Looking for advice: what collimation/optical system issues do you see in this image? - Forums

Topic starter

Jul 15, 2022 · Edited: Jul 15, 2022

Just trying to optimize my kit. It's an 8" newt with a coma corrector, mono camera and cheap LRGB filters.. I think I know some of the issues that I need to fix, but I don't want to start dabbling with my optical train if those of you with more experience than I can point me in the right direction.

Here's an almost (80%) full-res verion:

M51

Dan Watt

Jul 15, 2022

That's a lot of weight and focal length for a HEQ5. It's always going to be an uphill battle if you keep that combo.

As far as the stars go, it looks a bit of a mix of miscollimation and a lot of diffraction from the focuser tube. When imaging I'm guessing your focuser is racked all the way in and protrudes into the light path? That looks like whats giving you part of the messy star spike diffration pattern.

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Andy Wray

Topic starter

Jul 15, 2022

Dan Watt:
That's a lot of weight and focal length for a HEQ5. It's always going to be an uphill battle if you keep that combo.

As far as the stars go, it looks a bit of a mix of miscollimation and a lot of diffraction from the focuser tube. When imaging I'm guessing your focuser is racked all the way in and protrudes into the light path? That looks like whats giving you part of the messy star spike diffration pattern.

Thanks for that! I think you are spot on. I'm not so worried about the mount right now as having replaced its bearings and moved to an OAG it's coping OK. You are absolutely right that my focusser is racked almost right in and so its tube is protruding well into the OTA. I'm not sure what I can do about that, short of hacksawing it off (thoughts?). Also, I'm pretty sure my secondary is tilted, so will try and address that after this imaging session.

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Turix

Jul 15, 2022

Andy Wray:
Dan Watt:
That's a lot of weight and focal length for a HEQ5. It's always going to be an uphill battle if you keep that combo.

As far as the stars go, it looks a bit of a mix of miscollimation and a lot of diffraction from the focuser tube. When imaging I'm guessing your focuser is racked all the way in and protrudes into the light path? That looks like whats giving you part of the messy star spike diffration pattern.

Thanks for that! I think you are spot on. I'm not so worried about the mount right now as having replaced its bearings and moved to an OAG it's coping OK. You are absolutely right that my focusser is racked almost right in and so its tube is protruding well into the OTA. I'm not sure what I can do about that, short of hacksawing it off (thoughts?). Also, I'm pretty sure my secondary is tilted, so will try and address that after this imaging session.

The other option is it could also be the primary mirror clips, I had a similar issue but was able to resolve it by placing a circular mask over the edge of the primary to hide the clips (at the cost of ~0.5" of aperture mind you). You can see the difference in stars between my LDN 881 and Cocoon Nebula images, adding a mask really cleaned them up.

You can either DIY it as I know a number of people have done or there are some products on the market for that purpose too (alebit rather overpriced... but what isn't), for example:

https://shop.deepskydad.com/product/newton-reflector-mirror-aperture-mask/
https://www.firstlightoptics.com/misc/wega_mc_sw_200.html

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Dan Watt

Jul 16, 2022

The other option is it could also be the primary mirror clips,

Those don't help but will have a much smaller effect than a big 2" focuser tube protruding into 30% of the light path. That said using a aperture mask is still a great idea. Every screw, nut, etc that protrudes into the light path will introduce unwanted diffraction patterns, so eliminating these will go a long way towards having cleaner stars.

As far as modding the scope so the focuser doesn't protrude, there are two options. Either replace the stock focuser with one that has a much smaller drawtube, or saw off the back of the tube and drill new holes for the mirror cell so it sits an inch or two further up (whatever it takes to offset the focuser intrusion). Chances are you'll have to do both methods to get optimal results.

Keep in mind that if you use the scope for visual you will need an extension tube for the eyepiece to reach focus.

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DanRossi

Jul 16, 2022

Overall, this is a nice image. I just bought a newtonian (I'm used to refractors) and I'd be happy with this right out of the gate. I do think you focuser tube is in the OTA, so you'll have to figure out spacing scenarios with your coma corrector to rack the focuser out. Otherwise I'll let the more experienced newtonian crowd provide feedback and I'll be interested in their feedback as well. Nice work!

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Rodd Dryfoos

Jul 16, 2022

I think the stars look very good. The only one with messy difraction spikes is the big one. Guiding probably was a bit messy, the stars are not completely round. But that is only noticeable when zooming in. It’s an image of m51, not stars. I don’t think there is a collimating issue at all. Look at the clarity and details in m51. Very find details. That suggests collimation is good. Even the palette looks good— perhaps a touch greenish, but that is an easy fix.

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