• Welcome to Orpington Astronomical Society.
 

News:

New version SMF 2.1.4 installed. You may need to clear cookies and login again...

Main Menu

The fault in my stars

Started by JohnH, Feb 14, 2022, 11:47:39

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

JohnH

In the search for better stars I have run an uncalibrated Ha frame through Pixinsight. This is what I have got -



I hope that there is someone who speaks fluent PI who can interpret the result. It seems to me that I am off centre. This would make sense because I had a go at removing a tilt and did not think to re-centre when I collimated.

I also took an image of defocused stars while I was waiting for clouds to disappear (I suggest that those of a nervous disposition should look away now).

This is the image -



I think that the back focus is wrong (too close?). However, the solid black shadows appears to be a problem with the flattener/reducer. I had previously noticed a problem with the optical coating, it looks as though an over thick application of the coating has caused it to run, but I had thought that that would be removed in the flat field. Obviously, I was wrong.

Does my analysis make sense?

Have I missed problems?

Apart from throwing the whole thing away are there any solutions?

Thanks,

John
The world's laziest astroimager.

Carole

I am sure this defocused star image will be very informative to someone who knows what it means.  I can see strauight away that the shapes of the defocused star on the far left and the far right show that the centre of the cross hairs has moved and pretty sure this means your field is not flat.

Have you tried speaking to the retailed who sold you the telescope?

Carole 

RobertM

#2
Hi John,

Usually it's possible to see from the light gradient whether the scope is close or not but the moon skews that by adding yet another gradient.  Having said that the doughnuts in the last image indicate that collimation is well out, there may well be tilt as well but you need to be in focus to check and correct that.

I would suggest that you find a bright star, centre that then check the doughnut shape of the star out of focus.

I have various collimation aids that can help including a Tak collimating scope that's effective on fast Newtonians.

Edit: You should consider using ASTAP for interactive tilt adjustment.

BTW. What scope is this ?

JohnH

#3
Quote from: RobertM on Feb 14, 2022, 12:43:43
Hi John,

Usually it's possible to see from the light gradient whether the scope is close or not but the moon skews that by adding yet another gradient.  Having said that the doughnuts in the last image indicate that collimation is well out, there may well be tilt as well but you need to be in focus to check and correct that.

I would suggest that you find a bright star, centre that then check the doughnut shape of the star out of focus.

I have various collimation aids that can help including a Tak collimating scope that's effective on fast Newtonians.

Edit: You should consider using ASTAP for interactive tilt adjustment.

BTW. What scope is this ?

Sorry,

I should have said (but I thought that I had bored everybody rigid about it already). This is a Sharpstar 15028 HNT.

Because this is a fast Newtonian (f2.8) I was not sure if the central obstruction is dead centre of not.

I am working hard on this because I know it is, theoretically, capable of some fantastic images.

** Updated **

I have downloaded ASTAP (which is available for and works on MacOS!) and will get reading. Thanks for the suggestion.
The world's laziest astroimager.

Roberto

John

Your field shows obvious mis-collimation and potentially tilt.  Before using a camera and ASTAP, why don't you simply use a Cheshire eyepiece and try to centre the mirrors? You could do this on a table, inside and during the day.   I think fiddling with a camera, computer, new software at night, when it's likely to suddenly rain...it's a recipe for disaster and frustration or both!
Get visual collimation done first. Assuming there is no tilt in your focuser, you should at least then be able to test it with the camera for further refinement next time you are out.
Then you can use ASTAP to assess tilt.
My 2 cents

Roberto

Roberto

Also, do you have or can buy or make an artificial star?  I collimated all my refractors on a bench using an artificial star.  My Maksutov is of too long a focal length to reach focus with the star all the way to the end of my street so I have to use a real one but since your reflector is so much faster you could potentially reach focus with it at the end of the garden.

Roberto

RobertM

#6
An artificial star is good if you can reach focus.  An easy one is made using a piece of kitchen foil, a needle and torch.   Put a small piece of foil (enough to be able to fold over the front of the torch) on a hard surface then push a sharp needle into it; that will make a really small artificial star.  Fold the foil over the torch ensuring the hole is aligned with one of the leds.  A better way is with the ball of a ball point pen, about the smallest shiny sphere you can get, mounted on a pin - very tricky to make though !  Or of course you can buy one !

Ah it's one of the Sharpstar Takahashi Epsilon rip-off's ;)  the secondary would almost certainly need to be offset.  I would try to get hold of and read the Tak Epsilon series (E130 and E180 in particular) collimating instructions.  Mark Shelley has one of those and he did have a lot of trouble until he mastered the beast.

Edit: This thread on CN might make a good ASTAP read: https://www.cloudynights.com/topic/809050-tilt-discussion-astap/

Robert

JohnH

Again, thanks.

I have some very fine optical fibre (from memory ~0.25 mm) and am hoping to be able to use that for an artificial star. I can get focus at the bottom of the garden and so, when it is not raining, I will give it a go.

John
The world's laziest astroimager.

Roberto

Quote from: JohnH on Feb 14, 2022, 17:27:53
Again, thanks.

I have some very fine optical fibre (from memory ~0.25 mm) and am hoping to be able to use that for an artificial star. I can get focus at the bottom of the garden and so, when it is not raining, I will give it a go.

John

Good.  No need to take the scope out in the rain; only the artificial star  :lol: ;)

RobertM

Check that the primary and secondary are both spotted as that makes the collimation job much easier.

Robert


JohnH

Quote from: RobertM on Feb 14, 2022, 18:36:18
Check that the primary and secondary are both spotted as that makes the collimation job much easier.

Robert

The primary is spotted (as I understand it, the manufacturer masked the centre when depositing the mirror layer). The secondary is not spotted. I am reluctant to make things worse by taking things off and not putting them back accurately.

John
The world's laziest astroimager.

RobertM

If the secondary isn't spotted then I wouldn't add one but collimation would have been a lot easier if Sharpstar had.

Did you check out the Tak instructions also if sharpstar gave some then it would probably worth cross checking between them. 

Robert

Carole

Good luck with all this John, l hope you manage to get it sorted.  Sometimes l think Newtonians are just too much trouble.  Which was why l went back to refractors.

Carole

JohnH

Hi,

I had another go at collimation this morning (I have amassed an amazing collection of collimating devices). To my eyes hit is now collimated but that will wait for good weather.

In the meantime, this is a picture of my flattener/reducer:



:(

I think I may just have to replace it eventually but I cannot be sure until I can take the 'scope out under the stars.

John
The world's laziest astroimager.

RobertM

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  :cry:

Robert

JohnH

Quote from: RobertM on Feb 15, 2022, 17:32:54
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  :cry:

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
The world's laziest astroimager.

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

Quote from: RobertM on Feb 15, 2022, 19:14:23
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
The world's laziest astroimager.

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

Quote from: RobertM on Feb 17, 2022, 07:02:28
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!

:D

John
The world's laziest astroimager.

MarkS

#24
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

#25
Quote from: MarkS on Feb 20, 2022, 11:33:07
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
The world's laziest astroimager.

MarkS

Quote from: JohnH

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
The world's laziest astroimager.

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

JohnH

Quote from: Roberto on Mar 22, 2022, 12:20:34
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

Thanks Roberto,

That is a loooooonnnnngggg thread!! I have used ASTAP and it suggests that my tilt is now 8%. Perfection is to be desired but I do not know if I will really be able to improve.

I do not use (or know anything about) NINA or Hocus Focus but I will look them up.

I have been trying to fix backfocus, it seems a dark art.

I hope to get some RGB data for Cederblad 214 tonight with the same telescope collimation. I will then have another tinker and see if I can improve anything.

Regards,

John
The world's laziest astroimager.

The Thing

Hi John,

Good  luck. I'm also embarking on the NINA/Hocus Focus (plug-in) trek as I have similar problems with one corner being shown to be way out by ASTAP. I use NINA all the time but not this plug-in, the latest version of which looks very comprehensive for image train problem analysis.

Duncan

JohnH

Quote from: The Thing on Mar 24, 2022, 12:00:35
Hi John,

Good  luck. I'm also embarking on the NINA/Hocus Focus (plug-in) trek as I have similar problems with one corner being shown to be way out by ASTAP. I use NINA all the time but not this plug-in, the latest version of which looks very comprehensive for image train problem analysis.

Duncan

Hi Duncan,

From what I see this is not for me as it seems to be ASCOM based. I am Mac which does not run ASCOM, the equivalent is INDIGO which does not offer these functions.

I wasted Tuesday's imaging (not sure what I did but my Flats all have a mysterious ring which is not visible on the raw lights.

I might be able to have another go tonight and see what results.

Regards,

John
The world's laziest astroimager.

The Thing

If its a ring like a doughnut over the whole image thats another problem I'm trying to get a handle on. Apparently it may be caused by the flats light source colour being different to the subs causing a different Newtons ring pattern on each so the flat doesn't correct it out. Apparently.

Mac

QuoteI am Mac
Thats normally my line.

Can you not run a virtual PC with USB Access?
You might need usb to serial cables

The cpu usage when running the ascomm software on a true PC is negligible, a virtual PC on a mac would run any ascomm software with out any speed issues.

Worth a try.

The real Mac.  :cheesy:

JohnH

Quote from: Mac on Mar 25, 2022, 11:43:10
QuoteI am Mac
Thats normally my line.

Can you not run a virtual PC with USB Access?
You might need usb to serial cables

The cpu usage when running the ascomm software on a true PC is negligible, a virtual PC on a mac would run any ascomm software with out any speed issues.

Worth a try.

The real Mac.  :cheesy:

I can try (I do have emulator software), but Windows and Ascom are an undiscovered continent for me.

Regards,

John
The world's laziest astroimager.

The Thing


FYI YouTube video taking you through Hocus Focus in NINA.

Quote from: The Thing on Mar 24, 2022, 12:00:35
Hi John,

Good  luck. I'm also embarking on the NINA/Hocus Focus (plug-in) trek as I have similar problems with one corner being shown to be way out by ASTAP. I use NINA all the time but not this plug-in, the latest version of which looks very comprehensive for image train problem analysis.

Duncan