The fault in my stars

[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

[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]

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]

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. 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.

[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]

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

[Roberto]

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 

[RobertM]

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

Robert

[JohnH]

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

[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

[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 

Robert