The fault in my stars

[JohnH]

How did it get like that ? Are the marks on the surface of the reducer (either sided) or between lenses ?  I have some isopropyl alcohol and high quality optical tissue (recommended by Astrodon for their filters) if you've the need.

Any chance of a picture looking down the Cheshire  ?

Collimating tools... you can never have enough unfortunately 

Robert

I think that the marks are between lenses. I didn't take any pictures through my Cheshire but here is one through an Autocollimator (I have fiddled with the collimation since I took it. I think there is dust on the secondary and the camera focused on that):



John

[RobertM]

The collimation does not look right to me and to me it seems that your secondary mirror could be rotated, it's so difficult to tell without looking at it.  Using your Mk1 eyeball ...  If you take the corrector out then does the secondary mirror look to be on axis with the focused draw tube ?

Robert

[JohnH]

The collimation does not look right to me and to me it seems that your secondary mirror could be rotated, it's so difficult to tell without looking at it.  Using your Mk1 eyeball ...  If you take the corrector out then does the secondary mirror look to be on axis with the focused draw tube ?

Robert

I'll have a go!

John

[RobertM]

Only adjust if visibly out !  Sharpstar recommend that the gap between the secondary mirror holder and secondary spider assembly is 5mm (all the way around) so it shouldn't be to much different than that.

Could also be that you got it spot on and the camera shot made it looked skewed !

[Roberto]

Yes, careful with a camera shot in case it's not reflecting what you are actually seeing through the Cheshire.  If through the latter, the mirrors are concentric (as well as the dark rings), then you are close.  It if looks like picture, you are well out.

Roberto

[Carole]

I am sure you know what you are doing but many people found this guide very useful (including me).

https://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

[Roberto]

That's right!  I remember that guide; it's excellent. 

[RobertM]

I've not seen that but it does seem to cover everything in a simple straightforward way. It looks to be an ideal guide.

Robert

[JohnH]

I've not seen that but it does seem to cover everything in a simple straightforward way. It looks to be an ideal guide.

Robert

Simple and straightforward sounds perfect for me!



John

[MarkS]

I'm sure you already know this but with fast Newtonians you need to ignore any tutorials for standard Newtonians that recommend using out of focus stars.  The secondary mirror on fast Newtonians is deliberately offset quite considerably and therefore the central obstruction in the out-of-focus star is not central either.

You need to use a Cheshire - the type that has crosshairs.  This then forms your primary axis that everything else (primary and secondary) is aligned with. 

Mark

[JohnH]

I'm sure you already know this but with a scope as fast as this you need to ignore any tutorials for standard Newtonians that recommend using out of focus stars.  The secondary mirror on fast Newtonians is deliberately offset quite considerably and therefore the central obstruction in the out-of-focus star is not central either.

Mark

Thanks Mark,

There is a useful article by Simon Todd online in which he demonstrates collimating the same telescope and shows the finished view down the focus tube. As I have said, I have a problem centring the secondary (reflective area) as he shows it and making it round. I have obscured the primary mirror and put contrasting card behind the secondary to assist. Because of the thickness of the secondary there is still an offset on a defocussed star as well as looking down the focus tube.

I am not giving up!

Do you have any comment about the flat field? Am I correct that vignetting, if any, should be symmetrical?

** EDIT:

A really stupid question, how can I tell that a fast Newtonian is actually collimated?

**

Regards,

John

[MarkS]

Do you have any comment about the flat field? Am I correct that vignetting, if any, should be symmetrical?

A really stupid question, how can I tell that a fast Newtonian is actually collimated?

The vignetting will be symmetrical except at the edges of a large sensor where the shadow of the aperture vignetting begins to cut into the shadow of the offset secondary.

Once you have collimated the scope to your satisfaction, the only way to tell that a fast Newtonian is properly collimated is by examining an image of a star field.  In the corners of the image the stars should either look perfect or they should have identical distortions in each corner.  However, it is also possible that your scope is perfectly collimated but the sensor has tilt, possibly because of a tilted adapter or possibly because the sensor is tilted within the camera body.  Fast scopes are very sensitive to tilt and it's sometimes very difficult to tell if the tilt is a collimation issue or not.

The defect in the optical coating on your corrector is pretty bad - especially as its shadow shows up in the image of a defocused star and is probably causing that extra diffraction spike.  I would not be happy with that.

Mark

[JohnH]

With the help of Carole (thanks Carole) I have had another go at collimating the scope.

I have tested on Cederblad 214 in Ha. The attached image is a total of 2 hours exposure with a full moon in the sky.



Processed in PI (Calibrated, Registered, Integrated, Gradient Removal, Deconvolution, Denoise and Masked Stretch).

To my inexperienced eye the stars do seem more round, albeit still too many spikes. The full moon has probably reduced contrast and therefore more stretching than usual has been required. I hope to get some RGB data soon to turn this into a full image.

Regards,

John

[Carole]

Oh wow John.  Whilst still not completely right (extra spikes) that is Sooooo much better than before. 

Well done. 

Carole

[Roberto]

John

That is much better!  Your image still shows tilt which I am sure can be corrected.  If you are user of NINA, there's a new plugin that helps assess tilt and backfocus errors for imagers.  It is called Hocus Focus.  I very much recommend testing it out.  ASTAP also has a tilt analysis routine.   There's a (very long) thread on CloudyNights about both:  https://www.cloudynights.com/topic/809050-tilt-discussion-astap/page-19#entry11789196

Roberto