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F12, 4" newt Build.


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That will make a small difference. Not bored any more but. This is 4in F12 optimum 2ndry, 28mm 2ndry and 1/3 main mirror 2ndry. Which is which is pretty obvious.

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The 1/3 is outside Rayliegh's limit so wrong about the software. Silly me it's the axial one. I didn't alter the obstruction of the 2ndry on the 8in F6 otherwise it would show some sag.

The problems with the larger F6 mirror are nothing to do with the size of the 2ndry. The difference is purely down to aberrations. Like most things it's not that simple but if a modulation of 0.5 is picked the F12 mirror is better across the field than the F6. 0.5 loosely means that the actual contrast going into the scope is reduced to 1/2 of what it should be when it comes out. As stars are black on white that doesn't matter much, contrast is way way down at Rayliegh's diffraction limit anyway but on extended objects which generally have low contrast levels it has a noticeable effect.

One other thing that the table on the 4in F12 shows is the radius of the moons image so when at it's closest it will be about 13mm dia. Well within the scope of 1 1/4 eyepieces.

The side effect of all of this is that as scopes get larger and shorter the angular field they can cover well with simple optics gets smaller and smaller. If you read up on some of the big observatory scopes where they use extensive techniques to improve this they can still finish up with field sizes that are measured in arc seconds rather than degrees. This is why they also have smaller scopes about as well.

It's a curious area really. It doesn't mean that people shouldn't strive for bigger and bigger mirrors it just means that on things like the moon at some point it might not be as good as expected - also that the whole moon might not fit in a 2in eyepiece at some point.

John

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Edited by Ajohn
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Well last night I marked and cut the tube. Not perfectly straight, but not really an issue. Then today made the central part of the spider....might need to change the hardwood for aluminium, as i

Here is a picture of the overall scope before I fit the spider/secondary. Just looked through the complete thread and realised that at no point have you seen this yet.

Ok well the tape came off without it falling apart, and it actually seems fairly strong without any internal bracing. Looks nice sanded too.

Posted Images

Hopefully tonight will be the night for a test run.

Quickly go these shots before it got to dark. Sorry about graininess the iPad can't handle the dusk.

And yes, I know there isn't a counterweight.......yet.

And yes I need to grow a bit to view anything or take of my eq6 extension.

That is all.

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Now this is my type of scope build, congrats on a super looking job there Nebula. Its nice to see a wood tubed scope again, last time I saw one like that was in the Herschel Museum in Bath.

Look forward to first light report

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Well, things were not as encouraging as I had hoped for this outing. I was getting a kind of very close together triple image of Jupiter. It was more evident in jupiters moons. Each moon was showing as a triangle shaped formation of moons. See my drawing attached below. I would be interested to here comments from you guys as to why this is happening.

Note: the diagram eludes to the visual image being sharply in focus which it was to a degree as I could at times make out the lines on Jupiter, on at least one of the three jupiters! But each Jupiter was a different level of transparency, which when partially laid over one another made for a difficult image to focus anyway.

My friend Valefor believes it is the distance between the primary and the secondary, which needs to be shortened. This belief stems from previous viewing through the scope early on which revealed the same effect but with the images of the multiple Jupiters further apart.

I ask your advice because I want to know if it could be this, or a mix of this, a bad mirror and or off collimating, although I thought this to be pretty good myself.

All advice welcome, and thanks for the encouragement and compliments.

Thanks.

Andy.

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Edited by bluemaxroe
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Could be tube currents, did the triple images rotate or wobble? How loose is your primary mirror?, pinched optics can give a similar effect.   :smiley:          

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They say you should be able to slide a piece of paper between the mirror and mirror clips - they should not be tight at all. How are you holding the mirror in place - by a strap or other means?

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Scope looks great! Really lovely job. 

I think the focus issue might be your clips too. The mirror should "float" and not be held down by anything tightly.

The clips on most scopes are just a safety precaution to stop it falling forward.

If your clips are tight it could be forcing the central focus point out in three directions (where the clips bight) which is why you are getting three images.

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Wow never seen an image like that before, bet it was none too pleasing, but at least the guys above know what they are talking about, and hopefully you can get it sorted!

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Loosening the mirror didn't seem to help, although it was a very quick and dirty test. Not the best kind in astronomy. Maybe test again on Saturday as it is supposed to be clear.

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Try rotating the mirror if you can some how and see if the pattern moves with the rotation.

I doubt if mounting is the problem but you could improve the support - 3 screw heads for the back of  the mirror to rest on placed at around  70% of it's radius. For location it's hard to beat cork just touching it other than the difficulty of getting good cork. It needs to be pretty hard.

John

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You could try take out the mirror and test it focusing an LED on a piece of paper - see if it gives the triple image with its mounting taken out of the picture. You'll need to hold he LED and the paper about 2.4m from the mirror.

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