Conversion C8N into photographic OTA

[FrenchyArnaud]

A few years ago I retired my C8N (C for Celestron, 8 for 8", N for Newtonian - not to confuse with the better known C8) and replaced it on my CG5GT with a 130PDS. 

The CG5GT died and was rebuild using OnStep, nema17 motors and belt drives. 

The improvement in perfs was so dramatic, (RMS routinely <0.70) I thought : well maybe NOW this would be able to handle the C8N (aka "the Beast")  for AP? The 8" mirror has 234% the light gathering power of the 5" skywatcher and has the same parabolic primary than the 200PDS (Both produced by Synta)

The Beast is theoretically able of prime focus with a DSLR but in actual fact does not have enough inner course if using a coma corrector, which means, I needed to move the primary up. And while I was at it, try to improve all the rest! 

So first I disassembled the cell, and changed the 25mm bolts and collimation screws for 70mm ones. 

I also replaced the springs with 30newton ones. 

 

Then I painted both mirrors and focuser tube with a thick, gooey black vinyl paint.

 

 

Finally I replaced the collimation screws of the secondary by small m4+wing nuts. 

 

I also disassembled, regreased, readjusted and painted the focuser (crappy one!) best I could.

 

Then I reassembled everything, took the whole thing at the top of the hill to try and focus on a distant pylone (and find the correct height for the primary)

 

And finally put the Beast on top of the pier this morning! 

As luck would have it, apparently I will be able to test tonight... 

[windjammer]

How much did you move the mirror up?  If it is more than just a bit (scientific term) it might be worth getting an oversize secondary as well if you are missing a lot of the light cone.  You can lose a lot of light quite quickly with even small movements.

Simon

[FrenchyArnaud]

5 minutes ago, windjammer said:

How much did you move the mirror up?  If it is more than just a bit (scientific term) it might be worth getting an oversize secondary as well if you are missing a lot of the light cone.  You can lose a lot of light quite quickly with even small movements.

Simon

I installed rods that are 70mm long and in actual facts allow me to place the primary anywhere I want using the collim screws from original position all the way up to +40mm. When I did tests to find the focus point and place the mirror, it ended up 15/18mm above the normal position. As the secondary is quite massive (supposedly full frame coverage) and I am using an APSC I should be ok. But I won't really know before trying, I might end up with terrible vignetting. No idea yet! Let's say I am cautiously optimistic. But because it's in effect an F/4.5 OTA, the cone must be pretty shallow and that could end up getting off the secondary. Again... Only way to know is to try! 

[windjammer]

Well, suppose the focus moves 20mm forward. You have a 200 mm dia mirror and 1000mm FL.  Consider the half diameter:  the light falls 100mm in 1000mm, or 2mm in 20mm. Taking both sides, the minor axis of the flat is 4mm too small.  Putting the ellipse to one side, what % of the light falls in a circle 4mm wider the the circumference of your flat ?

Suppose for sake of argument flat is 50mm dia, then difference is % light lost = 100*pi*(27^2-25^2)/pi*25^2 = 16%

Only a rough calculation, but you can see how it goes!  Pretty soon you might end up in the same ball park as the 5 inch!!

 

[FrenchyArnaud]

14 minutes ago, windjammer said:

Well, suppose the focus moves 20mm forward. You have a 200 mm dia mirror and 1000mm FL.  Consider the half diameter:  the light falls 100mm in 1000mm, or 2mm in 20mm. Taking both sides, the minor axis of the flat is 4mm too small.  Putting the ellipse to one side, what % of the light falls in a circle 4mm wider the the circumference of your flat ?

Suppose for sake of argument flat is 50mm dia, then difference is % light lost = 100*pi*(27^2-25^2)/pi*25^2 = 16%

Only a rough calculation, but you can see how it goes!  Pretty soon you might end up in the same ball park as the 5 inch!!

 

Yes I know exactly where this is going 😅 I did the maths in reverse actually and the truncated cone by 20mm would result on paper in a primary reduced from the official 203mm to 187mm - which is still PLENTY more than 130. 

My interrogation on the reality of that comes more from the fact that I suspect strongly that the light cone does not fit exactly the secondary even out of the factory.  I suspect it's a tad too small anyway and I have no idea how the CC will do either (it's specifically for newtonians F4>5) 

I know too well that it is not a perfect solution... But I will not know how much of an improvement (or "de-provement"!) it will be over the 5", considering that other factors are at play : length, weight, momentum etc that can also degrade guiding performance. For starters, my pixel scale goes from 1.83arcsec/px to 1.18. So if my guiding in the same time, due to extra momentum, goes from RMS  0.70 to 1.30, the loss of light cone is really, really the least of my problems!

I will know soon enough, nautical dark coming in a couple of hours  I will have a shot at M101 which I did a few days ago on the 130PDS so I will have solid comparison data in terms of guiding, exposure length, good sub rate and IQ. 

And if it is a total fail... Well, It will not be because at the very least I will know a bit more tonight than this morning  And I can get back to the 130. 
And if it works as hoped (hope lives eternal) then I'll gain bragging rights  
There is no real downside, the Beast was collecting dust in the garage since 2018 anyway...

[FrenchyArnaud]

Ok, that did not really go as planned 🤣🤣🤣

On a positive note, the mount proved perfectly adequate, with RMS +/-1.00. So... there is that. 

However the images suffer terrible, terrible abberations, akin to extreme and almost random coma, to the extent that astap was never even able to plate solve. I was able to pass ficus back and forth without ever seeing the bhatinov pattern - that's how bad it was. 

Image luminous though, 5 secs at uso 800 clearly show the star field.

My guess is, combination of poor collimation, focuser sagging and possibly pinched primary. (The thick coat of paint made fitting it in the inner cell a very tight fit,)

So... not sure what's next. I am trying to figure out if there is anyway to test the assembly during day time. I guess I need to take it all apart and try to reassemble more precisely. 

[ONIKKINEN]

Word of warning for the painted mirrors: The coatings in my secondary mirror (OOUK hilux coatings, so not the exact same as yours but anyway) have started to come off around the edges after i blackened them much like you did here.

I cant be sure that the paint was the thing that did it, or the frequent dewing, freezing and thawing that the mirror undergoes around the edges. But anyway i only noticed this after having painted the mirror.

[FrenchyArnaud]

Ok so after looking into it in daylight...

I forgot somehow to tighten the spider supports. When I shoved the laser in the tube, there has been a "clong" and the red spot was bouncing from the laser, off the secondary and hitting half way through the tube  It's actually amazing that I got any image at all. I guess part of the light cone from the primary sneaked onto the secondary and up the tube. I have now very, very carefully aligned everything and collimated as well as is humanely possible and I need to wait for the next hour of clear sky to try... And it's forecast : 100% overcast for as far as the eye can see. 

To be followed!