:( :( :( what's next?

[EzyStyles]

frustrated.. just want to vent it out. bad night twice in a row.. yesterday i can't get my 8" to come into focus. ok so today i decided to go down to bunnings and got myself some longer collimation screws so i can move the primary abit closer to the focuser. got home and start fitting them in with some longer springs as well. Here's the breakdown:

6:00-8:00pm: setup all my equipment etc waiting for the sky to clear. I got focus alrite no problems but my stars from the top right to bottom left are ovals and don't come to focus? middle of the pic to top left seems to be ok. . alrite so i thought it has to be the tilt of the primary mirror to the top left. grabbed my laser collimator and rechecked. battery was flat.

8:00-9:00pm: drove down to bunnings again and got myself some batteries for the collimator. rechecked once again and seems to be ok. took another pic of M8. again the samething with the stars.

9:00pm-11:00pm : took the scope back into the house changed the collimation screws back to the originals and test the scope once again this time with the DSI II. samething. the stars are oval around the edges so it wasnt the collimation screws.

11:00-1:00am: packed up equipment called it a night and took a closer look at the scope. seems like mirrors weren't aligned properly according to my film canister method. after a few twist of the collimation screws from the secondary mirror holder, it had no affect. took secondary mirror out and noticed the screws were actually drilling holes into it!. The holder was made of hollow plastic!! :eyepop: no matter this can be fixed quite easily from a big nut washer.

2:30am drove down to local 24 hours K-Mart. they dont sell big washers on their own. Asked shop assistance if they have any, he suggested car baby seat holder kit has a big washer inside. $10.99 for the kit . Due to desperation i bought it just for the washer lol . Was just about to setup my equipment once again wasn't worth it in the end as it was already 3:15am.

Gee's... i feel like wasted the night.. oh well.. have to wait for another clear melb night.. not too sure when that will be again.

oh , my main question is what is wrong with my scope causing the stars only from the top right to bottom left to be like that? collimation? initially, i thought that might be coma, but coma should be spread evenly around the entire image and stars on the edge should be in focus.

oh its not polar alignment or mount tracking etc. a test pic from my guidescope appears fine.

thanks for listening.

[gbeal]

Wow Eric, what a night. You get top marks for persistence, I would have kicked something, and packed it in way earlier.
If I give you any advice, do these things on a wet/cloudy night, get it all set up, and then just the merest of fine tuning on THE night.
Hard to guess at the problem, but I suspect a combination of slight mis-collimation, (making the stars on one side "better", and coma/field curvature.
In the prvious images you have used the DSI no doubt, but the new toy is the 350D, correct? If so there is a whopping difference in chip size, with all those nasties at the field edge never seen with the smaller DSI chip. Now you have a BIG chip, this shows all those optical snags that you didn't know you had.
I could be wrong though, and there will be plenty to help here.
Gary

[Robby]

You may need a coma corrector, like tornado33 uses. MPCC I think they are called. The bigger 350D chip will show a lot more nasties than the small DSI.
Cheers

[Astroman]

By the looks of the image, I am no expert, but it does look lke coma to me.

[Striker]

Looks like Coma to me.

As mentioned the Baader MPCC will help with this..

I puchased one not long ago to go with my Meade 8" F4 Schmit-Newt.
I havn't tested it nor have I even put a camera on it I just new their recommended for astrophotography with these type of scope.

[h0ughy]

Agree with Tony on this one, Scott et al do use a coma corrector:D .

[Lee]

About the collimation screws.....
How long and what size/thread are they - sounds like a very good idea.... you did very well - is that washer made of titanium or something! :)

[Garyh]

Hi Eric,
Thats coma alright, and having a f/4 you will have plenty of it. I will be getting a Baader MPCC for mine in the future and it would be perfect for your f/4... Also there is some collimation error as well as the coma centre seems more to the left of centre in your image?..Do you rack the focuser in and out when you check it with the laser so that all is dead square on? I didn`t to start with and had similar pics until I got all perfectly centred racked in and right out!!
Hope you sort it out soon!!
Cheers Gary :thumbsup:

[ving]

see, now i would have thrown in the towel! :)

[GUIDESTAR]

Here is what i see based on the sequence of events;

1. Imaged before with a DSI -> No Coma Detected
2. Replaced Screws -> Mirror will tend to go out of collimation
3. Imaged with DSLR Camera using a wider chip -> Coma at side
4. Restored Original Screws
5. Imaged with a DSI -> Coma still present

I used your image as a reference to indicate the problem;

1. Your mirror has a very small "sweet spot" which causes the sides to induce a coma. Sweet spot basically is the part of the mirror where the image is sharpest. To solve this, either you need to replace the mirror (or probably the scope), or get a coma corrector (not sure if this will solve everything because you are using a DSLR with a very big CCD chip and might not cover for the coma adjustment)
2. The sweet spot has moved to an area towards the upper left (marked in cirlce) which indicates that your mirror has moved. An indication that your scope is not collimated. Use a barlowed-collimator to bring the sweet spot back the center field.
3. When you restored the original collimation screws and imaged with the DSI, it imaged the part (marked in square) where coma is still present. To bring it back to its original state, collimate the scope such that the sweet spot is placed at the center of the field.

Hope this helps.

[EzyStyles]

Thanks for your reply guys. I took my scope into Bintel today and got roger to have a good look at it. Got myself also a cheshire collimator. He suggested it might be the focuser sitting on an angle. I did some alignment method and placed a white piece of paper behind the secondary. took secondary mirror and focuser out. measured the radius of the focusing hole and placed a centering dot on the white paper. put back focuser and chuck in the laser. no problems with the tilt of the focuser at all. im still puzzled. I have attached a pic looking down the focuser with both mirrors well collimated. The only part i can see which isn't aligned is a black crescent as pointed by the red arrow. to me , it looks like a part of the secondary mirror. but both mirrors are dead aligned i can assure on that.

here's some pics with the longer collimation screws . i use the 35mm 6mm for lock screws and 40mm 5mm for collimation screws. and the big washer from the baby seat kit to stop and more drilling on the secondary.

[janoskiss]

The common Newt laser collimator will almost certainly not be accurate enough to complete collimation of an f/4 Newt with sufficient precision for your needs. Your secondary mirror looks like it is not properly centred and/or aligned in the focuser either. We cannot see the entire primary in it; only one mirror clip can be seen.

After centring the 2ndry in the focuser, I'd use a Cheshire followed by a star test to properly collimate the scope. Also recheck that secondary does not miss any part of the primary by looking for the 3 mirror clips in defocused star images.

[Starkler]

:scared: Ouchie-wawa! How tight are you doing up that secondary holder?
It looks like your already drilling into the washer too .

What Steve said is all good.
Once you learn to use the cheshire properly all will be good. Im amazed you have gotten by for so long at f4 just using a film canister.

[astro_nutt]

Hi EzyStyles,..just a question..how tight are the retaining clips for the primary mirror?...maybe I'm wrong but it looks as if the bottom of the actual clip isn't sitting flush on the cell..plus the metal plate on top of the clip is at an odd angle!!
PS I do admire your persistance!!
Cheers!

[Starkler]

Well spotted !

Those clips are waaaaaaayyyyyyy too tight. They should barely rest at the mirrors surface and be under zero tension. They look to be clamped down so hard as to distort their shape, not to mention what this is doing for the figure of the mirror.

Don't despair, once your scope is sorted your images will be 1000% better :thumbsup:

[EzyStyles]

great call astro_nut. I did tighten the mirror cell clips abit. but im sure it wasn't 'too' tight though. was kinda scared the whole mirror might flop over! ill have another look into it. Steve, was it hard to collimate my previous F/4? I recalled with the Optex i had similar problems showing the black crescent even though the laser was spot on with both the mirrors. i also had my laser tested for accuracy by Roger at Bintel. seems good.

[janoskiss]

I missed the mirror clips too! OMG! That's going to cause some nasty pinching. The clips should just barely touch the mirror, screws essentially finger tight. I use lock-tite on the screw threads to make sure they stay put w/o being tight.

No probs collimating your old f/4. I use a Cheshire/sighttube and star test to recheck.

In the shop they can collimate the laser only so well. That is probably good enough for medium power visual use at f/6 or slower, but beyond that they are not accurate enough. To collimate the laser itself any better gets very tedious.

Look up the barlowed laser method by Nils Olof Carlin (and published in S&T). It is a good one, and competely insensitive to laser misalignment.

[EzyStyles]

i forgot to mention, the attached pic showing the mirror with the clips , i have redid this and the plastic bit which holds the mirror is even now. i try not to rely on the laser too much steve. hmm might get you to have a look at my scope one day steve :D

[ausastronomer]

Ezy,

I can hear that primary mirror crying in excrutiating pain from here, as I look at those mirror clips about to snap off under the strain.

The clips are only there to stop the mirror falling out, not to hold it in place. You should tighten them so that you can slide a thin sheet of paper between the clip and the mirror.

CS-John B

[richardo]

Hi Eric,
some very good answers to your question been posted.
Judging by your image, using the 350D and your fast F4 system, coma is definately the problem here (go for the Baader MPCC coma corrector) and also, one thing that should be considered, what sort of 'offset' has been put into your secondary mirror, if any??
If there has been no offset put into your secondary, then the light cone when it hits the detectors plane will not be square to one side.
If your colimation looks spot on by eye and by laser colimation, then machanically it's colimated but optically it's not. This is what is showing up on your image, greater elongation to the R/H side.
These things wouldn't show up before with the small chip of the DSI, far less critical, but with the 350D's chip size it's a totally differrent ball game. Heaps more critical by a long shot.
The offset can be put in by adjusting your secondary mirror away from your focuser.... your probably aware of this already, but you never know!

Hope this might help sort your your probs.

All the best.:thumbsup:
Rich

[janoskiss]

Yes x 100. :sad:

Very good advice. Listen to the man, Eric. :prey: Use a bit of Loctite on the threads if you worry about screws coming loose.

I'd be happy to look at your scope. f/4 is very touchy with collimation. I can show you the barlowed laser method of collimating the primary. It's Ezy ;) and precise! :)

[ballaratdragons]

Eric,

I took your pic into PaintShop Pro and place orange circles where everything should be after collimation. All centred.
Then I placed a Green circle where you have currently set your secondry.
Then I placed a Blue circle where your primary reflection is.

As you can see nothing is collimated.

The only thing centred is the reflection of the focuser looking at itself.

My judgement is pinched mirror and collimation waaayyy out.

I hope the diagram helps.

[Don Pensack]

It looks like coma, and at f/4 is almost certainly that. Even a small chip will have a field of view large enough to show it. If the star images in the center appear focused, and the stars at/near the edge appear spread out along radial lines......it's coma.

Collimation: if you do not know how to collimate your laser, have someone help you who does. Collimating a scope with a miscollimated laser results in......miscollimation! I frankly don't like inexpensive point-source lasers. I prefer the "passive" and "repeatable" tools, like the 3 tools from www.catseyecollimation.com These are very accurate and will show you the inaccuracy in your laser (they did my Kendrick Laser. I had to spend an hour collimating the laser before it even came close to agreeing with these tools). If your laser agrees with the passive tools, consider yourself lucky--your laser probably came collimated (as Howie Glatter's always do).

Coma: easily (but not cheaply) fixed with a coma corrector. 3 good ones for photography that I know of--the Baader MPCC, the Lumicon Coma Corrector, and the TeleVue Photographic Paracorr. I'd look for a used one first if your budget is limited. Sometimes people sell them when they convert to a longer f/ratio scope.

Best of luck, and it looks like your image processing is pretty good. You'll be a pro in no time.

[iceman]

This thread has tonnes of information! Well done everyone.

2 thumbs up from me! :2thumbs:

[EzyStyles]

ok... i guess to solve my problem is to get a new scope or a coma corrector. myastroshop sells the Baader one's for $239. thanks ken for your pic showing the inaccuracy and i need to fix my collimation with a cheshire of course :P . thanks guys . it really helped me out.

oh forgot to ask, with a MPCC, that will extend the camera out? i was having problems with infocus and wouldn't the MPCC be an issue?

[AstroJunk]

Don't spend money on a coma corrector just yet - Coma effects are massively exagerated by poor collimation. Have another long hard look once everything is lined up properly.

Fix one thing at a time, and it will all come good!

[EzyStyles]

after playing with the collimation screw, here's a shot from last night. i think it is better.

[Lee]

Nice even coma on both sides now Eric - much better than before though.....

[EzyStyles]

thanks Lee. oh in regards to COMA correctors, i have found this:

http://www.telescopes4kids.com/products/vixen-coma-corrector-2-r200ss-27431.html

Quite cheap but will this do the trick? Also, my focuser just reaching infocus with about 1/2 cm left. Will i need more infocus with a coma corrector?

[ballaratdragons]

Well done Eric,

the Coma looks even all the way around the centre now :thumbsup:

[Starkler]

Im not sure how meaningful it is in this context, but the site states that it was designed for the vixen r200 scope. I believe its an 8" f4 just like yours ;)

[AstroJunk]

Perhaps our resident optical experts fresh from active discussion on such a topic may wish to comment on the differences between visual and photographic coma correctors.

[janoskiss]

No diff in functionality. One has an eyepiece in front of it to deliver light from the (corrected) focal plane to the observer's eye, the other has a sensor/film in the focal plane.

[EzyStyles]

I hope it will work ok. but would anyone know if i will be able to reach in-focus with a coma corrector?

[Don Pensack]

Typical coma correctors do require in-focusing--at least this is true of the Lumicon and the TeleVue.
The lens you've linked seems to be threaded to attach to something (the bottom of the focuser drawtube?), which is interesting. It wouldn't work attached to the bottom of an eyepiece (too close to the eyepiece focal plane), though, even if it could be attached there.
Since the active part of a coma corrector is the lens, there's no reason to have a tube if it isn't necessary.
I believe this is a Ross corrector, so it's intended for photographic use. But, like the Baader MPCC, so long as you pay attention to maintaining the correct distance between the eyepiece's focal plane and the lens, it should be usable visually as well.
The eyepieces most likely to require more in-focusing would be your short focal length ones, which are probably 1-1/4". There are 2"to1-1/4" adapters that drop the eyepiece down 12+mm, so you might be able to achieve extra in-focusing without resorting to a shorter focuser. More the issue is whether you can achieve the correct distance between the camera's focal plane and the lens. It might require experimentation.

[Garyh]

Hi Eric, your last pic is much better maybe a little tweek will get it spot on if you dare that is..:)
With the baader coma corrector will give you a extra 1cm outwards focus so should be ok. Can`t remember if the others change your focal plane postion much but some have to be much father from the focal plane too which would be a problem..
To your picture of the secondary having a dark crescent with your eye,focuser refle being shifted away from the primary in the very centre of your reflections. This is how it should look! That means your secondary is offset correctly to the focuser and mine is the same and you can see pics of the same on this site....
http://www.fpi-protostar.com/collim.htm..
So you are almost there...:thumbsup:
Look forwards to your new post pics...
Gary

[EzyStyles]

thanks for all your informed posts guys. really helped me out. if the coma corrector gives me more outer focus that is exactly what i need. Just ordered the Vixen R200SS from AStro Optical . Should be getting it tomorrow :) i'll post a pic with it on for comparsion. another question how would i mount the coma corrector and will it take filters?

[Roger Davis]

Hi Eric,
The collimation looks a whole lot better than when you had it in the shop. I didn't think to look any closer at the primary retaining clips, glad you've fixed that. But looking at your image the coma is magnificently apparent on only 2/3 of your field. That was proven by the fact that you could only see 2/3 of the primay mirror when you brought it in. You need to drop that secondary towards the primary.
Offset = minor axis of secondary*(Diameter of primary - minor axis of 2ndary)/4*(focal length-[distance of secondary mirror to focal plane)
Should be a couple of millimetres. However, having said that, I think you will find that your secondary mirror is slightly oversized anyway and offset will not be necessary if you have the secondary centred.
At f/4 coma is more than objectionable at 10mm off axis where the image of a star will be smeared over 5 or 6 times the size of the Airy disc. Your sweet spot is only about 5mm or so across. If you wish to take a photo of a larger field then you need to use a coma corrector.

[EzyStyles]

sweet spot only 5mm? nooo! i think i need a new scope. thanks Roger. The Vixen R200SS coma corrector arrived today. But it doesnt fit :( I will also need some Vixen adapter for the thread as well. I thought coma correctors were universal, though wrong. :( I wait for Merlin's one in the mail next week. Hopefully this will solve the issue.

[GUIDESTAR]

I already mentioned that at the earlier part of this thread. This is the first time i saw what a 'sweet spot' looks like after seeing your DSLR widefield image. :-) Anyway, the sweet spot can be fixed in place at the center of your FOV by using the barlowed laser collimator method. I have one and i can make the collimation adjustments in less than 30 seconds. That is the duration of placing the mirror's center spot sticker shadow on the collimator's center marker using the collimation screws.

[Starkler]

What I dont see mentioned often with the barlowed laser is the importance of the secondary alignment. You must use the laser unbarlowed first and point at the exact centre of the primary. Any error here may be doubled at the target after reflecting off the secondary twice.

This will align the optical axis to the focuser axis. The importance of this is that a barlowed laser target is never close to the focal plane, which of course is the place where collimation is most important.

[EzyStyles]

so a coma corrector will make this sweet spot bigger? i don't get the barlow laser method. if i move the primary collimation screws, wouldnt that put my collimation off? i tired putting a laser into a barlow, what i can see is just a big red rectangle.

[GUIDESTAR]

The big red rectangle is only half of the collimation process. The next half is getting the shadow of the center spot marker back to the laser source.

Below is detailed description;

http://gmpexpress.net/~tomhole/blaser.pdf

The COMA corrector basically increases the diameter of the "sweet spot". Up to what extent would depend on the quality of the corrector. At some point it may be not enough to cover the CCD size of your camera. Will wait for your test results.



For Starkler:

Yes you are right. Alignment of the secondary is very necessary. This assumes that the magnified "red rectangle" (as Eric would name it) would hit the center marker of the mirror and the return shadow is right on the middle of the laser source.

[EzyStyles]

thanks guidestar. i don't thik the barlow laser applies for my laser. My laser already has a primary centering screen but thanks anyway.

btw if you are the software developer for guidestar 1.1, what a remarkable software you have created. :)

[janoskiss]

Eric, the barlowed laser method will work with any laser that inserts in the focuser. You need a screen (paper sheet) at the end of the barlow, onto which the mirror projects an image of its centre spot. The screen should have a hole in the centre for the laser light to pass through. You align the mirror centre spot's image on the screen with the hole in the screen by adjusting primary mirror collimation screws.

Sorry if I'm not explaining it properly. It's easier to show than describe.

[EzyStyles]

yep i sure get it. but the barlow laser way will be similar to looking at the laser light reflecting off the mid hole in the laser?

http://home.earthlink.net/~celstark/sitebuildercontent/sitebuilderpictures/lcc.jpg

[janoskiss]

There is an important difference which makes the barlowed laser method much more reliable than using the laser on its own:

It is insensitive to misalignment of the laser in its housing, and the housing in the focuser. You can loosen and jiggle around the laser in the focuser and the back-projection of the centre spot will not move. Its position depends only on collimation of the mirrors.

[EzyStyles]

true .. hmmm might have to give the barlow laser another go. cheers steve.

Just received my Lumicon coma corrector today as the Vixen one didn't fit. thanks merlin. I have to return the vixen one. here's some shot with it in the camera with the modified t-mount + 2" barrel adapter and the baader uhc-s filter. should the filter be the last thing screwed in or the coma corrector? wil test this out on a clear night .. doesnt look like it is coming till the weekend though :(

[Striker]

Thats fine Eric....I have the filter on the end after my Focal reducer...and when I use my Coma corrector on my 8" schmit/newt it will look the same as yours.
Except for the step down ring.