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Old 12-09-2006, 02:47 AM
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EzyStyles (Eric)
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:( :( :( what's next?

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!! 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.
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  #2  
Old 12-09-2006, 04:31 AM
gbeal
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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
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Old 12-09-2006, 04:53 AM
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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
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Old 12-09-2006, 05:24 AM
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Astroman (Andrew Wall)
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By the looks of the image, I am no expert, but it does look lke coma to me.
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  #5  
Old 12-09-2006, 06:12 AM
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Striker (Tony)
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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.
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  #6  
Old 12-09-2006, 06:30 AM
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h0ughy (David)
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Quote:
Originally Posted by 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.
Agree with Tony on this one, Scott et al do use a coma corrector .
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Old 12-09-2006, 06:36 AM
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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!
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Old 12-09-2006, 07:12 AM
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Garyh
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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
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Old 12-09-2006, 11:24 AM
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ving (David)
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see, now i would have thrown in the towel!
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  #10  
Old 12-09-2006, 01:20 PM
GUIDESTAR
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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.
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  #11  
Old 12-09-2006, 03:57 PM
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EzyStyles (Eric)
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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.
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Old 12-09-2006, 04:11 PM
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janoskiss (Steve H)
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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.
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Old 12-09-2006, 05:25 PM
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Starkler (Geoff)
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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.
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Old 12-09-2006, 05:43 PM
astro_nutt
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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!
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Old 12-09-2006, 06:05 PM
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Starkler (Geoff)
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Quote:
Originally Posted by astro_nutt
Hi EzyStyles,..just a question..how tight are the retaining clips for the primary mirror
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
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Old 12-09-2006, 07:04 PM
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EzyStyles (Eric)
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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.
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Old 12-09-2006, 09:09 PM
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janoskiss (Steve H)
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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.
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Old 12-09-2006, 10:26 PM
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EzyStyles (Eric)
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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
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Old 12-09-2006, 10:50 PM
ausastronomer (John Bambury)
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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
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Old 12-09-2006, 11:24 PM
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richardo (Rich)
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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.
Rich
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