jasonharris
sage
Reged: 09/16/06
Posts: 253
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Well it looks like my old thread got archived andI wont see it in the 'live' forums again, too bad. If you are new to my build you can see the archive here. archived build for my 12.5" rc
I guess I will just continue on with a new one.. I spent way to long yesterday doing the two side rails for the scope. This is because I have started to save weight on the pieces as I do them rather then wait and hope that I don't need to reduce weight later on.. I only had 4" x 3/4" plate available and they weight quite a bit. I think I got about half the weight off them.
The main dovetail mount also weighed a fiar bit so I milled out four areas. I should have weighed before and after because it didn't feel like all that effort was paying off..
I am still itching to get the trusses cut to length and see what this guy is going to look like but I don't want to rush things, bad news if I cut them the wrong length.. Still, I might aim for this today and use tue/wed to reduce weight on the other components..
Also my solid round should arrive by then so I can make the counter weights up..
Edited by jasonharris (10/03/09 10:00 PM)
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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HiJason, It just amazes me that you are doing all this lightening manually and not with a CNC...
My hats off to you my friend 
Very Best Regards,
Preston
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A few I enjoy,
and a few more in the works ;<)
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DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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It looks like a work of art! It's just beautiful.
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Orion XT8 - Orion StarMax 102 EQ
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Thanks Preston and David! Yes Preston, it's hard work. I need a rubber mat next to the mill when standing on the concrete for so long.
There was a movie on TV which was ok to pass the time.. I really wish I had a rapid feed on the mill as well, it really is all done by hand.
I decided to cut the trusses, I just couldn't wait to see it all at the correct size. I measured and recalculated three times and only had to cut twice?! 
I was nearly 1" too short. I still don't know where I went wrong which is a bit sad for a guy who went to university to do maths. I guess it's been a while.. No matter, because of the internal length of the connectors I can afford to have 1/2" empty at each end on that single truss.. Lucky I checked after cutting one and not eight!
So once that was done it was out to the observatory and put it on the mount. It's the first time that mount has seen anything bigger then 4". It was getting an inferiority complex and I was wanting to see what a real scope looked like on it. So heres the results.. it's hard to take photos in a dark observatory but I gave it a go..
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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another
Edited by jasonharris (10/04/09 12:52 AM)
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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And one more.. The lens distorts things a little but it really is a lot more compact with the tubes having had 300mm cut off them.. 12"for you guys.
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dave brock
super member
Reged: 06/06/08
Posts: 165
Loc: Hamilton, New Zealand
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Well done Jason but... If the weather is like this just because you've got this far, imagine what it'll be like when you're ready for first light. 
Dave
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20" homebuilt truss dob
6" watson refractor
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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I wasn't going to get it anodised because of the cast parts so I was going to paint it myself.
Out of curiosity I asked a powder coating company around the corner from me how much it would cost. To get the whole scope done would only be $150nzd. I thought that was a pretty good price so thats the way I am going, much more durable as well..
I need extra counterweights so rather then the $230 USD for the paramount weights - and the $100 shipping I decided to do my own..
I got two pieces of solid stock the same size as the paramount weights and faced them off and threaded a hole for lock..
I copied how paramount did this . They have a brass pin at the bottom which is lifted by a spring. When you tighten the bolt this pushed the pin on the counterweight shaft. I thought that was a good idea so I followed suit.
Cost per counterweight - $31usd - Unless I but a knob to put on the end but I dont see much point when you only need an allen key when you set it up the first time.
Jason
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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Jason Just Beautiful
 
I only wish our new shop was up and running ...so many new ideas...
Fantastic work my friend
Very Best Regards,
Preston
ps...we could not see the wrinkles in the Primary Bafford in you posted pictures 
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A few I enjoy,
and a few more in the works ;<)
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mattyfatz
Pooh-Bah
Reged: 12/27/06
Posts: 1296
Loc: Boise Idaho
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A beutiful scope! And a noble project
Hats Off To You Jason Harris!
 
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**Matty**
Stimulating the economy, one piece of equipment at a time.   
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Thanks Preston and Matty, I am now going to do a bit of filing and tidying up so they are good for the powder coaters on monday.. Mid next week I can call it done if I am lucky.. Except for the tinkering and fiddling that will no doubt follow as I try and get everything collimated..
Thanks,
JAson
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Sky Captain
Metal Whisperer
Reged: 11/07/04
Posts: 7195
Loc: Issaquah, WA.
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Hi Jason, without a doubt a fantastic scope build!
Do you have any other good shots that you could post again of the secondary holder, vanes and the connection points?
I love how you did it from just looking at the photo's.
I especially like how you built the secondary and it's larger size. So many other scopes tend to use a relativly small secondary holder but use a pretty large secondary mirror and thus can create an unstable situation.
Thanks
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Equipment Overload!
Kerry
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Hi Kerry, actually the spider/secondary holder was the only thing I didn't machine myself.
Someone else who bought the same optics had all his parts machined in china. When I saw his plans the spider looked like it would work for me, same optics but he was doing a CF tube. For the price he was getting it machined for I decided to get one for myself. I can post a picture of the drawing if you like..
Here are a couple of pics I just took. The only thing I machined were the retaining ring that is not anodised a nut with a half sphere. This is used to hold the mirror cell to the plate with the collimation bolts. You can't see it in the photos.
The fasteners are just temporary to put it together, I will put in something the right size when it all goes together,,
Edited by jasonharris (10/10/09 04:10 AM)
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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other
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Sky Captain
Metal Whisperer
Reged: 11/07/04
Posts: 7195
Loc: Issaquah, WA.
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Thanks for the added pictures Jason, thats what I was after.
I was looking to do something similar to mine but found that it would add extra weight and overall length to my scope...both of which I didn't need since it is being used on the Atlas mount.
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Equipment Overload!
Kerry
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Dropped the bits at the powder coater yesterday and picked them up today..
The finish is nice. I bolted a few pieces together for a photo but I won't do the rest until I have the trusses properly fixed to the connectors..
I am starting to see what its going to look like - black!
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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Hi Jason,
Just Beautiful...what more can I say
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: 253
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Thanks Preston, I like the powder coating.
I often weld things up from steel tubing and paint it myself, it can be problematic with dust and it's time consuming.. For the cost of the powder coating I am not sure I will do a lot of painting again.
I also like that I don't have to clean up the parts much, their cleaning/etching process gets rid of all the grease and rubbish for me.
I thought at this point things would zip along quickly but I am finding little issues.. a couple of parts that were a 'good' fit before wont fit with the powder coat so I have a couple of small jobs to do there.
Of course there is the wiring for the fans, I have a computer fan controller I am going to use for this and it fit's nicely in the pocket I milled out of a side support.
I am now off to get some high impact styrene (like kydex) to make a tube for the lower section of the scope. I am not sure how necessary this is though..
I need to paint some other pieces myself and will cover some of the powder coated pieces with my black paint as it is more matt.
The only piece I need to make is something to hold the secondary baffle on. I am not sure if I need to make this adjustable or not?
Jason
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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Hi Jason, For the secondary baffold holder...unless you can make it perfectly concentric and that can be done, you still will want some adjustment to get it centered around the secondary...
And it needs to be seperate from the secondary, as when you collimate it, the baffold will move with the collimation and you will end up chasing it back and forth...
I modeled it in a CAD automation and saw this happening...
Thus we just started from the beganing with the simple adjustment that I think I showed...
If not, just ask and I'll attempt to find a picture.
Fit was one of the reasons we went with anodizing, as powder coating usually adds many thou's of inches to the thickness.
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: 253
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Progress is slow again as my wife is working very late and weekends at the moment. I am looking after the kids so I don't get much time in the workshop before the screaming and banging happens 
I had a go at making a ring that I need to attach to the mirror cell. I put some sheet metal on the faceplate of the lathe and tried to part it off. It's worked before but I wasn't having any luck this time.. Another piece of sheet and moving over the to the mill fixed the problem and I got the ring, just a bit slower then on the lathe.
The ring serves two purposes. It masks the bevel on the mirror and has just enough foam under it to stop the mirror from flopping forward. In reality you have to tip it near upside down for the mirror to fall forward but it's still better to be safe on this one..
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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other.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Just a thought - do you have access to a glass peener or sandblaster? You could roughen up the front and back surfaces of your new retainer ring shown above before anodization to reduce any specular reflections from it. You want it as black as possible for all illumination angles.
Mike
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Unfortunately I don't but it's been on my list for a while as a cheap sandblaster for this type of thing is only $20.
This guy will get a paint job with the tamiya flat black - very flat and black  I have to because it's actually galvanised sheet metal. I didn't have any aluminum sheet about and it was going to cost $$ to get the smallest size sheet.
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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Hi Jason,
Seeing your mill is causing me withdrawals...from my shop tools... 
Kindly quit posting those beautiful pictures taken machining things in your shop
Not going to happen... 
I'm watching with baited breath...
Very Best Regards, My Friend...
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: 253
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Thanks Preston, heres another shot of a mill for you. This time it's tidy!
I took the opportunity to clean it went I went in to the workshop and realised I wouldn't have any time to do work on the scope.
So, I set up the jig for attaching the connectors to the ends of the CF tube.. The stuff I used sets very quickly so I only had a handful of seconds to get it on and make sure the connectors were flat on the jig. If not they wouldn't line up and I would be out one piece of CF and a connector!
It all went to plan though. I also painted the pieces the trusses mate to on the scope..
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Preston, I am looking at your photos again and I can see how you mounted the secondary baffle.
What I can't figure out is how it actually works? It doesn't look lie you can tip/tilt it so I guess your thinking is that the spider will be at an angle which is good enough.
I also can't see how you can centre the baffle as there are 3 points at 120 degrees, given your points are glued to the baffle, they cant rotate so I am not sure how you are able to move it about.
I assume you do because your photo had a caption that you were testing centering the baffle.
Thanks,
Jason
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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Hi Jason,
The three points for the secondary baffold are on a sliding CF plane that has one fixed point and then a slot...and thus one can adjust to get everything concentric...
Does that make since???
Best Regards,
Preston
--------------------
A few I enjoy,
and a few more in the works ;<)
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Michael Miles
professor emeritus
Reged: 02/11/05
Posts: 677
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Quote:
Hi Jason,
Seeing your mill is causing me withdrawals...from my shop tools...
Preston
I agree completely. My (baby) milling machine is on the fritz, and I'm chomping at the bit to get back to making something - anything.
Pictures look great, telescope envy too...
Mill envious,
Michael
Michael
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LXD-75 w/
Stellarvue 102ED
Hardin 10" Newt
Antares 8" Newt
Meade AR-5
Meade N-6
Celestron 102mm refractor
Canon 300D, Meade DSI guiding
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Hi all, things have been a bit slow but I have been plodding away at some less glamorous pieces of the project.
I am near to glueing the secondary mirror in to it's holder but I was after some advice.. If you look at the pictures from the previous page you can see that the secondary holder is like a caketin.
I am not so sure that this is the best design when it comes to take out the mirror at some point in the future, for a re-coat maybe. Because it's a caketin there is no way to access any part of the mirror once it's in. I am wondering if I should drill a few holes in the back and possibly slot out a few sections of the sides of the 'tin'. I am not sure how you take the mirror out once glued? Pry it out? if thats the case I will need to alter it in some way.
The other question is what do I use to adhere the mirror to the cell? I see that most usually use a silicon. Mine will be glued on to anodised aluminium. Do I just go for any silicon that is used for aquariums?
Thanks,
Jason
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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Hi Jason,
I agree that the pie tin is not the best idea...
Go with something flat backed and if you want to attach sides than do that mechincally....
That way you can remove the secondary later easily without damage...
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: 253
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Preston, I talked to the guy who did the spider and the tin.
It all makes sense to me now, as to why it was done this way although I am not sure how much I like it..
The wall of the tin is quite thick, about .12". What he was going to do was to put three nylon screws at 120 deg around the outside, pointing towards the center of the tin. These were to center the mirror in the tin which has a diameter of about .12" larger then the mirror.
He was also going to use clips in front of the mirror to retain it. These clips would of course be touching the mirror to stop it falling forward.
Knowing that he never intended to use glue, my main complaint is no longer.. Should I also do this or not? I am not sure.. I cant see any problems and I like the idea of no glue but I would appreciate others thoughts about this one.
Thanks,
Jason
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Actually, I am talking myself in to his idea as I like the no silicone approach if there is a good one..
If I put down some felt pads and have 3 clips with a little bit of rubber foam under them then I should be able to apply just enough pressure to hold the mirror firm with no chance of it moving forward...
I think I will play around with that but please jump in of others see problems with this approach1
Thanks,
Jason
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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Hi Jason,,,
I do see major problems...
The mirror needs to be supported at roughly the the 70% of radius ...
By putting the felt pads at 70% and retaining at the edge you will be supporting at the edge and warping the secondary slightly...
You have done so well so far, why fail now???
Just My Humble Thoughts MY Friend,
Very Best Regards,
Preston
ps....you do not also want the 3 side screws touching at 120 degrees as they too will cause stress and show in the final images or testing IMHO and everything that we have tested.
--------------------
A few I enjoy,
and a few more in the works ;<)
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Yeah, I think I agree and some of that is one of the reasons I went for the primary mirror cell the way I did...
Anyway, it's all chopped up now! In a clean way I milled off the sides but left 4 small sections of wall at 90 degrees. I did this becaue its convenient to mount the baffle on and it's pretty nice for centering the mirror with, I just place 4 shims of the right size between them and the mirror and it's done..
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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OK, Once again, I had hoped to get it finished this weekend but I can't. I finished up the last few things but misplaced the silicon to put the secondary on.
No matter, I still wanted to put it on the mount
I also found a way to make it look like it's 16", not 12. Put a kid beside it
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Lessee, what graemlins express my thoughts on looking at this picture?
That's one nice lookin' scope dude. How did the baffles work for you? Stray light shielded properly? Or have you actually viewed through it yet with the secondary mounted?
Like Hancock says, "Good Job!"
Mike
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Mert
Carpal Tunnel
Reged: 08/31/05
Posts: 1804
Loc: Spain, Pamplona
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Jason, that's a mean looking machine
Also the mount!!!
Hey Preston, nice to see you here, how's your
Spanish lessons going along? 
All the best!
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Mert
42º49"N 1º38"W
3" Polarex refractor
6" F12 SW Maksutov,CS2-S
EQ6 + EQMOD 
SPC900NC/DFK21AU04.AS
My web-page don't laugh, I should make it better! 
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Thanks Mike, thanks Mert!
Mike, I haven't tried anything yet, I still need to get the mirror in. The weather is looking to close down again, the forecast until next weekend looks bad so it may be a little bit of time until it gets a run.
I am happy with it though. At worst it will be a good ornament
Still, the only thing I worry about is the field curvature. I can't afford a flattener at the moment and I have no idea what to expect from a 6.7 RC on a full frame chip??
Jason
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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Ola Mert, Bien bien me amigo...
No , no necesitas ayudar esta momento.
Pienso necesito mas traera (homework???)
And I'm back in the USA for a couple of weeks and that does not Help...
Best Regards My Friend,
Preston
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A few I enjoy,
and a few more in the works ;<)
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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Hi Jason,
I just love that Tak as a balancing weight
Very good minuture kid next to that 16 inch RC 
Best Regards,
Preston
Quote:
OK, Once again, I had hoped to get it finished this weekend but I can't. I finished up the last few things but misplaced the silicon to put the secondary on.
No matter, I still wanted to put it on the mount
I also found a way to make it look like it's 16", not 12. Put a kid beside it
--------------------
A few I enjoy,
and a few more in the works ;<)
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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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: 1171
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: 253
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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: 253
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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: 1171
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: 253
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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: 1171
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
--------------------
A few I enjoy,
and a few more in the works ;<)
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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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: 1171
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
--------------------
A few I enjoy,
and a few more in the works ;<)
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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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: 253
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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: 253
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other end
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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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: 253
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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: 253
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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: 253
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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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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Thanks guys, it sounds like this is the cause, one or many of them!
I will take some off the cell later on today to give it some room.. This is where I half wish I went for a more conventional cell that had the mirror supported by the center with an o-ring.. then I wouldn't have this issue. As I take more off the inside diameter of the cell then I have more lateral movement and I am still not sure how sensitive the RC will be to this movement.
I certainly have the tilted focal plane, I do need to collimate but when I went to look at jupiter there were no stars at the time, then the clouds rolled in.
I hope that the differential between the primary and the secondary will not be a major issue. I have fan cooling for the primary but I don't know the temps of either to try and keep them in sync.
I imagine the primary will never keep up with the secondary in terms of cooling rates..
Thanks,
JAson
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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Hi Jason,
Good to see you getting to focus...
You do really need to have all 9 points of contact on the back of the mirror.
I did not fully follow just how you delt with keeping the Primary Mirror centered on the Through Tube ...was it something like we did with the O-ring?
Mike or Paul can further tell you the criteria on having the Primary and secondary concentric, but you are very close at a couple of thou and it is EASY to get errors in trying to do this...
What we did, was after making the back plate and rings for the truss tubes...was to measure very carefully from the edge of the back plate to the Primary Mirror to be certain that it was centered and if off to which direction...as that could not be corrected other than by rotating the mirror around the Mounting Tube...
And we found that in fact the distance from the central hole to the outer edge varyed by over 0.040" as did the optical center of the mirror...ie was not the center of the mirror blank
Once one determines the concentricity of the Primary with the Optical center and then translates that to the Secondary Mirror position one can then replace the Secondary Mirror and adjust it to be concentric with the Upper Ring...
The one overlaying assumption is that the Optical Center of the Secondary Mirror is that Physical center of the Secondary Mirror...which may or may Not be the case...
It becomes a very fine dance to get everything adjusted and ferret out any errors...
You will get there my Friend, that I am sure...
But the journey may be taxing
Very 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: 253
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Thanks Preston, my method was a little bit different as the mirror has no central support. The tin that it sits in uses an o-ring to center it on the throught tube. The mirror sits inside of that with only a small distance between the wall and the mirror edge. I.e. the mirror is 12.5" and the inside diameter of the cell is 12.502. Just enough to drop the mirror in and not have pinching with good even support around the edge..
The current version after powder coating may just be too smug..
Thanks,
JAson
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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Preston, heres a pic for demonstration..
You can see the hole in the mirror and behind that the hole in the mirror 'cage'. The hole in the cage is smaller then the hole in the mirror and sits on the through tube with the o-ring.
The Mirror doesn't touch anything in it's center hole. I did this because I had heard of issues when the support is on the center of a mirror like this. Yours is conical so its much stronger in the center.
The walls of the cage give it even support around the edge and stop it moving laterally by being a good fit. Thats the theory anyway.
The mirror sitting on the through tube is a lot simpler and I may not have this issue if I went this way.. too late now I guess, oh well..
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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I see, I had thought that it too was a conical...
Hope you get in another session tonight.
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: 253
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I managed to pull it all apart and get it back together the same day.
Unfortunately I wasted the couple hours of clear night I had by trying to find a good star to use with ccdinspector. With no other scope attached and only the had paddle it's pretty hard to find what you are looking for with that focal length. Now I am familiar with the process I will do it again after I get the mirror spacing correct.
I talked to Paul Jones about the tolerances for the mirror spacing. He said that it's more important to get the back focus correct, i.e. adjust the spacing until the focus is at 9"
Whats a method I can use to do this? I don't know for any eyepieces where they should be. Could I rig up something using clear acrylic and see it focusing on that?
Is there a way to figure out how far away from the focal plane an eyepiece needs to be so that it looks in focus?
Thanks,
Jason
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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OK, dont worry about that last question either.. I didnt realise the plossls focused inside the barrel so I was able to measure it..
I should be able to do the trick now..
Thanks,
Jason
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1171
Loc: Houston,Texas
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Hi Jason, You should be able to measure the distance from the Primary Mirrors front edge to where the CCD chip would be within a few tens of thou's of an inch...
That should get your spacing of the primary to secondary within a thou or 2...
No need to use optical means, or so I believe...
Your chip is much easier to calculate the final position IMHO...
Any other opinions??? Out there???
Very Best Regards,
Preston
--------------------
A few I enjoy,
and a few more in the works ;<)
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