Screwed up my SCT, big time :(

[Minhlead]

I decided to take out my Nexstar 8SE for the cleaning and see a lot of dust inside the tube. I disassemble the corrector plate (CP) to clean inside without marking the original position of the CP. When I put everything back and did a collimation, everything seems fine but a quick search on the internet reveals that the angle of CP relative to the PM is critical to SCT. So without the marking of original position, my CP is now out of sync with PM.

Could anyone please take a look at my out of focus artificial star and see if everything is okay? I attached below both the inside and outside focus of the fresnel rings. Also, a few infocus star test with the airy's disk (since this is the first time I did the infocus fine collimation, I couldn't tell if I'm good.)

Thanks a lot. And sorry for my English, I'm not a native speaker.

[dweller25]

Does the secondary have a serial number on it, if so it should point to the dovetail - by rotating the corrector - assuming you did not move the secondary

Otherwise the collimation looks ok so just enjoy

[Minhlead]

Just now, dweller25 said:

Does the secondary have a serial number on it, if so it should point to the dovetail - by rotating the corrector - assuming you did not move the secondary

Otherwise the collimation looks ok so just enjoy

Thanks for your reply. Well I did remove the secondary mirror out of the CP  so I'm afraid that ship might has sailed

 

 

[dweller25]

Like this

[dweller25]

Might be ok - check under the stars ?

[Minhlead]

Just now, dweller25 said:

Might be ok - check under the stars ?

Yep, Waiting for the sky to clear. Hopefully everything will work out. Thanks again. 

[Minhlead]

11 minutes ago, dweller25 said:

Might be ok - check under the stars ?

Can you look at my infocus pic and tell me if my fine collimation is okay? I did not see the airy disk as described on the internet even with 300x magnification. When I took the photo using my camera i can see a few colored ring and not sure if they are the diffraction rings

[dweller25]

Another thought - Celestron used to put subtle alignment marks on the edge of the corrector, if the scope does not perform under the stars then it may be worth looking for these 

[dweller25]

The infocus pic is a bit overexposed but I think it looks ok - star test will confirm

Good luck

[Minhlead]

7 minutes ago, dweller25 said:

Another thought - Celestron used to put subtle alignment marks on the edge of the corrector, if the scope does not perform under the stars then it may be worth looking for these 

I looked very carefully on the edge of the CP but see no marking at all. What kind of marking to be expected?

[dweller25]

Perhaps a small number engraved on the front face of the glass - although i think this may no longer be done

[Minhlead]

2 minutes ago, dweller25 said:

Perhaps a small number engraved on the front face of the glass - although i think this may no longer be done

Thanks. I'll look again if the star test failed I'll try to look for the marking. My scope is over 10 years old so maybe?

[Ben the Ignorant]

Hi, Minhlead.

Any engraving could be the mark, especially if it looks more or less like an arrow.

About your collimation: you should defocus less, and by the same amount on both sides. See the tiny dot at the center? It is critical to alignment. Defocus till you see only two or three rings around that dot, or maybe four just to approach collimation if conditions don't allow less. Defocusing more makes any collimation look ok when it's not. Your focused patterns are overexposed, find a weaker star or reduce the exposure.

Really, a defocus pattern made on one side of focus is enough, but showing both allows a comparison that tells how much spherical aberration the scope has. I'm a scope review junky so I'm taking your problem as an excuse to have my fix. ?

[LightBucket]

Looks spot on to me with those images...

[Peter Drew]

As a "retired" SCT specialist I would say that the collimation is close but would benefit from further fine adjustment. The first and second images show the central obstruction shadow to be slightly displaced to the 4'oclock position which  causes the slight flaring to the 10'oclock position as shown in the nearer to focus.

There is still debate as to whether the radial orientation of SCT optics is critical, this certainly used to be the case in the early days but my current view is that the individual optical components are now so well made that this is no longer so important. What is important is that the three optical components are matched, such that if one component is broken it is not straightforward to obtain a replacement.

[Minhlead]

5 hours ago, Ben the Ignorant said:

Hi, Minhlead.

Any engraving could be the mark, especially if it looks more or less like an arrow.

About your collimation: you should defocus less, and by the same amount on both sides. See the tiny dot at the center? It is critical to alignment. Defocus till you see only two or three rings around that dot, or maybe four just to approach collimation if conditions don't allow less. Defocusing more makes any collimation look ok when it's not. Your focused patterns are overexposed, find a weaker star or reduce the exposure.

Really, a defocus pattern made on one side of focus is enough, but showing both allows a comparison that tells how much spherical aberration the scope has. I'm a scope review junky so I'm taking your problem as an excuse to have my fix. ?

Thanks, I'll try to defocus less and test again. A quick research reveals that if something wrong with the CP then spherical aberration perfomance will be affected so I think it makes sense to post both side of outfocus here. I'm using my DIY artificial star so seeing condition is not really a problem. Thanks for your reply

[Minhlead]

4 hours ago, Peter Drew said:

As a "retired" SCT specialist I would say that the collimation is close but would benefit from further fine adjustment. The first and second images show the central obstruction shadow to be slightly displaced to the 4'oclock position which  causes the slight flaring to the 10'oclock position as shown in the nearer to focus.

There is still debate as to whether the radial orientation of SCT optics is critical, this certainly used to be the case in the early days but my current view is that the individual optical components are now so well made that this is no longer so important. What is important is that the three optical components are matched, such that if one component is broken it is not straightforward to obtain a replacement.

Yeap, I think I'll get a shorter EP and try again. Deep down I still feel there is something not quite right. Thanks for your advice.

[Minhlead]

13 hours ago, Ben the Ignorant said:

Hi, Minhlead.

Any engraving could be the mark, especially if it looks more or less like an arrow.

About your collimation: you should defocus less, and by the same amount on both sides. See the tiny dot at the center? It is critical to alignment. Defocus till you see only two or three rings around that dot, or maybe four just to approach collimation if conditions don't allow less. Defocusing more makes any collimation look ok when it's not. Your focused patterns are overexposed, find a weaker star or reduce the exposure.

Really, a defocus pattern made on one side of focus is enough, but showing both allows a comparison that tells how much spherical aberration the scope has. I'm a scope review junky so I'm taking your problem as an excuse to have my fix. ?

 

12 hours ago, Peter Drew said:

As a "retired" SCT specialist I would say that the collimation is close but would benefit from further fine adjustment. The first and second images show the central obstruction shadow to be slightly displaced to the 4'oclock position which  causes the slight flaring to the 10'oclock position as shown in the nearer to focus.

There is still debate as to whether the radial orientation of SCT optics is critical, this certainly used to be the case in the early days but my current view is that the individual optical components are now so well made that this is no longer so important. What is important is that the three optical components are matched, such that if one component is broken it is not straightforward to obtain a replacement.

Okay so per @Ben the Ignorant and @Peter Drew suggestion, I tried again with my DIY artificial star (god I wish I make this sooner, tons of time saved and make collimating a whole lot less pain in the butt). This time with less out-of-focus. And god dang you are right, the Fresnel pattern clearly shows a slop on 4 o'clock that wasn't visible before. I corrected it and try the other side of focus. But no matter how hard I try, I could only get 1 side of the focus perfectly in collimation. Eye observation of the Fresnel rings via the EP at 500x magnification looks quite okay but when I whipped out the camera, there's always a side of focus not quite concentric. I guess the mis-alignment of the CP is now in play.

With 500x of magnification, I was able to see the airy disk, the disk looks round and concentric. 

I have to settle with one side of focus perfectly in collimation.

While the other is a bit out.

Being an engineer, I'm not satisfied with eyeballing the result so I whipped out my AutoCad and draw some concentric circle into the photos and it confirm that one side of focus is indeed a bit out of alignment. Not much but it's there. After a few hour of trying and failing to get both side of focus collimated, I called it quit and settle for the above. Will this kind of mis alignment affect my scope image quality much?

A bit curious, on 1 side of focus, the Fresnel rings shows a lot of Chromatic Aberration. What gives? Is it my Barlow ? Because when I remove the Barlow the CA is gone.

And here is my Airy Disk. Any thought? 

[Peter Drew]

That's much better!. I think other factors such as seeing conditions which are beyond your control would affect the image more than any minor collimation issues.  ?

[Ben the Ignorant]

Don't worry, your scope is good and very well collimated. The difference in Fresnel rings brightness between the inside-focus and outside-focus patterns is normal for a Schmidt-Cass. The barely visible residual miscollimation is normal, too. When you overmagnify, all optics show some defects. 400x is the theoretical max for an 8" scope but because of the large secondary and optical tolerances in that price range, 340x to 360x is more realistic.

You're using 500x so some residual appears, that's nothing to care about. And yes, your Barlow can cause chromatic aberration if it's not apochromatic. Most but not all 3-lenses barlows are apochromatic, and not all doublet barlows with ED glass are fully apochromatic either. Check 365astronomy's barlow page for an honest description of which ones are indeed apos.

The second Airy disk is not round but that could be the shape of your homemade artificial star. Is it made of precisely cut fiber optics, or is it just a pinhole? Pinholes can never be strictly round, unless they are brought to a large distance. An irregular star is okay for checking centering, even if it's quite gross, but optical quality checks require a truly round star.

All in all you scope seems quite usable as is.

PS. The faint "radiation-danger" pattern around the central dot seems to indicate pinched optics. Maybe you tightened the plate a little too much. I had the same pattern in my C5; I made an oversize screwdriver to tighten the threaded ring, and within three or four degress only, the radiations came back or left. I was surprised at how sensitive this is.

[Minhlead]

3 hours ago, Peter Drew said:

That's much better!. I think other factors such as seeing conditions which are beyond your control would affect the image more than any minor collimation issues.  ?

Thanks. I hope so. Gearing up for the mars opposition and really bumped out when I see Mars is being covered in dust storm. Hope things will settle in time

[Minhlead]

3 hours ago, Ben the Ignorant said:

Don't worry, your scope is good and very well collimated. The difference in Fresnel rings brightness between the inside-focus and outside-focus patterns is normal for a Schmidt-Cass. The barely visible residual miscollimation is normal, too. When you overmagnify, all optics show some defects. 400x is the theoretical max for an 8" scope but because of the large secondary and optical tolerances in that price range, 340x to 360x is more realistic.

You're using 500x so some residual appears, that's nothing to care about. And yes, your Barlow can cause chromatic aberration if it's not apochromatic. Most but not all 3-lenses barlows are apochromatic, and not all doublet barlows with ED glass are fully apochromatic either. Check 365astronomy's barlow page for an honest description of which ones are indeed apos.

The second Airy disk is not round but that could be the shape of your homemade artificial star. Is it made of precisely cut fiber optics, or is it just a pinhole? Pinholes can never be strictly round, unless they are brought to a large distance. An irregular star is okay for checking centering, even if it's quite gross, but optical quality checks require a truly round star.

All in all you scope seems quite usable as is.

PS. The faint "radiation-danger" pattern around the central dot seems to indicate pinched optics. Maybe you tightened the plate a little too much. I had the same pattern in my C5; I made an oversize screwdriver to tighten the threaded ring, and within three or four degress only, the radiations came back or left. I was surprised at how sensitive this is.

I'm using 500x with my 20mm EP and a 5x barlow. But the photos is taken with a A7s and 5x barlow so I figure the magnification is more likely 350x (I know the magnification in digital imaging is meaningless but the angular equivalent would be the same). 

I'm using a cheapo chinese no-branded 5x barlow so most likely it is to be blamed for the CA. Will invest in a better barlow.

And yes, my DIY AS is a pin hole that I poke on the aluminium foil with a neddle so I think that's why the "star" is not round. Did not expect that since I thought if the pin hole is a "point source" the shape of airy disk will be a circle. 

And also yes, I believe I did over tighten the screws on the rim of my CP. Kinda nervous about having dust fall in so I gave the screw a few good twists and I also afraid it would bend the glass and afect the OTA. Seeing your comment kinda confirm my fear. Thanks a lot. You are really an expert arent you. You do this for a living?

[Ben the Ignorant]

6 hours ago, Minhlead said:

 You do this for a living?

No, cause I didn't charge you anything. ?

[Minhlead]

6 hours ago, Ben the Ignorant said:

No, cause I didn't charge you anything. ?

Touché

[LukeSkywatcher]

Collimation looks good.