Bad collimation or defect on Apertura 8" RC

To check the collimation you always have to shoot the stars out of focus. There you can see the state of the mirrors better.

I also picked up the RC8 carbonstar. I do have pulled stars in the top two corners as well but I’m pretty sure it’s collimation on my part. It’s also my first RC coming from newts and learning about what it wants. Using an OCAL 3.0 to get close then fine tuning with defocused stars.

below is one of my screenshots through the ASIAIR with poor seeing conditions.

Ethan Pearson:
I also picked up the RC8 carbonstar. I do have pulled stars in the top two corners as well but I’m pretty sure it’s collimation on my part. It’s also my first RC coming from newts and learning about what it wants. Using an OCAL 3.0 to get close then fine tuning with defocused stars.

below is one of my screenshots through the ASIAIR with poor seeing conditions.

Thank you. Your picture looks similar to mine. I ordered the collimation focuser plate  so  I can do what you are describing

Rodrigo Roesch:
What is not clear to me is what to see in an out of focus star in this telescope.  Assuming the star is in the middle of the field of view, if the secondary shadow is not concentric to the white disc of the star, does it mean the secondary needs to be collimated? I have seen two videos: One tells to move the secondary to center the shadow in the middle of the bright disk, the other video tells to move the primary to center the shadow in the middle of the bright disk. What is the correct way to do it?

Thank you

Neither. Centering the secondary against the primary with respect to the image plane (e.g., the Cheshire crosspiece) should done ahead of tilt correction which is what you do either using a tilt plate.

Freddy Meiresonne (Fmeireso)To refer to the first post : triangled shaped stars mean pinched optics. I noticed i easily can get triangled shaped stars by just tighten the screws of the sec a tad too much. From the moment you feel a little resistance do not screw it any further. The word 'tightening' is almost an overwhelming word for my screws on my sec.

Thank you, I most have tightened too much when I collimated the secondary

Well I went ahead and bought the Apertura rack and pinion focuser from highpointscientific, hopefully it will actually fix my focuser droop with my heavy imaging train.

Someday I'll learn how to collimate this thing, not today, but someday. Everything looked great inside, not so much now. I thought my defocused issues were just thermal since the scope had only been outside about 30 minutes when I checked, coma and astigmatism looked pretty good though. Now I have these ugly looking star shapes. I do like testing just how much blurX can fix. 

I cropped off the guiding graph, but I'm at 0.4 to 0.5 tonight so I doubt that's causing my issue. 

Tony Gondola:
That really looks like guiding error to me. At 1800mm 0.5" RMS might not be good enough, especially if the delta error is high between RA and DEC. I shoot at 900mm and can certianly see the effect if guiding gets much above 0.6" RMS.

It's with the reducer at 1100mm, but the stars look the same blobby and spiky in the sharpcap live view at 1 second exposure length. I really think it's just my collimation, plus ASTAP is showing 23% severe tilt, that's probably not helping either. I think I'm just going to start over once I get the new focuser, maybe try it one night without the tilt plate at all.

At least blurX can compensate for my failure. This is correct only mode. 
​​​​
​

Freddy Meiresonne:
That still looks liked pinched optics to me...typically triangled shapes stars....

So I just checked it and again the secondary is a good bit off center even though I checked it inside just before taking it outside to put on the mount. Something is happening in the few minutes between those two points. Maybe it's the temperature change from 70f to 40f, maybe it's the way it clamps down, I don't know. I do know that from now on I guess I'll do the collimation after I clamp it down on the mount. 

It's a pretty significant shift, from dead center to black dot below the first ring looking through the cheshire (RCKOLLI), takes about a half turn on the secondary screw to get it back, crazy for just moving the scope from inside to outside. I'm sure it's my fault whatever is happening. 

Pinched optics could still be possible, I'll double check my primary screws. 
​​​​​

Well, I've confirmed my indoor method does not work whatsoever for collimation, at least not with the camera attached. After collimating with my TS RCKOLLI and putting the camera on, I decided to check with the DSI method. It was very much way off, secondary and primary. After about 20 minutes of work with the DSI method, I'm really close and nearly all my star issues are gone. A 5 minute exposure from my blue filter from just now.

Someday I'll learn how to collimate this thing, not today, but someday. Everything looked great inside, not so much now. I thought my defocused issues were just thermal since the scope had only been outside about 30 minutes when I checked, coma and astigmatism looked pretty good though. Now I have these ugly looking star shapes. I do like testing just how much blurX can fix. 

I cropped off the guiding graph, but I'm at 0.4 to 0.5 tonight so I doubt that's causing my issue. 

I got this kind of star shape with my RC10 when the polar alignment is not perfectly ok ;)

Also I am using ocal to collimate the RCs (I have 10,8 and 6')  it is working very whell but you also need to remove the primary baffle to have clear view of the spider and all the circle  inside. Then just follow the tutorial ocal has done for collimation of a RC6  because it works for all size of tube.  The only thing I do more is to adjust the tilt ring to center the black dot that remain not centered in the RC6 tutorial from ocal. After that I have nothing to touch  , the residual tilt is less than 9%.

The picture below show a residual tilt before I adjust the tilt ring, the spider is not perfectly aligned 


at the end you have something like this