Help with f/4 Newton: what does this star test show?

[radiofm74]

Hello all!

I've recently gotten me a scarred old Vixen R200SS. I brought it to a respected tech in Italy who ran over it and collimated it. He said A-OK. Under stars, however, it has performed horrendously, with even stars at the center of the field not being in good focus, despite 2+ hours of acclimation and using orthoscopic EPs (with and without a coma corrector). I've wasted close to 7 hours of good nights on it and always let the eyepiece frustrated. 

It's my first "fast Newtonian" (I had no issues operating a 6", f/5 before it), so I thought "operator error". However, a first round of comments on CN unanimously questioned whether the mirror was good and/or well collimated and suggested I do a star test. I've done so, and being relatively new to fast Newts I was wondering if more experienced members might help me determine if anything is wrong with the mirror or collimation, and if so what.

I am actually trying to determine whether I should keep the scope or move it on. And even if I move it on, in order to do a honest sale, I think I should first determine whether something is crucially amiss. 

Here's how I've done the test, at home (with a pile of tube extension so the dratted camera could reach focus on such a close object…). I've shot 12 exposures documenting the (artificial) star image from full outside focus to full inside focus going through (what I hope was an) at-focus image. These are very short exposures (20 milliseconds, and 10 milliseconds on the image at focus). I adjusted the histograms a little so the rings would show, and cropped  just so the image would be large enough for your appreciation, trying to respect roughly the proportion from full outside to full inside but cropping more the images close to focus. 
 
Hope it's all clear and I've made no major mistakes. I am a complete newbie as far as f/4 newtonians are concerned so will refrain from offering any thought as to what these images show. I am very much looking forward to reading what you may have to say!!
 
Thanks in advance for any comments!

 

The first is with focuser fully racked out of focus. The next ones move towards focus, then to fully racked in.

From here on I darkened them so rings would still be visible

[radiofm74]

[radiofm74]

Here we get very close to focus, then through. Note how the image is "messy" at focus: not a nice little airy disk but a bit of a rayed blob… that's what I also saw when focusing on bright stars under the night sky. 

[radiofm74]

That's all… again, thanks for sharing your thoughts if any!!

[Backyardscope]

Is the object centred in your fov? if so - the collimation is out, also you might want to check the retaining clips on the mirror. The clips should be just resting on the mirror, then forced down or tight. You will find astigmatism will be apparent.

When dealing with F4 Reflectors, mechanics, optical parts must be all aligned best as possible.

Don't rely on cheap lasers to collimate fast scopes.

A 2" sight tube for the secondary and a Chesire eyepiece for the primary.

PS. If you get fed up with scope, ping me a PM about the scope, thanks.

 

Edited February 29 by Backyardscope

[radiofm74]

14 minutes ago, Backyardscope said:

Is the object centred in your fov? if so - the collimation is out, also you might want to check the retaining clips on the mirror. The clips should be just resting on the mirror, then forced down or tight. You will find astigmatism will be apparent.

When dealing with F4 Reflectors, mechanics, optical parts must be all aligned best as possible.

Don't rely on cheap lasers to collimate fast scopes.

A 2" sight tube for the secondary and a Chesire eyepiece for the primary.

PS. If you get fed up with scope, ping me a PM about the scope, thanks.

 

Thank you so much! Yes, the artificial star is carefully centred in the FoV (I used AsiAir's grid and made sure it was dead center).

Your comment is extremely valuable. I'll try to recollimate and go easy on the retaining clips: I follow astro-baby's methodology and do collimate, in general, with a Cheshire and a collimation cap.

With my older f/5 newt it was much easier since there was little offset. Here, I even have trouble understanding why collimation is off – though I trust your judgment 100%! Given the offset, which I read should be normal, the rings seem pretty concentric. It would probably be good for me to have a reference image of how the star test in a well-collimated f/4 newtonian of this kind should look like… I'll look around!

Be that as it may, I'll try to better the current collimation, and if I throw in the towel you'll be the very first to know! 

Edited February 29 by radiofm74

[Backyardscope]

I forgot to ask, was you using a coma corrector as well, generally the centred the object would be round, even without a corrector and the coma would be further as you go out of field.

The secondary collimation screws should be tight and also check the secondary spider is not loose the secondary screws are good and before i forget again the spring tightness on the secondary holder and on the primary collimation knobs.

[radiofm74]

No coma corrector during the test. Everything was screwed down tight and firm. Perhaps even too tight (clip screws on the back, I need to check as soon as I have the scope under my hands tonight). Thx!

[radiofm74]

Dear all, especially dear Backyardscope,

following your suggestions I recollimated at home, then took it out on the first available clear night yesterday in Lausanne (Bortle 6-ish). Good transparency, pretty bad seeing but I cannot be picky these days (it's been cloudy for the longest time…). 

Star-testing it, I found it was still a little off. After quite a bit of work, and getting to understand much better than before the push/pull system of the R200, I got decent collimation. I suspect that the center spot is off and that that is what made it so difficult for the previous owner to get the scope collimated. I'll check today if I have a minute, and I suppose I'll have to trust star tests rather than Cheshires… 

In use: Mizar was a nice view, though the main stars were still a little bloated; same for Algieba, which I could split in spite of the bad seeing (unthinkable on my first outings). In the Leo Trio, M65 and M66 were evident, and the Hamburger galaxy was clearly visible, intermittently even with direct vision. No mean feat under urban skies!

I brought my beloved little C6 – it's a pretty sharp one – for a comparison. I could confirm that the stars were indeed bloated due to seeing, not to imperfections in the R200, and the superior reach of the 8" duly showed (M65 and M66 were much fainter, and the Hamburger galaxy nowhere to be seen).

Last point, the R200SS sat comfortably and without any shakes on the Advanced Polaris.

All in all, it seems I have a functional, lightweight 8" capable of going with my "trip to dark sites" mounts. Boy am I happy ;D and it's all thanks to your showing me my blatant mistake when collimating my fast newt. Thanks again!!

[Backyardscope]

Mark the tube and primary mirror cell position. Make a circle paper template for the primary circumference, fold it to get centre point and place that gently over the primary mirror, then you will get a pretty accurate idea where is centre point.

You can get a cotton bud, slightly dip it with water (some use nail polish remover) and dip the bud on the centre spot and slowly remove it. Use a corner of soft tissue,  lightly touch the corner to soak up any bubbles up left on the mirror.

Buy a Cats Eye centre spot and using the Cats Eye template stick the centre spot onto the mirror.

Check that the focuser and secondary are all aligned, then re-collimate, then Star test.

Testing and Patience is the key!