary). With my QHY8, only the central 20% or so is free of noticeable input-aperture-related excess diffraction on my 200f4 scope (with the offset secondary). The offset required was about 4.5mm from memory (used formula in Suiter's book). Without it, I guess that the input column would have been about 4mm closer to the edge of the OTA.

You can see similar spider support intrusions and clip screw on the 150mm scope in the front-on image in the link that Barry posted http://stargazerslounge.com/topic/19...ing-newtonian/ That image also shows that the secondary on the 150f4 does not appear to have an offset and it also shows the other potential diffraction source that commonly appears with a small fast scope - the intrusion of the focuser into the light column. The test image in that link shows a bright star on the left hand side with a messy diffraction pattern around it. It looks a bit like I used to get before I added secondary offset (and repositioned the primary and increased the secondary size to get the focuser out of the light column). I came to the conclusion that getting the secondary offset and placement right is mandatory if you want nice looking stars from a small fast Newtonian and found that back projection is the best way to get optimal longitudinal and skew alignment of an offset (and slightly oversize) secondary. It will be very interesting to see what Barry finds with his new scope.

excellent graphic explaining optimal eye placement for secondary alignment - thanks for posting

[Jason D (Jason)]

Sounds like the interior of your OTA is an obstacle course for the photons
You did a good job in explaining why your scope needs the offset. Good luck to you and Barry.
Jason

[Amaranthus (Barry)]

Thanks Ray and Jason. I'll report back here when I get some results!