SCT optical/mechanical testing

[matt]

Anyone know of anyone in Australia who'd be equipped to carry out this sort of testing and repair/fixing?

For example, where would I send my 9.25 to have things like the orientation of the corrector/secondary checked or the mechanical side of the OTA (mirror housings etc) looked over?

Your help much appreciated

[casstony]

You might try Bintel - at least one of their people has trained at the Meade/Celestron factory. Does something about your images give you cause to worry?

Tony

[matt]

Thanks Tony

Bintel are not authorised Celestron dealers/repairers. The scope is currently under warranty.

[acropolite]

I think York Optical are authorised Celestron dealers. [1ponders] had some fungus removed by Bintel on a C14 recently it may be worth checking this thread (http://www.iceinspace.com.au/forum/showthread.php?t=11258&highlight=Fungus)

[matt]

Phil.

I can't send it to Bintel. Period. Not yet.

They are not authorised Celestron dealers/repairers and therefore any work they might carry out would void my warranty. That would rule out the chance of a replacement should that be necessary.

The scope still has about 18 months warranty left.

At this stage it looks like York.

[ausastronomer]

Matt,

What's wrong with the scope, or what do you think is wrong with it ?

Re Bintel:

I can appreciate that you are not able to go to them whilst the scope is under warranty, so at the moment that is not an option to you.

Something to keep in mind for future reference is that Bintel used to be a Celestron Dealer prior to switching to Meade. On top of that Don Whiteman, the head telescope technician at Bintel, actually worked for Celestron in the USA for a couple of years as a telescope technician. He is probably the best person in this country to pull apart a Meade or a Celestron SCT.

CS-John B

[matt]

Something I'm very aware of John.

Don't want to go into detail as to what may or may not be wrong, at this stage. As much as I'd appreciate the many and varied diagnoses.

Cheers

[casstony]

Matt, you could entertain us all with collimation/star test images, photo's down the front of the tube, etc :-) I've been following the threads by Jerry Wise on the cloudynights forums - better than watching TV.

Tony

[matt]

Already done the former:D

Not sure what a photo down the front of the tube would be worth other than a photo of a big shiny circle with a black tube poking out of it.

Sorry Tony. You'll have to find your entertainment elsewhere, I'm afraid.

Where is Ken ... by the way:lol:

[Johno]

Matt,

Try asking your original dealer or Extravision. If it's under warranty they will have someone to deal with it.

[matt]

Johno

Have been speaking with Extravision today.

Fingers crossed everything will work itself out.

Gotta have that Plan B in place though...:thumbsup:

[matt]

Here's a doozy.

What sort of equipment would you need to test the optics, particularly misalignment of the corrector and/or secondary, which might cause a soft focus in an SCT????

[Rigel003]

Hi Matt,
I've had a Celestron Nexstar 11 for 3 years and have more recently bought an 11" CPC. I've also had initial misgivings about the optics in each case (I suppose it's natural when you spend a lot of money on a new telescope) - even tried an alternative tube assembly the 1st time. Compared with my 4" refractor, star images were not so crisp and at first I though the in and out of focus diffraction patterns seemed to suggest some form of spherical aberration.

What I've come to realize after lots of time with these optical tubes is that they are pretty consistent in quality. Star images and planetary performance aren't as crisp as in a premium refractor but they're ok and they compensate with much more aperture. Also, proper cool down is very important - vast difference in the star test when the optics are at ambient temperature and this can take hours unless you have some form of active cooling (I've just ordered the Lymax SCT cooler). Also need very steady atmospheric conditions for a sizable aperture to perform at its peak. Just some thoughts that might encourage you not to be too quick to judge yours if you're unhappy.

[matt]

Yep. Am aware of all that and I've had the Lymax Cat Cooler for many weeks.

I've been struggling with this scope for about seven months, so my misgivings are not "too quick" in formation;)

I'm having it optically checked tomorrow. Will let you know whether your advice is on the money or whether I've had reason to doubt.

You never know Graham, it might have been rash to dismiss my apprehensions?

[Rigel003]

Sounds like you've covered all the bases then. Good luck - I'll be interested to hear the results.

[matt]

So.... back to the question currently before us?

Is there an optician in the house?:D

[RAJAH235]

Matt, try DAW Optical Supplies. 10A Henley Road, Homebush West. 2140.
Ph..02-9764-1998. Wolfgang may be able to help.
:D L.
ps. Be careful if you 'Google'.

[matt]

Cheers Raj....

[Merlin66]

Using a Ronchi screen near focus position should show the misalignment and any other optical issues. Have you tried this??

[casstony]

Matt, at the risk of providing another answer you don't really need, checking for concentric circles from the front of the OTA should tell you whether the elements are all aligned on the same axis, or are you referring to their relative rotational positions to one another?

Hopefully one of the experts will become sufficiently tortured by our answers that they will chime in :-)

[matt]

Yep. Their relative rotational positions to one another.

I've got a strong feeling, and it's been backed by a few folk who've looked through my scope, that perhaps the corrector is not optically aligned for peak performance.

I dropped it off today to be checked and fixed, if needed.

Merlin - a Ronchi test will definitely pick up optical misalignment???

Thanks guys.

I appreciate you taking the time to post ideas/suggestions:thumbsup:

[Merlin66]

I think, Yes! The distortion of the Ronchi bands shows the nett effect of the optical train, whether the error is coming from the secondary, primary of any other optic between you and the reference star ie correctors, atmosphere, etc etc.

[matt]

I've been told the Ronchi Test will only reveal optical defects in the primary alone?

It won't show whether your primary-secondary-corrector are all aligned in the optimal configuration.

Who knows:shrug:

[asimov]

Who knows? > http://schmidling.com/ez-testr.htm

[iceman]

I guess you also have to be confident that whoever you take it to can
a) perform the tests accurately
b) analyse the results accurately
c) Give you honest and accurate recommendations

[matt]

You're right Mike... on all three counts.

Finding someone to carry out the first 2 is particularly troublesome if not impossible and the third? Nigh on impossible if you're dealing with anyone remotely connected to the product (ie where you bought it from, dealers etc).

Thanks for that link John, although from my initial reading of that page it looks like it's intended for assessing the "figure" of the optical components and not their alignment/ orientation???

I fear the problem with my scope is the corrector has rotated/moved out of alignment.

Could be a worthwhile investment though.

I'm just about at the point of giving up, to be honest.

No-one seems willing or capable of testing and/or offering a guarantee it will be ok, even if the problem is correctable.

I could go into more detail but that would just depress me more, and I'd need to pass negative comment on a few astro industry "sacred cows".

I just don't need the grief:(

Bottom line (for me, anyway)... unless you can get a replacement, you are on your own kid.

[casstony]

Doesn't Celestron have a service agent in Australia who could pull the corrector out and check the index marks? This is a very simple and quick proceedure for someone who is practiced at doing it, well worth it for Celestron to ease the minds of forum members who may be considering a purchase. I've seen enough bad reports on Meade and Celestron recently that I wouldn't purchase either right now without going through a dealer who would check the instrument up front.

Tony

[matt]

You'd think so, wouldn't you Tony.

One of the big problems I'm facing is finding anyone who can actually

a) Check to see if the corrector has moved relative to the index marks

b) If it hasn't moved, test if that orientation is the correct (optimal) rotation for the corrector

On the face of it these seem like simple enough jobs, for which I'm willing to pay a fee.

But so far all I'm able to find are so-called "experts" whose big idea of optically checking my scope is to stick it in front of an artifical star and tell me whether or not my scope is collimated.

BIG DEAL!!!!!!!!

I can collimate a telescope and I don't need to spend $170 for the privilige of someone telling me my collimation is OK.

I'll say this one more time: It's not the collimation!!!:mad2:

An SCT with misaligned optics can still produce perfect in and out of focus star tests.

It's the OPTICAL ALIGNMENT which i want tested and if necessary... corrected:help:

[Merlin66]

I can assure you 100% that testing the telescope complete against a reference star ( or real star) with the Ronchi test will show the effect of ALL the optical surfaces and the impact of any misalignment, poor optical quality ( ie surface accuracy) of both the corrector ( in a SCT) secondary, and primary. If the system is 100% then the bands will be ABSOLUTELY straight and equally spaced and equal widths with no tapering ( mis-aligned optics) or waviness ( poor optics).
I base this on almost 35 years experience!

[matt]

Thanks Merlin

I'm interested in your comment about the "tapering" in the Ronchi test and how that would indicate mis-aligned optics.

What do you mean by tapering? Got a pic or an example of what you mean?

[Dennis]

Pretty Ronchi pictures can be found here (http://www.atm-workshop.com/ronchi-test.html), along with other links.

This particular link seems to be testing just the main mirror though, not an assembled SCT.

Cheers

Dennis

[Merlin66]

Tapered means the lines appear wider apart at one side of the image than the other. A perfect image/ system will show perfectly straight and equally spaced ( all over ) lines.
Try it, you'll quickly see what I mean. A 120 line grating would be a good starting point.

[merlin8r]

If you have concerns regarding the optical alignment or configuration of your telescope, and its still under warranty, why not just take it back to the original supplier for service or replacement? Then you wouldn't need to worry about cost, as nothing comes from your pocket. Worst case scenario, you get a brand new scope! As soon as you have someone else work on it, you have voided your warranty.
Just out of curiosity (coz I believe there's no such thing as a stupid question), what level of magnification are you using?

[matt]

Yep.

That's exactly what I'm doing in talking with Extravision.

Gotta be careful of voiding that warranty.

Re: magnifications?

All sorts. Depends on seeing etc

I have (on occasions) checked it with the ToUcam in the barlow...watching the image on the laptop screen. Otherwise various eyepieces offering a wide variety of magnifications.:)

[merlin8r]

I don't have a ToUcam, so don't know what focal length it equates to.

[matt]

It's the equivalent of a 5-6mm eyepiece, I believe.

When in the 2.5x Powermate or 3x Barlow... that's quite a bit of grunt in a scope which already has a focal length of 2350mm

[merlin8r]

So, doing the maths on the pessimistic side;

6mm/2.5 = 2.4mm focal length

2350mm SCT focal length / 2.4mm optical focal length=

979X!!!

You don't think this might be overpowering the scope just a teensy bit?

[shredder]

Just a thought. You mention the corrector might not be "optically alligned". If this is the case then you have one of two problems, 1. The corrector has rotated (in which case the alignment markers are going to be misalighed. or 2. The corrector was always out of alignment and the alignment markers are not going to indicate anything.

I would suggest that 2 is probably extremly rare, and probably not an issue.
1. However can be checked by simply taking off the corrector locking ring and checking if the markers line up. Another handy approach is if you still arent satisfied, to loosen the locking ring, line up the scope and by hand turn the corrector until you are happy with the results (mark the new position) and then tighten it all down.

Anyway if Extravision cant help (and hence the warrenty is not an issue) then I would ask Don at Bintel to do this as he suggested this to me for mine.

Cheers

M

[iceman]

Not while imaging.

I have had my 10" newt (FL 1250mm) at over the equivalent of 1400x magnification using a ToUcam, with a 5x powermate + extension tube.

Below is the image I got (Jupiter and detail on Ganymede).

[matt]

Like I said... I use a wide range of powers with the barlowed ToUcam the upper limit and that's for collimating on screen when the conditions allow.

It's actually quite a common and accepted practice.

I am also a planetary imager, and they are the focal lengths I work at:)

[matt]

Cheers Mike.

Beat me to it by a minute;)

[iceman]

It would've been 3 minutes but I had to find that image where I used the extension tube ;)

[merlin8r]

Just trying to help. With my 8" I can rarely get above 250X, even though "in theory" it should go to 400X. I would guess that you could only bump up to that kind of power in EXTREMELY still conditions. If you're not happy with the optics, take it back. Problem solved!

[matt]

Appreciate your help.

We're just pointing out the same rules don't apply to magnification between visual and imaging:D

[matt]

Shredder. Would you carry out this procedure at night, on a star?

What about the four cork shims that are wedged between the corrector and the tube? Would they rotate with the corrector? I imagine their placement is also criticial?

Cheers

[shredder]

Um, well, in short yes, how else are you going to tell if its ok unless you do it at night?

The scopes arent quite as fragile as people seem to make out (at least mine isnt). The cork shims are really quite small and will probably just move with the corrector. Basically I am not saying remove the locking ring, just loosen it a little so that you can rotate the corrector.

I would suggest this:
1. At home take of the locking ring and make sure the alignment marks are there, and aligned, and note the position of the shims.
2. Put back the locking ring.
3. Tighten it up, but not 100% so that with some pressure it can be rotated. But certainly not so loose that it can fall/wobble etc.
4. Take it out at night, sight your star, test it out, rotate it till you think its right.
5. Take it down / inside, and open it up.
6. Note the new position of the alignment marks.
7. Move back the shims to the original position.
8. Tighten back up the locking plate.

An SCT is really not that hard to take apart, and once out of the warrant period well worth doing so you know what people are talking about. Many people wont touch them in case they break but with reasonable care they are ok, I mean after all they survive the postal service from the US to Australia so they are reasonably tough.

But in answer to your question, yes if it really bothered me I would do this, just check the position of everything first in case you make it worse and want to go back to the original position. Oh and the shims are really small, be careful not to take the corrector off at night or you will loose them (they too are also easily replaced). Try it at home first, in the light so you get the feel for what you are doing etc.

And looking over your questions again, no I dont think their (the shims) exact placement is such a big deal, just so long as they are evenly spaced and dont end up all in the one quarter etc.

M

[matt]

Um, well... ever heard of an artificial star?

It's what technicians use 24/7 to check scope collimation etc

Admittedly a real "night time" star is best.

Thanks for the other info

[shredder]

An Artificial star? you mean like Danny Minogue?

Ok so I wasnt thinking so clear when I wrote it. Yes it should work just as well with an artificial star (I dont have one so didnt think of it). Would suggest you check up on how you disassemble your SCT as what I have said is only a rough guide, but its basically accurate.

M

[matt]

Your instructions are very good, shredder.

Much appreciated.

I've got plenty of bookmarked sites on how to disassemble the little beasty. All for a rainy day:)

However, none of it will be used while the scope's under warranty, for the aforementioned warranty reasons.

Thanks heaps for your suggestions and input.

I'm waiting on good news from the distributors this arvo.

By the way... I cracked up at that Dannii Minogue comment!

[Merlin66]

Caution!!!!
The magnificaton quoted was for photographic imaging NOT visual!!!!!!
The aperture in MM usually give a fair guide to visual maximum magnification, photographic/ CCD well that's another matter!!!!!!!!!!!!!!!!!!

[matt]

That's right