SCT Questions

[04Stefan07]

Just a few quick ones.

1) I have seen many different ways to clean the corrector plate on an SCT. Some use solution, some say not to, some say to use a microfibre cloth in a circle motion while others say to wipe outward.
How do you clean your plate?

2) Have read a lot about collimating one of these and it seems pretty straight forward however all guides don't mention the best time to collimate the OTA. Daytime or night time? I know you can buy artificial stars to collimate during the day but for the unfamiliar user looking through an SCT, how can you tell if the star or planet you are looking at requires collimation where poor seeing conditions can be the cause?

3) Got a ShortTube 80 that can be used as a guidescope, what would be the best way to attach it?

Cheers!

[csb]

I bought used C8 XLT with smudging all around the outer edge of the corrector plate, on the inside. After removing the plate, I used demineralised water only with tissue, as per some internet instructions. I mainly wiped in an arc from inside to out. Worked great! Also cleaned the outside with same technique.

This is first time I have done such so I was a bit apprehensive. However it was as simple as the instruction guides said it was. I read many guides and just chose the guide I preferred.

[multiweb]

Does your corrector really need cleaning? It would have to be pretty dirty to really need a clean. Otherwise learn to live with it ;) It won't affect your views. It doesn't have to be spotless. Too much care can be more harmful than anything. My 2c.



I'd clean outwards radially no concentric motion. Microfiber might be a little rough. Think of it as a big lens, not window glass. It is coated and it does have a profile cut in the glass. More importantly put the shims back exactly where they were and also keep the same orientation for the glass. DO NOT TIGHTEN the retaining ring back. Just "touch" pressure. One member tightened his on a C14 when it was cold. When the glass expanded back at room temperature during the day it split in half. Correctors are usually paired with primaries and secondaries so all the elements are figured to work together. You cannot replace a broken corrector. Not with a Celestron anyway. Maybe Meade are more generic, I'm not sure. But I know that with Celestron usually the last bit getting figured is the smallest one: the secondary. And the optical path is tested for aberrations with the primary and corrector in place. So the corrector cannot be swapped or you'll need a new secondary as well.




You can use an artificial star no problem to collimate during the day. You'll need at least 20m or 40m line of sight on a grassy field to minimise seeing. It will get you ball park. A star test is best done at the zenith with the mirror rested in its cell. Out of collimation will show the star flaring sideways. If you want pick a double star then collimate until the separation is nice and clear and the two stars look nice and round.

When you use an artificial star because there is no drift you'll be able to go to very high magnification to the point you'll see the airy disc and the little dot in the middle with the first (maybe the fainter second) concentric ring. It will be obvious when you're collimated.

[g__day]

Must say to the second part of the question I like to collimate at night time, scope cooled and pointing near the zenith - but I greatly prefer the way out of focus method - create a doughnut on your monitor or eye piece and adjust your Bob's knobs or whatever until the ring in the doughnut is perfectly centred.

Each time I adjust collimation I tend to re centre the doughnut. Either method works - but I find the doughnut method (with a software tool if required) is a bit faster and a bit better for me.

Once the hole is centred and very circular I just then get the tightest focus I can and observe the star diffusion ring pattern. If you have a lot of upper air turbulence I find the doughnut method is far better to use!

[sharptrack2]

:thumbsup: to Mathew's approach. I did a crash course in star collimation out of necessity and have developed a similar approach.

I collimate at night as well. The doughnut method works great to get started using a 20 or even 15mm EP. I then quickly move onto 9mm and 6mm EP's, adjusting as I go. Use very fine adjustments, you can get skewed off to one side very quickly if you over adjust and try to compensate (learned the hard way ;) )

I currently have managed to get mine down to a 4mm EP and collimated to a Mag4-5 star. The Airy disk is very stunning when the seeing is good.

Oh, and I have not "cleaned" my corrector plate since purchasing the scope (4-5 months or so now), I simply wave a eye glass cleaning cloth around a few times to blow away any dust or bits that might have settled before I put the cover on.

[04Stefan07]

Yep it is no way dirty enough to clean in my opinion so I might leave this for now, don't want to risk damaging it!

Thank you for the helpful advice, will take this up.

[04Stefan07]

Forgot to ask something else!

Added question 3 :)

[julianh72]

Celestron make a piggy-back camera adapter (Part No. 93609) which will fit pretty well any SCT (I've got one on my Meade 8" LX90), and you can mount a camera or a guide-scope using a standard camera-mount base.

http://www.celestron.com/browse-shop/astronomy/astroimaging-accessories/other-astroimaging-accessories/universal-piggyback-mount

They're a bit hard to find in Australia, but you might be able to find one in the IIS Classifieds or similar, or you might find something similar made by another company.

Alternatively, you could fit a Losmandy rail to the top of the OTA and mount your guide-scope that way:

http://www.bintel.com.au/Astrophotography/Astrophotography-Accessories/Losmandy-8--br--Piggyback-System/1388/productview.aspx

[04Stefan07]

Thanks.

I already have a Vixen dovetail and tube rings for the OTA. Would something like this work?

http://www.bintel.com.au/Accessories/Dovetail-Plates---Adapters/Losmandy-Curved-Block-Set-VCB8/1257/productview.aspx

[DarkArts]

I agree with Marc for cleaning etc. I have had cause to use a 50/50 mix of IPA and de-min water on sap and oily residue stains (?) from God-knows-what on a corrector, but only once. I think regular tissues are harsh, so used a super-soft MF cloth (fresh from the packet).

I collimate roughly in the day with an artificial star (e.g after major change to the optical train) then fine-tune at night. I've always done it visually with an eyepiece but want to try it with a high-speed camera, now that I have a good one.

HTH.

[sharptrack2]

If it's any thing like my black body Celestar 8, there might not be a position to install it in. You may need to have it perform double duty as a finder and guide scope. The better choice would be the long bar on top.

[h0ughy]

http://www.iceinspace.com.au/forum/attachment_browse.php?a=186712

http://www.iceinspace.com.au/forum/attachment_browse.php?a=166706

http://www.iceinspace.com.au/forum/attachment_browse.php?a=145339

[ngcles]

Hi Stefan,

I was gifted a used 20cm Celestron CPC several months ago. After I brought it home I discovered a very ugly/nasty-looking stain on the inside of the corrector plate (see attached photo). The previous owner had drilled a small hole in the corrector plate mounting cell to attach something and a fluid had made its way inside the tube at some stage and dribbled down the back surface of the corrector. It looked as though the coating was flaking and or corroded.

I had never attempted to clean a STC corrector before (I'd never owned an SCT) and was naturally very apprehensive so I approached Don at Bintel for his expert advice -- and sent him a photo. He felt that it was probably only a water stain and would be very surprised if the coatings had been damaged. He advised to clean it inside and out with a wad of clean cotton waste and Windex. He said the coatings were very hard and durable and that Windex was just fine for the job.

I found there are many youtube videos demonstrating how to disassemble the corrector plate and it took less than ten minutes to remove after I created a couple of registration marks so that, upon re-installation, it would be back in precisely the same spot.

It took just a couple of wipes to remove the offending stain and after a thorough cleaning that took about five minutes it was re-installed -- squeaky clean like a brand-new one.

Star testing that night revealed collimation was still A1.

Don't know about whether that cleaning advice could be applied to corrector plates and or coatings made by other manufacturers, but it worked a treat with the starbrite coatings on the Celestron.

Best,

Les D

[multiweb]

OAG is the only way to go with an SCT. Plenty of back focus anyway.

[brian nordstrom]

:eyepop:man thats UGLY !!! , glad it cleaned up ok and yes the coatings are very sturdy , just dont rub them dry and you will be fine , windex is good stuff.

Brian.

[bigjoe]

Thanks ngcles.

I was just about to attempt to clean my 10"Meade SCT so thank you for your timely cleaning method .:thumbsup:

bigjoe.

[g__day]

For guiding at full focal length on a large SCT - on or off axis guiders should be your primary solution. Differential flexure will kill your guiding every time if the mirror shifts or flops by even the slightest amount varying with elevation. The tolerances are miniscule - so most folks find out after a lot of pain your have to go with a guider splitting light (on or off axis) from the primary imaging train to get great results.

I use an Off axis (giant Lumicon) prism and get great results normally. Only when I can't find any guide stars (using a not too sensitive Meade DSI II Pro mono guider to I switch to a side by side (actually piggy backed side by side) 90mm refractor with a Lodestar guide camera (but that is my last result fall back - only when the OAG's camera can't find a guide star)!

[04Stefan07]

I found the required adapter for a guidescope, needed a VCB8 set. I think I know which screws I can take out to mount it but I just want to confirm with someone who has done this.

The last thing I want to do is disconnect the mirror/glass from the tube!

There are a bunch of screws with a round top and a flat top. I am guessing the flat top ones are holding the mirrors and the round tops you can remove.

[The_bluester]

All my answers..

I occasionally clean mine, the only time it really needed it was after the bushfires went through our place two years ago, the smoke left a light haze on the corrector. I remove the corrector, remove the secondary holder and with the corrector on a felt base, clean using a straight motion, inside to outside with a soft cotton ball and some of the Bintel solution.

Collimation is not difficult, I do it at night with the scope properly cooled and pointing near the zenith. Usually star images tell the tale, if it is a little off, bright stars loose that "just so" sharpness and look muddy, same for planetary images.

Connecting a guidecope, I deforked my CPC925 and fitted a dovetail, probably the simplest method to fit a guider would be to fit a second dovetail bar to it and use dovetail clamps or similar to mount it.

[Sylvain]

Unless your corrector is visibly dirty, I would refrain from cleaning as suggested above. You have to realise you are chasing a very tiny and most likely unnoticeable gain in transmission while running the risk of mishandling your corrector, scratching it, bumping it, reassembling it not perfectly aligned. Just not worth it in my opinion, unless the plate is significantly dirty, but that is rather unlikely if you keep the lid on the scope while not un use.

Collimation is alwaus best done at night, but to achieve an optimal result you will require good seeing conditions, which you cannot control. A good alternative is an artificial star (have a look online) which lets you do all the work comfortably during the day. Depending on the pinhole diameter, you do need to be relatively far, hence why the optical fiber ones are great.

Regarding the piggybacking, Celestron used to sell a mount for piggybacking a camera, but that would not be the best option IMO. Your best bet is to have a dual dovetail plate (have a look online) and mount the 2 in parallel. Lets you add some equipment to it too because it's very sturdy.

Good luck!

[kens]

An alternative form of artificial star, which sounds easier to make reliably, uses a ball bearing.
https://stargazerslounge.com/topic/264094-artificial-star-alternative-method/

[julianh72]

Yes, a ball bearing works beautifully.

You can also use it for getting some nice calibration spectra with a Star Analyser SA-100 grating. Use a variety of light sources (incandescent bulb, halogen bulb, compact fluoro, "white" LED, etc), and in each case, the pinpoint "star" will generate a beautiful test spectrum.

Use a white light source such as a halogen bulb, and fit your various filters in front of the grating, and you can instantly see the absorption characteristics of each filter.

[Nikolas]

how far away do you place the ball bearing from the scope??????anyone do a diagram?

[julianh72]

I use a 10 mm ball bearing at a distance of about 20 metres, as that is as close as I can get my 8" LX-90 to focus. I attach the ball bearing to a piece of black cardboard, to give a nice dark background to work against. At that distance, the Sun's image on the ball bearing is a brilliant pin-point of light - by my calculations, about 0.25 arc-seconds across, so easily fine enough to do collimation etc.

My 90 mm Mak will focus down to about 8 metres, which makes it easy to use indoors, by setting up down a hallway or in a large room, and using a halogen light pointed at the ball bearing as an "artificial Sun".

[Renato1]

I find collimating my C8 to be very difficult using the doughnut technique spelled out in my manual.

Instead I use a 5mm eyepiece to give me 400X on a bright star, and look at how concentric the diffraction rings are, and adjust if they are not. Only problem is I might have to wait for one or two weeks before I get a night when the tiny diffraction rings stand out well rather than being a mushy blur around the star.

For cleaning the front corrector plate I use distilled water mixed with the proper isopropal alcohol (not the foot rubbing kind), together with lots of little lens cleaning tissue or DSLR CCD cleaning tissues, gently dragging the tissues out from the centre - with the plate being nearly perpendicular to the ground.
Regards,
Renato

[Nikolas]

Cheers
That may be an issue for my c9.25 though...

[julianh72]

Most SCT's will focus down to about 20 metres or so - if you can reach focus shooting across your backyard (or across the street), you can use a ball bearing for a daylight star test.

Preferably, set up with the sun behind you, fully illuminating a fence post or similar that you can focus on. (It's not essential to have the Sun at your back, as you will see a "glint" image of the Sun on the ball bearing from almost any position, as long as the ball bearing is not shaded. (If you set-up with the telescope facing South, you can actually view the Sun's image in the ball bearing from sunrise to sunset without ever needing to move anything, and without any risk of direct sunlight entering the OTA.)

Mount a piece of black card (for contrast) with a 10 mm - 20 mm chrome ball bearing on the post, and then focus on it in full sunlight.

Focus on the ball bearing, and you will see a minuscule "glint", which is the image of the Sun. Because the ball bearing acts as a reducing mirror, the image of the Sun's reflection as viewed in the telescope is MUCH smaller than the Sun itself. When you do the maths, the glint is approximately 25 microns across for a 10 mm ball bearing, and at a distance of 20 metres, this translates into a pinpoint that is about 0.25 arc-seconds across, which is plenty good enough to do your collimation tests etc.

The method works just as well inside or at night, using an artificial light source, such as a halogen light, as long as you can focus on the ball bearing.

Point the light at the ball bearing, but try to keep the light source some distance away from the ball bearing (to keep the image as small as possible). (If you imagine a light source that is right up close to the ball bearing, the reflected image of the light could be almost as big as the ball bearing.) If the light source is up near the telescope, the image of the light source will be minuscule, as desired. I find a halogen desk light works well, as the source itself is small and brilliant, and it can be placed near the telescope, directed at the ball bearing, without too much distracting light spillage coming back to you at the telescope.

[Nikolas]

Thanks
Will give it a go :)

[5ash]

I believe collimation using an artificial star can be used to precisely collimate a SCT if it is done correctly. The distance of the artificial star is of importance as the diameter of the star in arc minutes must be smaller than the resolution of your scope, after all we can't resolve a star as a disk. For this to happen the physical diameter of the artificial star must be unresolvable at the distance it is observed. For an 8" scope using a common diameter star this would be in excess of 40 metres. Seeing and convenience are the best reasons for collimating this way. It is rare for seeing to be good enough to observe a star at high power for collimation .
Philip

[julianh72]

It depends on the diameter of the artificial star - and that is what makes the image of the Sun on a small ball bearing such a good artificial star.

The Sun's image on a 10 mm ball bearing at a distance of 20 metres is about 22 microns diameter (about half the size of the hole in commercial "artificial star" devices), which translates to a subtended angle of about 0.25 arc-seconds, which is below the theoretical resolution of even a 12" SCT. It is a brilliant pin-point of light, which is easily observed when the ball bearing is placed against a black card or similar.

[multiweb]

Sorry, got to disagree with this one. When you have an SCT in excess of 10" looking horizontally at an artificial star there are a couple of things to look at. First the distance between the mirrors will be less than ideal when you're in focus at 30m or so. If you use extension tubes at the back it will only make things more difficult. Second, the scope is not pointing up and the weight repartition is all wrong. If you think you're collimated accurately when you do a star test at the zenith I can guarantee you'll be off by a fair bit. So it gets you close enough to do a star test. I see people tend to over complicate things as well. Ball bearings, diameter of the reflection dot, etc... Truth is that it doesn't matter. Just get a small length of discarded fibre optic or even a small fishing line, stick it through a piece of black cardboard and shine a led torch at the back. That's all you need to get you close. Do a star test to finish off and pick a star right up.