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Reflector star size(s)

ZWO ASIAIR Plus ZWO EAF Reflectors Collimation
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Buyman3375 avatar
Buyman3375 avatar
Hello everyone.

I was wondering what your average star sizes are, or even the best you've ever been able to capture with a Newtonian?
The reason is, because I am wondering at what point should I try to give up on the perfection of my collimation efforts. 

Before I replaced my secondary mirror, I remember seeing my ZWO EAF resulting in focused stars down to 3.5, though I'm
not sure if this is/was in arc seconds or something else - because when "detecting stars", the sizes are smaller and go down
into the 2" arc second sizes. All of this is captured in my ASI AIR plus.

However, my current setup is giving me grief and I have tried to collimate, calibrate and calculate the hell out of my new
secondary mirror, but cannot see to get below the size of 4 in my EAF focussing efforts. I am now wondering if this is due
to the mirror or other issues, but wondering if I should keep trying, since I am close enough to my previous 3.5 size stars.

So back to my initial question: What star sizes are you, or were you able to achieve with you Newts? My refelector is a 
12" Newtonian from OOUK. And right now, the best star size I was able to achieve recently, was a 4.16.

Thanks in advance for your feedback.

Clear skies and good luck with the Eclipse!

Chris
Engaging
andrea tasselli avatar
andrea tasselli avatar
It isn't quite clear what the number refers to. Is it arcseconds or pixels? If the latter then we would need the size of those pixels.

If the former then my best so far is 1.7" with a 12" OOUK f/4.
Buyman3375 avatar
Buyman3375 avatar
andrea tasselli:
It isn't quite clear what the number refers to. Is it arcseconds or pixels? If the latter then we would need the size of those pixels.

If the former then my best so far is 1.7" with a 12" OOUK f/4.

Well, it isn't quite clear to me also. When I'm running my automated focus routine with the EAF in the AsiAir plus, the stars get this number, but it doesn't specify if this is acrseconds. However - when I am looking via the "detecting stars" tab, they show smaller sizes and this time, they have the arc seconds sign next to them. For example, last night I was only able to focus down to 4.16, but the stars showed lower 3" arc seconds. With my previous secondary mirror setup, I got down to 3.5, but never "dected stars" to know what their real arc seconds were back then. 

It appears we have the same scope. And my camera is the ASI2600MC pro. But that hasn't changed to my prior setup. That said,
I am assuming, I still suffer from my mirror exchange. 

Despite the catseye collimator (thanks fo the tip, Andrea), I have not been able to achieve perfect collimation. Yesterday, I had it seemingly perfect collimated. I followed all instructions and had no ghost images. However - the stars were still not satisfyingly sharp and when looking at the out of focus stars, I realized the donut wasn't placed in the center. I was sure everything was alligned perfect - yet, the star disapproved...

Btw - if I would like to collimate via star, how does one go about that? I tried it via focus and video mode on the AsiAir, so I can see real life changes of my twisting primary mirror knobs. But the turns only moved the star (which I slew back), but the donut stayed the same - no matter how aggressive I turned the knobs...

I am getting really frustrated with this collimation issue. Never had this kindof trouble before!!!

Chris
andrea tasselli avatar
andrea tasselli avatar
I think is best to clear out exactly where you are at the moment. Now, if you have PI it is dead easy to get the actual FWHM (FWHMeccentricity script). Let's suppose the 4.2 is in fact pixels. This would yield a FWHM of 2.75 (4.2 x 0.65) which is nothing to sneeze at if your typical seeing is just fair (which holds for most of us). Or you can post on sample raw image somewhere where it can be downloaded so I can do the measurements myself if you don't have PI.

As for collimation in focus: you'd need a high power EP which yields around 300x (so 4 mm) such as a superploessl or similar variants. Bring a bright star at the center of the EP and achieve as good a focus as you can and then just slightly defocus it so that you start to see one ring around the central core. If the ring is entirely symmetric then your are bang-on collimated. You must wait until you catch a moment of good seeing (and hence the star is best placed near the zenith). If the ring around the core is slightly offset then you would need further tweaks. Move the star toward the edge of the FOV (but not quite at the limit) and move it around the EP in a circular fashion. You'll find a place where the offset is less or null. Then, acting on the primary cell collimation knobs very very gently move the star back toward the center of the EP. Repeat until you have a perfectly circular pattern of the first defocused ring w.r.t the core. It is imperative that the amount of defocus to be just slight. Most of the folks starting with collimating a newt commit the error of looking at the image of the exit pupil to check for quality of collimation by massively defocusing a bright star with their camera. This is NOT the way to assess it!
Helpful
Buyman3375 avatar
Buyman3375 avatar
andrea tasselli:
I think is best to clear out exactly where you are at the moment. Now, if you have PI it is dead easy to get the actual FWHM (FWHMeccentricity script). Let's suppose the 4.2 is in fact pixels. This would yield a FWHM of 2.75 (4.2 x 0.65) which is nothing to sneeze at if your typical seeing is just fair (which holds for most of us). Or you can post on sample raw image somewhere where it can be downloaded so I can do the measurements myself if you don't have PI.

As for collimation in focus: you'd need a high power EP which yields around 300x (so 4 mm) such as a superploessl or similar variants. Bring a bright star at the center of the EP and achieve as good a focus as you can and then just slightly defocus it so that you start to see one ring around the central core. If the ring is entirely symmetric then your are bang-on collimated. You must wait until you catch a moment of good seeing (and hence the star is best placed near the zenith). If the ring around the core is slightly offset then you would need further tweaks. Move the star toward the edge of the FOV (but not quite at the limit) and move it around the EP in a circular fashion. You'll find a place where the offset is less or null. Then, acting on the primary cell collimation knobs very very gently move the star back toward the center of the EP. Repeat until you have a perfectly circular pattern of the first defocused ring w.r.t the core. It is imperative that the amount of defocus to be just slight. Most of the folks starting with collimating a newt commit the error of looking at the image of the exit pupil to check for quality of collimation by massively defocusing a bright star with their camera. This is NOT the way to assess it!

Below are my FWHM results from last night. And thanks for the star test explanation!





andrea tasselli avatar
andrea tasselli avatar
If you are shooting @ f/4 that's 3" FWHM at worse which isn't too bad at all.
Buyman3375 avatar
Buyman3375 avatar
andrea tasselli:
If you are shooting @ f/4 that's 3" FWHM at worse which isn't too bad at all.

Well, after hearing I could achieve something like your 1.7", I guess I cannot give up on my collimation frustration quite yet. Thanks!?
andrea tasselli avatar
andrea tasselli avatar
That was my best ever with the 12", normally I'm between 2.5" and 3". And no amount of collimation perfection will make the seeing improve, I can guarantee that!
Oscar avatar
Oscar avatar
Well, this is from a 2x drizzled linear image, from my C8-N, with a 533mc pro, with supposed "better than average", and sometimes "excellent" seeing and transparency that night, according to Astrospheric:

(this is from an RGB image turned to grayscale)


And that's from this image: (only stretched, low quality version)


Which is this image: (Fully processed, with color, BXT applied to stars and non-stellar)


M51 - The Whirlpool Galaxy - Reedited
Buyman3375 avatar
Buyman3375 avatar
andrea tasselli:
That was my best ever with the 12", normally I'm between 2.5" and 3". And no amount of collimation perfection will make the seeing improve, I can guarantee that!

So, I noticed something yesterday, when collimating - again. I was utilizing the catseye, when I started to run out of sunlight (no pun intended), and had to move the scope in another direction - to verify "ghosts" or no ghosts. Before I moved the scope, I was pretty sure that the "ghosts" should be either minimal or gone. But after moving the scope, everything looked a lot worse than before. I used my collimation cap to verify the donuts position and it was out of center. 
After collimating it completely - again, I tested it by moving the scope back into the old position again and the same issue occurred.
Does that mean I have a lose primary mirror? That could explain my fruitless endeavor to collimate the scope. I may have to tighten the clamps of the primary or something else may be causing this...
Engaging
andrea tasselli avatar
andrea tasselli avatar
You can check it by moving the scope and at the same time observing the reflections in the autocollimator, most likely if there is a movement in the primary you'll see it right away. And yes, you might need to tighten the primary but be mindful of over constraining it, as this will cause three-foil coma (pinched optics).