jasonharris
sage
Reged: 09/16/06
Posts: 252
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Thanks again all for the comments 
Now I am in the tweaking stage, starting with getting the collimation of the basic bits..
I was never going to put an eyepiece in this thing, just a camera so I made a fairly large adaptor that matched my tak fsq so I could swap the camera easily.. Now I find I really needed to put in my laser and an eyepiece so I spent a bit of the day making something to hold all of these.
So, I come to center the primary baffle and something I feared might happen did. I didnt have enough adjustment because I didn't think the taper of the cone would be as much as it was when I made the part.
Here is a pic of where I can get it to. Does this look acceptable? It doesn't block any of the mirror, which is not installed - neither is the secondary baffle.
If the baffle doesnt block any light from the mirror what side effects can it have if it's not 100% centered. I will modify the parts if I have to but I don't really want to this close to being done!
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
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Hi Jason,
If you gave Mike the correct spacing between the mirrors, the conic constants and the back focus the baffolds should be correct.
In order for the primary baffold to fully be filled with the secondary baffold you will have to get your eye or camera back to the Back Focus Distance.
It appears that you are TOO close to the secondary and thus are seeing the annulas around the Secondary Baffold.
Once everything is correctly alinged, you should not see ANY light other than the reflection of the Secondary.
Are your Baffold Lengths correct and is the spacing between the front edge of the Primary Baffold to the front edge of the Secondary baffold as per Mike's specs?
There is lots that can and may go wrong here.
Check all of the Baffold's measurements and spacings against Mikes specs...it is easy to get just one or two off a bit and then it does not work.  
Best Regards,
Preston
ps...we had to tweek and remake some of the collimation baffold parts a couple of times to get it all correct.
--------------------
A few I enjoy,
and a few more in the works ;<)
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jasonharris
sage
Reged: 09/16/06
Posts: 252
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Hi Preston, when I measured them after I cut them they were within 1mm so the sizes are good.
When I took the picture above it was just for the purpose of centering so you were seeing a lot of light around the secondary holder. In fact the secondary holder wasnt in, just the plate for the collimation bolts.
I assembled it with the correct spacing and took a couple more photos at the backfocus distance. Indeed there is only black! The photos show a lot of light as it's daytime and a long exposure pointing down the back of the scope.
This is what I have found out. The mirror is 6" and the primary baffle shows me an a circle with a diameter of 6.496", so the space you can see either side of the mirror is 0.25". Dont get fooled by the bright areas, thats where I have milled off the anodising when I was removing the walls.
So, if I have issues centering the primary baffle, how critical is it as long as all the mirror is visible. I can center it roughly like the previous picture as far as concentricity goes.
The secondary baffle takes up the rest of the view, you can't see outside it. The secondary baffle is 3.24" deep. I put a marker inside it to see how much of it I am actually seeing... That thin strip you see is about 2.3" deep with the first inch hidden by the primary baffle.
Thanks,
Jason
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jasonharris
sage
Reged: 09/16/06
Posts: 252
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One more thing, the photo above is way out of alignment, I can do much better then that even thought the whole of the mirror is visible..
I was thinking about making a light shroud for the bottom half of the scope - much like RCOS do. I have a sheet of .035" thick black acrylic I was going to use Of course, its too shiny and I would have to paint it. Flocking paper is not sold over here and I dont have the $$ at the moment to import any.
Is it neccessary to do this? I am wondering if I don't get it right that I may just be setting up a big reflector on the sides as I am not sure how black it needs to be so it's effective and not having a negative impact..
I am in a dome so I don't have lots of stray light coming from the sides. Other then that id does have one practical aspect and that is for keeping dust out. I could possibly fashion a top of two halves to go around the baffle .
Any thoughts appreciated.
Thanks,
Jason
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
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Hi Jason,
As long as no light gets to the Focal Plane than you are good to go...
Perfect concentricity only endures that the very least amount of light would be able to affect the image and may be a bit over the top.
Look at you chip size and make a clear plastic or little window the size of the chip centered in the field of view.
Then, again go back and look to see that no light will hit the outer corners of the chip. If no light shows at the corners you should be good to go.
As for the Baffold around the back of the OTA and Mirror area, what about a sewn baffold out of some black cloth that would not collect dust like a rip stop nylon or perhaps even black felt??? 
Looking very good my friend 
Best Regards,
Preston
--------------------
A few I enjoy,
and a few more in the works ;<)
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jasonharris
sage
Reged: 09/16/06
Posts: 252
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Well, I just put the silicone on for the secondary and as I look back at my questions it seems that in my haste I didnt follow all the advise..
No practice on something else, the blob was just over .5" and the washers I used were about 0.04" for the spacing.. I roughly had them at 70% but didn't mark it..
The trouble was as soon as I opened the silicon I felt rushed and I should have done the prep first..
No matter.. I can look between the glass and the back and see three round bits of silicon ~ 1 inch diameter.. I thought it would spread more when it was squished but apparently not.. I guess I leave it for a few days now..
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
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Hi Bud...No worry here...
What we recommendeed was the Cream...
What you got was the CCrreeaam>>>>>>>>>
Without the last m>>>>>>>>>>>>>>
You Are So
Close!!!
Very Best Regards,
Preston
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A few I enjoy,
and a few more in the works ;<)
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jasonharris
sage
Reged: 09/16/06
Posts: 252
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Someone just told me that I didn't in fact use a silicon.
Selleys All Clear is a copolymer adhesive and not a silicone.
Hmmn, sure looked like one, hope the results are ok because my guess is it's on pretty solid by now..
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
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Hi Jason,
Kindly post the information on the copolymer...and I'll advise you as to any likely problems if I see any...
Very Best Regards,
Preston
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A few I enjoy,
and a few more in the works ;<)
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jasonharris
sage
Reged: 09/16/06
Posts: 252
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Hi Preston, this is the product page, I know it has been used before - probably by other unknowing people - with no ill effect though.
http://www.selleys.com.au/Selleys-All-Clear/default.aspx
Thanks,
Jason
Edited by jasonharris (11/05/09 11:23 PM)
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jasonharris
sage
Reged: 09/16/06
Posts: 252
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I made a light shroud from the .8mm acrylic I had. It looks ok but I realised after putting the primary baffle in that it wont be possible to do any adjustments on the baffle with the shroud in place.
Not the end of the world, I will just have to get it lined up before installing the shroud - it only becomes an issue if I want to tinker with it once its on the mount..
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jasonharris
sage
Reged: 09/16/06
Posts: 252
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other end
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jasonharris
sage
Reged: 09/16/06
Posts: 252
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Hi all, I am a bit stumped! The weather has not been any good so the scope has been on the mount for a few nights awaiting a star to do some collimation. Right now things are roughly lined up by eye..
I thought why not see if I can look at a tree and get first light! So I got my DSLR ready and to my surprise the back focus was way behind where I expected it to be.. So, what about my understanding is wrong here?
Firstly, a couple of things that I thought correct.. One is that the 9" back focus I have is measured from the front of the mirror. Two is that if the mirror spacing is correct then the sensor from my DSLR positioned 9" behind the front surface of the mirror should start to come in to focus.. right? or wrong...
Here are a couple of stats.. the mirror spacing is not quite right as I havent made a jig to set it up properly. Instead of 26.97" it could be more like 27.3" - i.e. longer but I thought that would move my back focus forwards if anything..
The tree I am lining up on may not be at infinity, it is about 130 - 160 feet away..
Given all of this I am not sure how I need to place my sensor more then 13" behind the front of the mirror to get focus when the BF should only be 9"
I must misunderstand something here right? it's my first cass so I won't be ashamed if I have!
Thanks,
Jason
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jasonharris
sage
Reged: 09/16/06
Posts: 252
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To save you guys typing, I got hold of Paul Jones who said I do need to look at something ~infinity to get the right back focus, so I guess I have learnt something today!
Thanks,
Jason
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skyward_eyes
Carpal Tunnel
Reged: 12/12/06
Posts: 2104
Loc: Arizona
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You should go into business making these things! Awesome looking scope my friend!
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sixela
Postmaster
Reged: 12/23/04
Posts: 10874
Loc: Boechout, Belgium
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Quote:
To save you guys typing, I got hold of Paul Jones who said I do need to look at something ~infinity to get the right back focus, so I guess I have learnt something today!
If you know the distance to the tree, you can use 1/f = 1/distance_object + 1/distance_image_plane to calculate how much extra back focus the closeness of the tree created.
I did that on my Newt before I cut the truss poles (it's easier to focus on something close than to focus an eyepiece between the UTA ring and the secondary...)
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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jasonharris
sage
Reged: 09/16/06
Posts: 252
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Thanks Sixela, I will have to do a calc based on my guesses and see if that takes the backfocus to about where it was.
Thanks to you also Skyward. I would love to make them for a living but I don't think I would make any money off it to give up the day job, unfortunately.
I don't think I posted how much this all cost me either.. I did a rough calc the other day and there is about $4000USD in it. The optics and coating must have been $2600 or more.
Jason
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jasonharris
sage
Reged: 09/16/06
Posts: 252
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So, I got to see something real in the scope..
It looked like it might clear so I got ready. Jupiter was the first thing visible so with a 20mm eyepiece I put it in focus. At least, as well as in focus as I could given it is just a sliding tube with some tape to keep it in the right position, which it wont stay in...
Without any further collimation I could make out the bands, just, and the moons easily but something strange was happening and it got me thinking that perhaps there is more work to do 
When out of focus I had a nice donut that looked ok. As I was coming in to focus it started to take on a strange shape, like three evenly overlapping coins. Going in to focus further they seemed to resolve in to the single disc of jupiter.
I have no idea what would cause this? I was chatting to a friend about it and we started to think about other things. One that came up is could I be pinching the optics..
I had a look at the cell and noticed something that is potentially an issue. The mirror was only sitting on 7 of the 9 pads.
I think the inside diameter of the cell is slightly too small for the mirror. Prior to powder coating the mirror would go in easily and had a little bit of lateral movement - not sure how much but I thought a bit of tape around the edge would get it ok..
After powder coating the mirror must go in exactly vertical. Any tilt and it gets stuck against the wall. if you were to turn it upside down and vertical it would fall out no problem either..
If the mirror wont sit flat on all pads then obviously the wall of the cell is holding the mirror.
The last thing I want to do is pull it apart to skim something off, its too sad!
Now I have to remember how much lateral movement is allowable, from memory I have heard only a couple thou of an inch .. thats not going to be completely easy to set up and remove..
If I don't do it will I regret it, at worst could I damage the mirror if the aluminium shrinks too much on a really cold night ..
Oh well, I did see jupiter though..
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RedIrocZ-28
Pooh-Bah
Reged: 09/18/05
Posts: 1187
Loc: Grand Rapids, Michigan
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Jason, though I have not ever made a telescope before (and I am quite enjoying watching your progress in thiss!!) I noticed that you said something about "3 coins" as you were focusing.
I have been told that this could be due to tube currents or an improperly cooled mirror where there is a temperature differential boundary in front of the primary.
However, I could be wrong in this, and pinched optics may do that as well, but in my 10" newt, it seems that I get the same 3 disk looking things when I am just out of focus if the mirror hasn't reached ambient temp yet.
Take this with a grain of salt though, as I said, I am no expert on the matter, just relating what I have read.
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12230
Loc: Los Angeles
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The 3-coin appearance, when focusing, indicates there is astigmatism somewhere.
Causes of astigmatism:
--differential cooling of optics
--pinched mirror(s)
--tilted focal plane (poor collimation)
--astigmatic optical surfaces.
I suspect that the first 3 may all pertain, in this instance.
Definitely give the mirror more room in the cell.
Definitely figure out a fan system for temperature differentials.
Definitely spend time on achieving perfect collimation--don't trust the machining.
I will presume that #4 is ruled out.
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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