skybadger
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Posts posted by skybadger
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Is that an auto collimator, ie a mirrored internal surface ?
I have a 1.25 and a 2" I use as part of my checks, alongside a Cheshire and a baader laser.
When you mention that they don't need collimating, you are assuming they are sufficiently well setup at time of sale...
I'm not sure that's always true.
But they are useful.
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Not at all flimsy but ... The rubber cups on the end of the feet allow some squishing as weight moves, so need swapping with the hard feet. Mines carrying about 10kg.
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Not using an oil bath then, for 2c ?
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21 hours ago, callump said:
Personally, i'd just use whatever exterior grade ply you can get and not stress too much about it.
A good coat of varnish will seal against the elements.
All depends on whether you want to look through it, or at it...
/callump
I use a 15% or so thinned varnish as the primer varnish coat - penetrates more and seals all the edges before you start putting normal thickness coats on it.
I have a few scopes treated this way, including parts built from kitchen worktops.
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Got my 6" blank that's been ground to a f/5 on the bench.
While digging it out and getting stuff ready, found that my 12" tool I acquired which was bonded by pitch to the blank was actually a mirror close to being polished out. I separated them using a few taps from a mallet and recycled the substantial amount of pitch. That's now been put on one side for later..
In the meantime, I poured a sub-diameter lap for the MoM polishing of the 6" . The reason I stopped a while back was because the lap pour wasn't working for me. In this case, it poured easily into the masking-tape wrapped tool mould and once firm enough I pressed onto the cerium-wiped mirror on top of a sink protector sheet to create the lap grid. That went so well, I took the grid off and continued pressing for a good contact. The squares closed up so the pitch was still warm internally and soft. So I put the grid back and let it continue to press on the mirror.
I then went around the lap edges with a chisel to just tap off the overlap.
Tomorrow I'll put it on the machine for a spin for a few hours and see how it goes.
While that happens I'll get on with making a new part for the foucault with the source and the window closer together.
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1 hour ago, JeremyS said:
Has anyone on SGL built a DPAC system yet?
I have used an oil bath and a frame that allowed me to invert the scope over the bath with adjustor feet to get it properly vertical.
I used an oag to inject the light source through the prism before I made a ronchi eyepiece with the led built in.
I'll look for some pics.
https://www.skybadger.net/equipment/12inch Cass.shtml contains a dpac arrangement at the end which was used to acquire a set of ronchi images. It stands on the levelling frame and the box contains the oil pan which is a 20" cake holder pan with veg oil in.
The image below is a simple oag used to create ronchigrams
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5 hours ago, CraigT82 said:
That’s good, because my next thought about DPAC testing was that it won’t be long before we start seeing threads with people complaining that the flat they’ve bought is not of the advertised quality!
But surely that's the first thing you do when you get your flat.? I.e. put it in a bath of water under a monochromatic light and look for the fringes ? Not that it's a dpac test but youi mght be interested
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Has anyone made the point yet that you can test DPAC using a water bath so everyone everywhere can have a flat of the same accuracy ?
I use it to test and setup my cassegrain scopes but use a white light led.
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On 21/02/2023 at 23:52, Astrobits said:
I agree with Francis.
If the camera is in the black disc on top of the circuit board then the separation between that and the source is far too large. You will need them to be as close as possible especially when you start figuring faster mirrors. Ideally the source and viewing point should be on the same axis. this can be accomplished with a beam splitter cube. When I figured my 16" F/4.7 mirror a separation of about 20mm showed significant astigmatism caused by that separation.
Nigel
I read this again. The camera is in the centre of the plastic rectangle. The black round disk is the original microphone from the camera, tucked out of the way, so the actual separation is about 10mm less than it look and may be about 15mm overall.
I'll see what I can do to reduce it.
What effects should I expect I'f the source and sensor are not the same distance from the mirror ?
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I bought a replacement filter from AliExpress and then did it myself (to a Nikon d5000)
The first time I managed to break a Flexi PCB clip resulting in no longer being able to use the rear switches but controlling it remotely was no problem .
I did a second one and it works perfectly. There was no problem with realigning the chip since the chip module just bolts flat as it seems pre-shimmed. Images taken since have no obvious flare.
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I currently run 4 vixens,
The vc200l
Vc110l
NA140
ST80
but have moved the mounts I had on to consolidate them too.
The vc110 is meant to be a grab and go scope and lives in the car.
The 140 and 200 are on the main mount together until I replace the 200 with the 315 RC . The ST80 guider isn't getting any use at present.
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Sorry for the delayed response.
Maybe the term slump is the wrong term. What is happening to cause mirrors mounted on edge to give astigmatism that's removed by rotating ?. I guess it's bending ?
I've checked out the mirror I matic. It still functions. So next step is to cast my lap and polish the 6" to a sphere. Next post should be after a week or so.
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It does seem rather pointless though.
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I believe it will, there's slump and there's slump..
The images are of a high quality f/6 mirror. Texereau and Bartel's talk of slump in edge supported mirrors creating astigmatism, especially in thin mirrors. I'm told there is astigmatism in these images. Just guessing as to the cause. One could be due to the distance between source and image. One could be due to edge support.
Rotation of the mirror would tell the difference.
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7 hours ago, davidc135 said:
Plenty of astigmatism coming from somewhere. David
You can see it Davide but you'll have to point it out to me, if you wouldn't mind ?
That mirror has been on the bench for a few weeks so may have slumped a bit.
What am I looking for in those pictures ?
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That's a good point about the mirror being silvered.
Thanks.
The separation between source and camera is about 15mm. I moved the source from horizontal to in the vertical plane to allow source and image to be at the same distance from the mirror but that has increased the separation a little..
Looking at the stellafane examples I didn't think I was pushing any distance boundaries here.
I can always try it and see.
Need to do some more machining.
I'll polish out the 6".f/5 to see how that affects the image brightness.
Cheers
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I suggest something a little more rigid, dams like that don't just fall into a circular shape they will take on all sorts of variances.
Cutting the rim off a garden plant drip tray did the job for a small grinding tool. Held its shape well.
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There is that. It's surprisingly stable but I haven't got a big bench I can dedicate to this.the image itself is stable, it just needs improving.
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2 hours ago, Astrobits said:
The problem with the Foucault test is that the focus of the side zones is different and not on the optical axis. If you draw the light beams from a parabola out on paper you will see that the focus of, say, the left side 70% zone is on the right of the optical axis (OA) and the right side zone focus is on the left of the OA. With your knife edge on the OA you are cutting the beams away from their focus points. This doesn't pose a problem with large F/ ratios but becomes increasingly obvious as the F/ ratio gets smaller. To improve your accuracy with fast mirrors you will need to measure both X and Y positions of each focal point of each zone. These will lie on the Caustic curve.
Nigel
How do I finesse getting my tripod on the OA so that I can move backwards and forwards along the OA and know I'm sufficiently perpendicular to the mirror so my left and right knife measurements are comparable ? Otherwise I'll have to find the centre each time and take an estimate.
In my head those annuli/zones from the parabola are focusing along the OA and I'm finding where each comes to a focus to measure the parabola.
I've got to get to being able to do that reliably yet. Then I'll worry.
Contrast and repeatability ? Necessity for a pinhole ?
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8 minutes ago, bosun21 said:
A 12” travel scope is too big for my travels.
I plan for it to be quite light..bar the weight of the mirror.
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Hi all
I'm starting to learn this Foucault testing lark for testing ATM mirrors. The purpose is to able to grind and polish mirrors on my mirror o matic machine. I have a 6" ground to polishing stage and want to do a 12" f/5 for a travel scope and a 16" f/4 for a back yard visual scope. The latter o acquired part finished, hence wanting to get fully into the whole mirror grinding experience.
I built/rebuilt a moving source slitless Foucault tester and I'm using a known good 12" f/6 mirror to test against to learn the ropes.
I've got texereau and a Foucault testing handbook as my manuals. I'm looking for advice on how to improve contrast and repeatability. This is because I'm finding it {too} bright in the camera but fainter and low contrast to the eye and difficult to equalise the bulls eye and more so to repeat it to take measurements against.
Similarly I have real problems distinguishing the axial focus point. Is this other people's experience ?
How can I improve contrast ?
How do you come reliably back to the same equal zones at the same measurement point in the mirror ?
Does the knife edge look rotated or not central? I suspect I need to drop the height a smidge to centralise the reflection but it's not easy!
The source led is ground flat down to the zone where light is created in the led, so has a natural diffuser and is about .5mm across (?) The knife lies across the centre as close as I can estimate. I considered using a pinhole but there were some CN testers producing beautiful images which didn't even grind the led down or use a pinhole so I'm not sure of the value of that. I guess I can test the use of one but I'll struggle to put half a 200um hole across the knife.
Some pictures are below.
The tester in the foreground and the mirror in the distance. The tripod carries a coarse xy adjuster and on top of that is the led source with single vertical knife on left/right micrometer and fore/back Vernier . On the hole where the eye goes I've put a small usb board camera and can slide it out on a dovetail. The camera connects to my phone for the pictures hence the lack of complex controls. The blue cast in the pictures is the camera led I'll have to remove. The image sequence moves from outside focus towards the mirror to on axis focus. Knife from right, moving source, so appears illuminated from left.
Mirror in its cell and scope box in the distance.
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You could of course create a new chart and then Search to cause the search to centre on the new sheet. Should work with all versions. Flip between sheets as needed.
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I haven't experienced dew with my hydreon, the internal heater being co figured on. What I have noticed is that it doesn't see snow.
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Or put an off the shelf skywatcher dob drive kit on it. Ah, they've stopped selling them.
Interferometers/Mapping
in Physics, Space Science and Theories
Posted
You could look up the dtfringe software by Dale Easton and its use with a home made Bath interferometer. There's lots of info on the web.
I have one in bits waiting to be assembled on the bench right now, behind a long list of other projects.