Celestron c70 Project

[happy-kat]

My winter project (well one of them) is to modify my Celstron c70 so that it loses the inbuilt 45 degree erecting prisim thing and I replace the back of it with a new flat plate and directly mount the 1.25 eyepeice adatpor to it. I have seen this done on a archived for sale thread on another forum.
I have a friend who can make me the flat back plate but can't do the cutting a thread to make a male fixing for the adaptor.
Are there any ideas on how I could get the right thread cut please so that I can screw the adaptor in?

This is the 1.25 adaptor showing the female thread. (the adaptor is upside down)

Very crudley the back plate would be round with a hole in it.

The side view idea is if I made a raised area, could I then afterwards get a thread cut into it?

Edited December 1, 2015 by happy-kat

[michael8554]

I assume the item you have is a "SCT Visual Back".

If you had one of these or similar:

http://www.bluefireball.com/product-p/c-07.htm

You could make a hole in your new backplate that the male thread on this item would just pass through from behind.

Then when you screw on the female-threaded SCT Visual Back from the other side of the backplate, it will hold the two together against the backplate.

You might have to shorten the male threaded part if the female part runs out of thread before it tightens, or make the backplate thick enough.

I guess there is enough focus range on the scope?

Michael

[happy-kat]

Thank you for replying.

The c70 is not a mak like it's big brothers and the rear end does not have the expected arrangement.

The c70 has after the back plate an inbuilt correct image 45 degree prism. My plan is to remove the inbuilt prism and replace with a diagonal of my choice. The c70 comes with the ability to use 1.25 eyepieces and I hope to use the adaptor fixed to a new piece of rear plate. There should be enough focus travel as I an swapping an inbuilt item to a separate item. The c70 lives is a moulded rubber sleeve that incorporates the prism I am just striping it back to reveal the mak.

I will add photos as I go which should explain things.

[happy-kat]

So whilst the American version (or perhaps a year or two older model) sports a metal body under the rubber mine is plastic.

Made a start today to understand how the C70 is covered.

It is a rubber sleeve which is in two parts, the orange metal band covers the join.

The sleeve has been stuck with a very hard dried glue down the centre seam. This could proove very hard to break the glued seal. Have been trying to pry the rubber sleeve with a plastic splodger seeing as the tube is actually plastic I don't want to use a metal splodger. One side has much less glue and lifted with a bit of a fight but did lift. The other side has a wider glued area (about 1cm) and it does not want to bodge.

The plastic body fels quite strong it does not buckle.

The length of my diagonal looks to be the same as the inbuild affair so I am not too concerned about focusing once the changes are done.

Due to finding a plastic body I may have to rethink the end design but I will decide once it is all exposed.

[happy-kat]

Seeing as maks have to cool down for optimum use does make me question the thick insulating rubber sleeve!

[happy-kat]

Took the rubber sleeve off today it is quite thick.

The tube is plastic.

The rear prism looks to be two torx head screws holding it on.

A crude measurement does look promising that a diagonal will be in the same place for presenting the eyepiece.

[happy-kat]

The rear element was held on by two small alen keys. There is a lens in the rear element as well as the prisim in the back of the C70.

I am very hopeful that removing all this lens stuff and replacing with a more typical diagonal will improve the view.

Next job is remove that prisim it is held with a couple screws.

Then I hope to fashion a cardboard back for my diagonal and an eyepiece to test focus.

[happy-kat]

Removed the inbuilt prisim held on with three small screws.

Made a cardboard test of the back so that I can test if focus is still possible on distant objects. The cardboard is holding the 1.25 eyepiece adaptor.

[happy-kat]

There are 6 collimation screws on the back of the C70 in three pairs of two.

[happy-kat]

Initial test shows that the optics all line up as I could see my vegetable bed some 20 feet away but as expected far too close to reach optimun focus

[happy-kat]

Just had a test, can focus on the stars and there is loads of focus travel left so I am reducing the number of card board rings and going to test again with the hope I can then focus on the bird feeders which are quite close in the garden.

Though I have assumed to shorten might find out it was the other way!

Edited December 8, 2015 by happy-kat

[happy-kat]

I have always found putting my eye in exactly the right place very annoying. Tonight I thought I would do an exit pupil test.

Looks like there is a problem that is a fan shape.

Colimating the heritage is easy compared to what I think this might be!

I am still going ahead with the changes as I believe this issue has been here from day one.

Before I twiddle the screws can anybody confirm please if a fan shape is normal for a Mak?

Edited December 8, 2015 by happy-kat

[Joseki]

Wish I'd seen your thread a couple of days ago - I've got a Acuter Mak-70 (C70 clone) - from astroboot ~£30 - though it came without the 1.25" adapter.  I bought my own 1.25" adapter but the threads were wrong.  Given a need to keep total spend on improving it down to a sensible level I took it apart (stripped off the rubber seal & removed the back like you).  With the back off (and the protective glass removed) I super-glued the 1.25" adapter I had to the original back keeping it away from the optics until teh glue had dried (I'm leaving the prism in place at the moment, pending tests).  - seems to be strong enough and nicely square.  The other mod I did was to slice off a rectangle of the rubber sheath and super-glue on to the plastic body a SW quick release finder bracket.   - okay I guess I should upload a pic.

edit:

Edited December 8, 2015 by Joseki

[happy-kat]

Finder bracket, nice idea, I'll probably add a red dot though I think mine will get more day time use.

That looks neat how you mounted that bracket.

The mak looks the same.

When I can get a single bright object in view then either side of focus it is still round not that I see all of it.

Confusing...

Could you test please the shape you get through your one I used a 25mm and a 16mm both have the fan and not a circle.

Edited December 8, 2015 by happy-kat

[Joseki]

Thanks - yes I want it mainly for lunar and white light solar (and if it seems okay I'll get it a camera tripod so I can keep it handy) - and the zoom ep it came with was rubbish.  I've just had a go with the projection, though it's not something I've done before.  I shone a bright desk lamp down it and used 20mm and 32mm plossl eps, with some thin paper wrapped over the top of the ep to form a screen - it looked pretty circular I'm afraid.  Did you try it before removing the prism? also, does flexing the cardboard adapter make any difference?  btw I may borrow your adapter idea - I think maybe I could make something cheap and usable with discs of plywood.

[happy-kat]

Thank you for testing.

Mine has always been very hard to use so feel fairly confident it was always there as eye position has always been very hard.

The cardboard is for testing.

I will design a back and get it 3d printed that is the plan.

Unless you intend to remove the prisim you won't need an adaptor.

Edited December 8, 2015 by happy-kat

[Joseki]

A 3d printed back sounds smart  Good luck with the twiddling

[Ruud]

I have this little scope. I use it with a 32mm GSO Plössl and for a finder I use the tube of a pen, like this:

Thanks for the thread. I'll bookmark it!

[happy-kat]

The good news is out might be human error, hope so.

Took the diagonal out and just used the 25mm eyepiece, nice round light circle on the test card.

Re attached with the diagonal and adjusted how it sat and the shape improved, not perfect but better. This gives me confidence that a pepper center square back will be fine.

I shall carry on with the current endeavor can I make it focus close then 30 feet away depending on position of the diagonal.

[happy-kat]

SketchUp does not look like it like my 10 inch tablet so now downloading 123D from AutoDESK to see if that is happy to run on the tablet it didn't like XP either much like SketchUp.

[happy-kat]

SketchUp is alright on the tablet I just have to occasionally rotate the tablet to get to the button I want, mainly the one to select the template to use at the beginning. I didn't like 123D it worked but didn't rate it as good as SketchUp.

I've changed my design since I started this project but all is good now is the best time to make design changes.

[happy-kat]

The size of three print is going to be 91mm in circumference.

85mm to cover end plus an over hang of 3mm all around.

The suggested print bed size is smaller.!

[happy-kat]

Getting to grips learning SketchUp, current target is testing how to poke a hole through the side wall of the eyepiece holder correctly.

[happy-kat]

My brass m3 threads for plastic inserting came today, need to use one for the eyepiece holder.

It's amazingly powerful this SketchUp and free for the hobbyist, lots of free extension add ons as well.

[happy-kat]

My nice m3 screw arrived yesterday, longer shank and nice head to turn with.

Probably aiming to make the eyepeice holder walls quite thick but the brass thread fitting is 5.2mm long anyway and I need a small coutersunk entry.

Not sure yet what features I will lose when the SketchUp pro trail runs out in 21 days time.

Currently doing a measure three times approach to design the part.

YouTube videos to the rescue for some bits like a cyclinder passing through another cylinder whilst on the same pane.