RC 16" collimation

[Robindavid]

I All,

 

I have spent several months in trying to collimate my RC16" TS but there are strange things hapenning.

Is it normal that in a Ritchey Chretien telescope the outfocus and infocus image of stars are elongated and perpendicular to each other?

I can make good images FWMH between 1.5 and 2 arcsec but as soon as I am slightly out of focus the stars are elongated.

Here is a picture of the elongated stars outfocus and a picture of visually aligned spider vanes;

 

Robin

[vlaiv]

Should be normal - but only far from optical axis - it's signature of astigmatism (inherent in RC design). Depending on your sensor size it should be noticeable in the corners, but not near the center.

[andrew s]

I think you will find is that what you are seeing is due to astigmatism. It is common to see this with an RC at the edge of the field even when in focus https://starizona.com/acb/basics/equip_telescopes_ritchey.aspx . From this it may be the collimation and or primary/secondary spacing are out or the mirror set is astigmatic.

Sorry but I don't have any practical advice.

Regards Andrew

[Robindavid]

Thank you for the answers. These elongated stars are in the center. That's why I don't understand. I think the primary/secondary spacing has an influence on the spherical aberration but not on the astigmatism because the error would be on the axis of the mirrors. But I may be wrong.

[vlaiv]

Not sure about this, but I think that spherical would be a problem with mirror distance / spacing if they are collimated properly. If not, wrong spacing can amplify astigmatism which is caused by miscollimation.

Don't know if this is going to be much of a help, but I did collimation of my RC8" - visually. It only needed collimation of secondary (quite a bit) - but I managed to do it under 15 mins. I collimated on in focus star image by inspecting airy disk / first ring - until it was more/less perfectly concentric. Still haven't tried it on larger format sensor (4/3 is hopefully coming soon), but for 9mm circle it's perfect.

[Peter Drew]

I would think that the centres of revolution of the ellipsoids would need to be accurately coincident, any lateral displacement could introduce anomalies. Triplet objectives for instance are very sensitive such errors. Not sure how you would check for this.

[Robindavid]

I will try to check the focal length after a plate solving so I will see if the focal length calculated is in the specification of the supplier.

Concerning lateral displacement I can see there is some play for the pulling screws of the primary but only one mm. I will try to move slightly the primary to see any difference.

 

Thanks both for you comments.

[tony210]

Your picture does not show perfect collimation as the mirror outline is not central in the view and will affect star shape - it is fiddly- I have a 12 inch RCT and use a Tak collimation scope but laser collimators out there are good- also beware focuser not being at 90 degrees to optical axis always a worry with these scopes and throws off everything - best wishes Tony

[Robindavid]

Hi All,

I think my problem is solved. As Andrew suggested I checked the focal length and it was 3275 instead of 3250. I could increase the distance between the mirrors by screwing the supporting screw of the secondary. I estimate that I moved the secondary some 5 mm. The results was a decrease of the focal length. After that the collimation took 15 minutes following the DSI method. The stars are now round in the center even when little defocused.

Thank you everybody.

 

[Peter Drew]

Nice one. Good result.  

[sloz1664]

That is a great result. I always feel particularity pleased when  things go right. I am terrified when the time comes to clean the mirror on my Tal 200K.

Steve 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[southup]

On 22/02/2017 at 07:54, Robindavid said:

Hi All,

I think my problem is solved. As Andrew suggested I checked the focal length and it was 3275 instead of 3250. I could increase the distance between the mirrors by screwing the supporting screw of the secondary. I estimate that I moved the secondary some 5 mm. The results was a decrease of the focal length. After that the collimation took 15 minutes following the DSI method. The stars are now round in the center even when little defocused.

Thank you everybody.

 

Hey Robin David,

The other day I recollimated the Focuser with the Takahashi collimator being threaded from 117mm to 36mm (takahashi thread).Much better results Than The cliplock ,howiclatta combination. By the look of you image your focuser is out.This has to be on axis before anything else is done.I'm still seeking the perfect image out my 16"rc.

Focus should be around 310.9 mm from the backing plate of the cell on a gso  rc16a.You may need to recheck your focal lenght.
hopfully not to painful

Best david

[southup]

1 minute ago, southup said:

Hey Robin David,

The other day I recollimated the Focuser with the Takahashi collimator being threaded from 117mm to 36mm (takahashi thread).Much better results Than The cliplock ,howiclatta combination. By the look of you image your focuser is out.This has to be on axis before anything else is done.I'm still seeking the perfect image out my 16"rc.

Focus should be around 310.9 mm from the backing plate of the cell on a gso  rc16a.You may need to recheck your focal lenght.
hopfully not to painful

Best david

Move your primary before touching the secondary.1/4 to a 1/3 of a turn in on the collimation bolts should do it.