Collimation- grateful for some advice

[acey]

I agree the images speak volumes and the collimation issue is probably not worth further worry.

Ourt of interest, which "myths" do you debate?

[TopHouse]

Ohhhh, I'm going to have to draw a diagram now, gimme 5 lol

[TopHouse]

This one though, pic coming shortly

myth: You have to square the focuser very accurately

I'm not quite sure of even what "square" is supposed to mean - likely it means set perpendicular to the tube, or possibly to the optical axis - or both, always assuming you have made them coincide. There is nothing wrong with doing it, of course, but the secondary is optically flat, and the angle of reflection is not critical. Most secondaries are made to look circular when tilted 45 degrees (to reflect 90 degrees), but if the angle deviates from this by a few degrees, the only consequence is that the secondary will appear slightly elliptic - it won't affect the image.

[TopHouse]

OK, firstly, the author says it's a myth but then admits he doesn't know what square is supposed to mean, how can you discredit something if you don't know what it is?

Anyway, in the pic below, the red dashes represent the sweet spot, mirror angled right and focuser squared.

The dark green, is if the focus tube was moved to the left, in other words it would miss a 'lump' of light.

The pale green is if the mirror wasn't angled just right, again, not all the light is going up the tube.

The first rule of reflectors, "aperture is king" meaning you need to gather as much light as possible, now you then need to focus as much of that light as possible into your eye.

Everybodys done this on a summers day, the sun is out, there's a 'sun fairy' on the wall, your watches reflection. You move your wrist a little and that point of light goes across the room. Draw an imaginary disk on the wall the same size as your watch, now don't move and you are completely lighting that whole disk, move only a little and you are no longer illuminating the full disk. Keep still, and move the disk a little, you are no longer illuminating the whole of the disk.

Now, go one step further, in true blue peter fashion, instead of an imaginary disk, replace it with a piece of toilet roll tube, now light the wall through the tube, be at just not quite the right angle and instead your sun fairy is lighting up the sides of the tube, a little spilling out over the sides? You have to get all your angles precise, stand in the right spot, andgle your wrist exactly right until you are sending your sun fairy squarly down the tube and illuminating the wall.

[astromerlin]

Hi Guys, Sorry to cause some debate here. I have been imaging for exactly a year to this month, all my images have been like the first pic in the above post. Firstly I fitted 3 fans at the end of the OTA to aid cool down quickly. I then persevered with it thinking perhaps it was my technique with the webcam and seeing conditions or somthing else.

I recently turned to collimation to see if I could improve it. I have read nearly all the info I have found on the forum and many others, all suggest that what I should see when I look down the focuser with a cheshire is as in Jason D's above pic that is marked "Correct". However what I saw was different hence the plee for help and advice.

When I took the images of Saturn I waited for the tube currents to cease, I understand this to be visualised by slow moving eddys in the image of an off focused star. I also realise that the seeing is unpredictable but gets better in the early hours.

Both images of Saturn were taken at around 2-3am in the morning when things had settled down. The settings of the webcam were approximately the same.

The thing I still dont really understand is, I have followed all the standard collimation info and also used TopHouse's method of aligning the focuser and the secondary to get the image down the focuser as in Jason D's above pic that is marked "Correct" except I could see the "cresent anomaly.

The only way I could remove the cresent was to move the secondary nearer the spider and tilt the secondary away from the focuser to get the edge of the primary reflection centered, it still isnt centered exactly I can almost see the edge of a mirror clip on the side away from the secondary holder stem but if I adjust more the cresent problem shows it's ugly head again.

Seeing that the next few days are going to be wet I'll have plenty of time to recheck the adjustments.

[TopHouse]

@ Astro, your pic in post #36 looks spot on to me and to me is what you should be seeing.

[acey]

TopHouse, your diagram agrees with what Carlin says, which is that if things aren't exactly "square" (or, equivalently, if the secondary is slightly rotated), then you see an elliptical secondary and lose a little light. And obviously we don't want to lose any light if we don't have to, so we straighten things until we see a perfectly circular secondary. But the point is that it doesn't affect the image, other than possibly dimming it by a fraction of a per cent. The scope is not out of collimation, star test will show the little donuts it ought to.

Also worth remembering that losing light is not always necessarily a bad thing per se. It is not the case, in the real world of less than perfect telescopes, that "aperture rules". Masking the edge of a bad primary or introducing some vignetting with a focusser baffle can give better views.

[TopHouse]

The picture I got through my focuser and primary is really interesting, did you see the one that appears to show the 3 ccd's in the camera, now THAT is aligned lol, will post it again here, I'm not 100% sure what i caught, I'm unsure if the silver ring is the amount of secondary thats bigger than the camcorder lens or if it's the primary, but I think it's a good guess that the 3 colours are the CCD's :-

[Doc]

Looking at the three CCD's is it my eyes or are they not slightly off centre. If the mirrors were perfectly aligned then why are the three ccd's slightly off centre

[TopHouse]

Because to make this happen i was shining a torch down the tube which was off side due to shining around secondary.

That's my theory anyway

I think that's what the bright bit is on the left, torch glare

[Jason D]

First, the author of the included links is Nils Olof Carlin – a renowned and well-respected expert in collimation. He is the inventor of the barlowed laser technique.

John, it is not clear to me why you are fixated on that crescent. As I have explained, that crescent is expected and a good indication that you are aligning your secondary mirror correctly. Eliminating it will only make things worse.

Comparing two images taken in different nights could be misleading. Maybe seeing was better; maybe thermal stability was better, maybe the collimation was better. Do not be hasty to draw conclusions from comparing these images.

Furthermore, your secondary mirror seems to have been mounted with the proper offset away from the focuser. I can tell from the spider vanes relative location to the secondary shadow. Do not attempt to move your secondary mirror further away from the focuser.

TopHouse, I am afraid your diagram and explanation is inconsistent with the established collimation theories. Light received at the focuser end from stars is a cone – not a cylinder. The geometry of aligning the secondary mirror with respect to a cone is different from that of a cylinder.

I would like to make few more points:

1- Proper secondary mirror alignment under the focuser is meant to distribute illumination symmetrically around the FOV. It the secondary mirror is slightly oval or off-center then its impact is negligible visually. Bear in mind that a properly sized rectangular secondary mirror will work as well as an elliptical one. The fact a rectangular secondary mirror would look squared from the focuser end will not make stars look squired.

2- Squaring the focuser “precisely” is not critical. Squaring it “reasonably” is adequate. Collimation is about optical alignment – not mechanical alignment. Even if the secondary mirror and the focuser are little off, following the proper collimation protocol will align the optics. Getting the mechanical and optical alignment to agree is only desirable if the scope has a “GOTO” , “DSC”, or tight OTA/UTA opening.

3- The most critical alignment is the primary mirror alignment. It is not the secondary mirror alignment under the focuser. “Coma” (image sharpness) is strongly correlated to how well the primary mirror is aligned. Typically, misaligned secondary mirror will not introduce “coma” or impact image resolution.

4- The secondary mirror does not have to be at exactly 45 degree and the primary/focuser axes do not have to coincide at exactly 90 degree angle. Following the proper collimation protocol will set the optimal angles automatically.

Jason

[astromerlin]

John, it is not clear to me why you are fixated on that crescent. As I have explained, that crescent is expected and a good indication that you are aligning your secondary mirror correctly. Eliminating it will only make things worse.

Comparing two images taken in different nights could be misleading. Maybe seeing was better; maybe thermal stability was better, maybe the collimation was better. Do not be hasty to draw conclusions from comparing these images.

Furthermore, your secondary mirror seems to have been mounted with the proper offset away from the focuser. I can tell from the spider vanes relative location to the secondary shadow. Do not attempt to move your secondary mirror further away from the focuser.

Jason

Hi, To be sure I dont have enough experience to agree or disagree with your explainations. What I do know is that for the past year, every nights imaging has produced fuzzy out of focus details. When I imaged planets the image seemed like there was a luminous fog slightly in front of the planet obscuring the detail, thats the only way I can explain it.

When I first checked the collimation using a cheshire every thing looked spot on as per all the drawings and images I have seen on the web but with the "cresent" of the focuser showing. I have yet to see another picture or drawing showing that some reflector OTA's are like this.

The image below is as the OTA was set up when taking my first good image of Saturn. I cannot describe my excitment when I took the image and the relief that I'm finally getting somewhere.

[TopHouse]

I have to say I agree with jason re the crescent and to me it should be there, since you have a webcam, how about centering your secondary with the webcam, much the same way as the camcorder method? I wish you lived near me because I'd happily have a go at it and set it up like mine, have also just checked it with a barlow collimation and it's absolutely spot on, and this cloud is driving me nuts!

If you want guiding through camera centering that secondary I'm around most of the time.

[TopHouse]

Also, I have the crescent, doing the stealth job on the scope has made a HUGE difference, and it's now hard to get good photos down the focusser even shining a torch in, but here's mine and as far as every test I've done, collimation is PERFECT. See how my crescent is black? That's because i blackboard painted the sides and back of my secondary. the crescent is actually that rough silvery edge of your secondary nearest the open end.

[TopHouse]

The next stuff is all guesswork but here goes. Your secondary is at 45 degrees (ish) if you move your primary back towards the open end of your tube, because it's at 45 degrees, you are actually moving the reflecting surface center AWAY from your focuser. SO, I 'think' you are also then changing the focal length, and doing so in 2 planes, both between the primary and secondary, AND between the secondary and focuser. SO, 'could' the lack of clarity in previous photos be something related to focal length?

[TopHouse]

Thinking further, again all guesswork and very open to input from others, the change in focal length between secondary and focuser, created by moving the secondary back is irrelevant since this would be corrected by winding the focuser. SO, this only leaves the focal point between secondary and primary, does that make a difference? i don't know.

[TopHouse]

Just had a silly thought but you never know, there's not an area of damage on your secondary at about the middle is there? by damage i mean, a scratch or marks of any sort, because if there is, then moving the mirror you would be using a different 'bit' of the mirror.

[astromerlin]

Also, I have the crescent, doing the stealth job on the scope has mad a HUGE difference, and it's now hard to get good photos down the focusser even shining a torch in, but here's mine and as far as every test I've done, collimation is PERFECT. See how my crescent is black? That's because i blackboard painted the sides and back of my secondary. the crescent is actually that rough silvery edge of your secondary nearest the open end.

Hi TopHouse, The "cresent" I was referring to is not the rough part of the secondary mirror, I also have blacked mine recently. The "cresent" that I'm trying to describe is a reflection in the secondary mirror. It showed on the opposite side to the rough bit of the secondary. When I loosened the focuser from the OTA tube during my feeble attempts to discover the problem I noticed that the cresent increased in width (showed more of it). Sliding the focuser to and fro I realised that what I was seeing was the left edge of the focuser in the reflection. I then moved the focuser to reduce the reflection of it (slid it towards the primary) this however did not remove the reflection completely so i've moved the secondary towards the spider and tilted the mirror so that the "cresent" has just disappeared.

Everything was centered in the cheshire once again all but the left edge of the primary as in my recent pic in post #62 I realise that it is still not perfectly collimated so still work to do.

You can see the "cresent" reflection in the image in post #35

Slight plee for help!! If anyone has a Skywatcher 250 OTA (the blue one) and can tell me if they have the "cresent" feature too then please put me out of my misery.

Still smiling that I've managed a half decent Saturn though.

[astromerlin]

Just had a silly thought but you never know, there's not an area of damage on your secondary at about the middle is there? by damage i mean, a scratch or marks of any sort, because if there is, then moving the mirror you would be using a different 'bit' of the mirror.

Ooo, several posts whilst i'm writing a reply. I've inspected the secondary closely and whilst there is a fair bit of cloudy spotting mostly round the edges and a black speck slightly off from the center, there are no scratches that I can see.

Being worried that I still might be doing things the wrong way I went out and measured the gap between the spider and the secondary mount. The gap is 5.05mm and the 3 setscrews are nearly all even.

I'm not sure if this info will help any but there it is.

[moocher]

Surely its worth investing in a Laser Collimator?

Alan:)

[acey]

Slight plee for help!! If anyone has a Skywatcher 250 OTA (the blue one) and can tell me if they have the "cresent" feature too then please put me out of my misery.

I have a Skywatcher Flextube 300P and I have the crescent feature. As Jason D has said in a very informative post, you have got absolutely nothing to worry about. If you follow the link I gave to Carlin's Sky and Telescope article it will tell you all you need to know about using your Cheshire to get things right.

The little spots on your secondary are dewspots and dust. They have absolutely no visible effect on image quality. Forget about them for a year or two until you decide to clean your mirrors.

[astromerlin]

Just had a silly thought but you never know, there's not an area of damage on your secondary at about the middle is there? by damage i mean, a scratch or marks of any sort, because if there is, then moving the mirror you would be using a different 'bit' of the mirror.

"Oh no there's no scratches" says I.

I decided to start again and set the focuser up as per TopHouses paper/card method, took a picture through the focuser. Doing all this close work required I put my reading glasses on, picked up the secondary with my usual care I give it and as the strong light I was using caught in the reflection there it was one straight scratch going diagonally right across the mirror, my heart sank.

So there it is guys thats got to be the problem after all. What to do now? A new one or get it recoated if thats even possible.

Cant even be bothered to put it all back together now, sulk sulk.

[TopHouse]

It was something that occured to me whilst thinking about your problem, that if your imaging was poor, and shifting secondary position made your imaging good, I just thought what if it's a 'bad' bit on the mirror.

I would have thought obtaining a fresh one wouldn't be difficult, but I'd post a new thread asking the question as there are a few telescope builders on here and they may know a cheap option for getting it recoated. Sorry to hear about it though, my heart would have sank too and I'd have to give the dog a right kicking!

[astromerlin]

Hi TopHouse, I dont have a dog to kick unfortunatly but the other half is off to her Krav Maga to beat up a few big guys for me.

Thanks for all the help and advice though, the handy tips with the camera down the focuser helps alot with getting everything centered.

New thread coming up.

Cheers

John

[TopHouse]

These have a good reputation and apparently do patrick moores

telescope mirror aluminising, silvering, coating, realuminising, resilvering, recoating

Looking at their site it looks like it's only a fiver or so to get it redone.