REmoving corrector plate on the EdgeHD scopes

[pmrid]

Has anyone done this?
I'm considering it in order to clean the inside of the plate.
It's a new scope (i.e. I didn't buy it second-hand) and the plate has ever been removed but I can see the marks of finger-prints on the inside and also a smudging around the whole thing where someone has wiped it, in a circular motion, leaving a thin film of gunge. I am more concerned about the finger marks and the possibility of long-term dmage to the coatings through the slight aciity (or is it alkilinity) of the oils from whever's fingers left these marks.

Anyway, the question is about the removal and re-installing of the plate. It is easy enough to put indexing marks around the circumference using cut tape or whatever, but I am a bit anxious about the centering of the plate. The older Celestrons have little cork wedges that are used for centering but I understand that is no longer the case. Are there centering grub screws or anything like it?

Anyone??

Peter

[DavidTrap]

Have you tried posting about this on Cloudy Nights?

DT

[pmrid]

Not yet David, but I will.
peter

[multiweb]

Don't stress about the centering. The orientation is more important. Go for it. It's a no brainer.

[OICURMT]

I'd take it back for an exchange... no way a new scope should have anything like that on the inside.

QA/QC problem at Celestron I'd guess, probably the same now for any company mass producing scopes. Bit of a shame really, considering the money we throw at this hobby.

OIC!

[casstony]

Hi Peter, I've read on CN that there are centering screws for the newer correctors rather than cork spacers. The 9.25 EdgeHD I have has 4 sunken hex head grub screws around the circumference of the corrector that appear to be for this purpose. I imagine one would back them off by equal amounts to remove the corrector, then firm them by equal amounts during reassembly.

The interior of my 9.25 Edge (few months old, purchased recently on IIS) is also quite grubby in comparison to my older C11. In normal daylight some marks can be seen on the corrector; with a torch more marks are visible and even the primary looks grubby in comparison to the older C11. Not-so-clean Celestron interiors seem to be a common complaint on CN in the last 2 or 3 years.

The optics of my 9.25 were very sharp on the one night I've had to use it so far, with good detail visible on Jupiter even at 120x.

A C6 I have between the ages of the other two scopes has a grubby corrector (oily marks or whatever) but a main mirror that is as clean as the C11.

[multiweb]

I got rid of the cork shims on my C11 and threaded small holes so the corrector is held by screws now. Shims are ok also. As I said before the centering of the corrector is no that critical. The orientation is important. I woudn't worry too much about dirt on the corrector. Unless it's really filthy don't sweat it. It won't degrade your image much.

[pmrid]

Thanks guys. I appreciate the input.
I think I'll live with it for a while yet but the idea of threading in some grub screws for my older C14 seems a good one. Next time I pull that one apart I'll do that. I'm thinking about installing a mirror-lock (anti-flop) collar on it which will necessitate removing the corrector so the two jobs can be done at the same time.
cheers all.
Peter

[multiweb]

I had a 3 point mirror lock but since I have changed the mounting of my primary I have zero shift or tilt when focusing up and down the baffle.

I always knew that the baffle tube was machined to extremely tight tolerances. I measured the depression in the middle to be 0.001th of an inch. This acts as a grease trap so you don't shave all the lube dry when inserting the mirror back in.

Last night collimation and viewing confirmed that mirror flop in those scopes (if any) must be caused by weight shift and compression in the rubber and cork that binds the glass to the baffle. I'm now convinced of it. As I say I have now zero flop and I don't use my mirror lock anymore.

If you intend to make a mirror lock I'd recommend doing one lock for the focusing arm rather than 3 bolts at 120 degrees holding the mirror like I originally did. It'll save you some material and a lot of work.

[mldee]

Marc, am I correct in assuming that the cork and rubber mirror attachment ring is now assisted by the bonding material you used to hold the mirror to the new plates, so that there is no flop? Sounds very interesting.

The reason I ask is that my "remote focussing" concept, using a rear-mounted Crayford moving an internal tube attached to the mirror assembly, might not then be needed if I just emulated your attachment bonding. Seems simpler.

My approach also probably removes the ability for the scope to operate in normal mode, as the focus tube would be internal to the baffle tube, partially blocking it. I'll use my RC8 for longer FL stuff, so no concern to me.

Finally, do you think it would be a relatively simple engineering task to machine three 120deg radial lengthwise slots about 5mm wide by (say) 50mm long in the baffle tube for (say) 4mm diam focus attachment points to connect to the inner (Crayford-attached) mirror-push-pull tube? Since the baffle tube basically provides all the mirror support and movement dimensional accuracy, damaging it would be a no-no. How does it attach to the rear OTA plate, I can't see from the outside?

Sorry for the endless questions. What I'm trying to ascertain while it's still fresh in your head is whether I should proceed with my concept or just bond up the mirror for rigidity and concentrate on motorising the existing FT focuser.

[multiweb]

No - there is no more rubber compound, wood or cork. The mirror and the stainless steel ring are now one unit bonded with a compound which is as hard as steel. The mirror does not touch the baffle tube anymore. It's suspended by three M10 bolts with enough OD in the glass to tilt. There is a 5mm gap between the baffle and the glass, plenty to move.

The mirror flop occurs because the glass mirror was originally held by 3 spacers made out of wood (yes wood :rolleyes:) positioned at 120 degrees, embedded in a lot of rubber filling between the glass and the baffle and cork shims at the back and front. I think this works ok for smaller scope (C8/9) but with mirror weights increasing in C11 and C14 the system has its limitations IMHO. I am now convinced this is the cause of mirror flop.


I'm not sure what you mean. You can't access the mirror tube from the back of the casting. Can you post a quick drawing? I assume you're using it with Hyperstar exclusively?

[mldee]

Thanks for the comments Marc. I apologise for my lack of clarity, I have been trying to keep the posts short and not hijack the OP's threads. Perhaps better if I start a new thread on my concept and put all the gory details there. Just handy to have all your comments in one place!

However...Just to clarify my previous mirror-tube comment, presently there are two concentric "tubes", the inner baffle tube that's attached to the rear casting (how?) and the outer "Mirror assembly" tube, apparently consisting of mirror, various bits of wood, rubber and metal focuser bracket, which slides up and down the baffle tube, moved by the one-point offset focuser threaded rod.

My concept is to add a third concentric ~40+mm OD aluminium "focuser tube" inside the "baffle tube", and of only slightly smaller diameter, and extending out the existing rear visual back, where it can be attached to the inside of a Crayford draw tube to move it forward and back. It's then necessary to attach the other end of this "focus tube" to the rear of the "mirror-assembly tube (MAT)" via new baffle tube slotted holes for 3x attachments to the existing metal focuser bracket, to move it back and forth for focussing, while still having the MAT basically supported as usual by the baffle tube. The old threaded focuser rod would of course be removed.

The new inner "focuser tube" would not support the MAT, just move it back and forth via the Crayford drawtube movement. The "baffle tube" slotted holes I was referring to earlier are to allow a connection from the inner focuser tube to the outer "mirror assembly tube" as it moves to and fro over a very limited (40mm?) range for focus. As they don't carry any of the mirror weight, these "connections" shouldn't need to be too hefty.

Clear as mud? I think a new thread is needed. Of course, if one was brave and ambitious, one could also incorporate your collimation feature........Sow's ear and silk purse for a C8 comes to mind though.

[multiweb]

I see. Might be worth starting a new thread. Going inside the baffle tube IMHO is a bag of trouble. You want to add to the existing mechanism and modify the existing stuff as little as possible. The main baffle is pressed intp the casting. I don't think it's even threaded. And it's machined to very tight tolerances. Not worth fiddling with.

[mldee]

OK, thanks Marc, your advice will be taken and we'll dream up plan B, probably a clone of your design as it sounds like it worked well.

[pmrid]

Good points being made here.
May I ask what advantage you see in having a rear crayford with a drawtube extension going up inside the baffle tube? I'm trying to picture this and seem to end up with the image of an unnecessary obstruction to the light path that doesn't give any added benefit.
A good crayford on the rear end is what I use now - using the original focus knob for coarse focus and thereafterusing only the crayford for fine tuning - requiring a travel of only a few mm either way.

And MArc, I can't picture what you have done. Any chance of a bit of a graphic?

I'm happy to have this discussion continue here without a new thread being opened.

Peter

Peter

[multiweb]

Cool - no problem then. Thanks. I posted a flash video which shows you a 3D visualisation of the two rings but I'll make a quick still rendering to show how the thing tilts.

[multiweb]

Here's two other links.

This one (http://www.astropic.net/astro/back_tilt/)from the back of the mirror. [3MB]

This one (http://www.astropic.net/astro/front_tilt/)from the front of the mirror. [3MB]

I made the back alu ring translucent so you can see the stainless steel ring (green) bonded to the mirror.

The blue bits are the existing focusing tube.

[mldee]

Peter, My objective is to convert the C8 into a Hyperstar-only widefield OTA with remote electric focus, and use the new RC8 for longer FL work as it seems to have a nicer field. Until I sold my other C8 OTA recently, i also did as you described when in normal C8 mode. Worked fine. At the moment the Hyperstar is the only scope I have that requires me to go outside to refocus. God forbid I venture outside too often at night:)

With Hyperstar, there is nothing attached to the back, so I could hang a cheap Crayford off there just to elegantly push and pull the focus tube.

The reason I was looking at something inside the baffle tube to do the job was so I could get a symmetrical 3-point attachment to the back of the mirror assy for focus movement, rather than the normal offset single point. But Marc's comments lead me to believe that with proper mirror support as he describes, the single point mode is firm in focus and is a far simpler approach. And as a byproduct, I like the collimation ability as well.

[multiweb]

You might want to look into a CF tube for your C8. I converted my C11 and it keeps focus a lot longer. With the old alu tube I had to refocus every 30min at the beginning of the night when the temperature drops most rapidly. I also now use the old alu tube as a dew shield as it obviously fits the inside of front corrector cell. The added benefit with the hyperstar + dewshield is that the heat flow generated by the back of the camera helps keep the corrector dew free and I usually don't have to use any dew heaters.

[mldee]

Yes, I was thinking of that also, but was reluctant to bring up the subject in this thread.. I know you have mentioned it in earlier threads, so I'll also try a search here. Google is our friend, too.

[mldee]

The IIS thread search was easy, and the US distributor website is now http://www.carbonscopetubes.com/Home_Page.html. Unfortunately they don't do an 8", but start at the 9.25 and upwards. Probably a bit of an overkill for a sparsely-used C8 anyway....
I'll just stick with Marc's mod for the moment. As I commented some threads ago, the C11 does seem to be the sweet spot in the Celestron range anyway. I'll keep an eye out for one on IIS.