A couple of SCT questions

[Leebert]

Hi All,

I've recently got my hands on a Celestron 9.25 and was observing Jupiter last night. I noticed that whilst focussing the image would move about in the EP. I did a fair bit of research before going for the SCT and knew it would happen but my question is what is the typical amount of shift and is there handy hints to reduce it?

The second question is regarding collimation. A star test looks pretty good but I noticed with the CCD in (for imaging Jupiter), it looked worse. Is this typical for an SCT? The EP was used with the star diagonal and the camera straight into the rear of the tube.

Many thanks.

[Peter Drew]

Image shift, to varying degrees is the consequence of primary mirror movement for focusing. It is effectively removed by fitting an external extra focuser such as a SCT fit Crayford type unit, you then do the coarse focusing with the mirror system and the fine with the Crayford. The optics in a diagonal or a webcam are not always perfectly square to the focal plane of the telescope and can produce an offset appearance by comparison, you need to either rectify if possible by adjustment or recollimate on the image that the webcam presents. The webcam produces a highly "magnified" image so the actual difference might be quite small.

[IanL]

There isn't really a "typical" amount of shift, it varies by model and individual scope depending on several factors (see below). As a rule of thumb your scope should be considered okay if your target stays within the field of view at low-medium magnifications for visual work. At high magnifications (for visual) or imaging at prime focus a fair amount of shift is normal.

For example, imaging at the typical f/10 using a small-chip webcam or low-end CCD, it is not that unusual to have the target shoot right out of the field of view if you go from inside to outside of focus; the scope isn't broken, it just comes with the territory I'm afraid. On the other hand if you are using a high-end CCD with a large chip or a DSLR and you can't keep the target in frame when focussing, you may have a problem.

As you probably know, the mirror is mounted on a 'sled' which slides up and down on the central baffle tube that you can see when you look inside the scope from the front. It's similar to a cylinder and piston in a car engine, only in the case of the SCT the 'cylinder' (sled) moves, and the 'piston' (baffle tube) stays put. In an engine, the tolerances between the piston and the cylinder are pretty tight, but in an SCT there is (deliberately) a larger gap between the outside of the baffle tube and the inside surface of the sled.

The reason for this gap is due to the way the focuser works. The main focus knob is attached to a rod which is screwed in and out of the scope as you turn it. The rod pushes on an arm sticking out from the side of the mirror sled, thus as the focus rod goes in, the mirror moves up, as the rod comes out, the mirror moves down. This force is off centre due to the focuser acting on the end of the arm, making it a lever. As the rod goes in to the scope, the rod/arm pushes the side of the mirror above the focus knob away from you, and as the rod comes out it pulls that side of the mirror back towards you. This push/pull on one side of the mirror tilts it slightly thus causing image shift.

If the tolerance between the sled and the baffle tube was too tight, the sled would bind on the baffle tube due to the off centre tilting force, and the mirror would move in a series of sudden jerks (or not at all), which isn't what you want when trying to focus! To try to alleviate this necessary compromise, there is a thick layer of grease between the baffle tube and the sled, which should keep the sled moving whilst minimising the tilt/shift.

Things to do:

a) As Peter has suggested, a secondary focuser is the only way to eliminate image shift. You focus roughly with the main focus knob, and (if the SCT has one) you then use the mirror lock to fix the mirror in place (or hope for the best if yours is an older model without a mirror lock). Then you can fine focus with the secondary focuser which should not produce image shift if it is working properly. More recent/expensive SCTs are supplied with a secondary focuser (often motorised) as standard, but you can purchase an after-market model for any Meade or Celestron SCT.

As ever you get what you pay for. The main thing to consider is the length of the secondary focuser. Cheaper models are a lot longer than the high-end expensive ones. If your SCT is fork mounted you may well find that the combination of secondary focuser and camera or eyepiece/camera will not pass through the fork arms as it collides with the scope base. This would make it impossible to use when pointing at or near to the zenith. It is slightly less of an issue if your fork is on a wedge, especially if you observe from a low latitude, but still a problem. If your scope is on a GEM rather than forks, it is not a problem. The second issue is that you may not be able to use a focal reducer with an after-market focuser (especially the long/cheap ones); SCT reducers are typically designed to go on the rear cell of the scope, and the focusser may add too much spacing between the reducer and the eyepiece or camera to work properly (or at all).

Don't let that put you off though; I bought a cheap Revelation Astro Crayford for my Meade 8" and I wouldn't be without it, though things did get a lot easier when I deforked it and mounted it on an NEQ6 GEM.

Something really cheap you can try is to re-distribute the grease on the baffle tube. It can tend to get too thick or too thin a layer in one place after a while. Simply screw the focus knob all the way in one direction and then completely unscrew it in the other; repeat five or six times. This will move the mirror sled over its full range of travel and re-distribute the grease more evenly. You may find this reduces image shift a bit and/or makes focussing smoother.

c) Depending on the age/model of the scope, you may want to consider upgrading the focus knob bearings. Sites like ScopeStuff offer kits and instructions to upgrade/replace the bearings on various models of SCT. I did this on my Meade, the kit only cost a few quid and took maybe half an hour to fit. The original Meade 'bearings' behind the focus knob were plastic 'washers'; replacing these with proper needle bearings make the focus action smoother and took out all of the backlash in the focus knob too; didn't affect image shift but certainly made focussing much easier and repeatable.

d) Work on your focussing technique. It is always advisable to try to approach final focus from the same direction. The usual advice this should be by turning the focus knob anti-clockwise as the last thing you do. This should push the focus rod in to the scope and thus push the mirror away. This has two benefits:

- The mirror will tend to 'fall' to one side or the other under its own weight depending on the orientation of the tube to the ground. Your objective is to have nowhere left for the mirror to fall after you have focussed; it is generally considered that pushing the mirror with the focus rod rather than pulling it will achieve this but your mileage may vary and you might find that finishing focus with a clockwise turn (pull) works better for you in particular orientations.

- Perhaps more importantly, if you always final focus in one direction, you will find it easier to predict the amount of image shift that is going to occur, and thus you can position your target in the eyepiece or on the camera chip to take this shift in to account so it ends up where you want in frame as you hit best focus. Practice makes perfect and it is a bit of an art (I remember plenty of hours of chasing Jupiter with a webcam, constantly losing it due to mirror shift and poor tracking).

Regarding collimation; you should definitely collimate for whatever your set up is going to be that night. If you are observing with a diagonal, collimate using the diagonal. If you are imaging straight through, you need to collimate straight through (I know that is harder than it sounds!) Also the point about final focus in one direction applies to collimation as well. The fact that the primary mirror tilts relative to the secondary will affect collimation to a greater or lesser extent; make sure that you focus (or set your defocussed star image for rough collimation) by finishing the focus action the same direction every time. This will ensure that the mirror is as close as you can get it to the position it will be in when you are imaging and so collimation will be better.

[Freddie]

You can used metaGuide with your cam to help with collimation. Works really well.

[GlassWalker]

Great thread, and kinda answers/confirms my understanding as a relative newcomer to a SCT myself. I found the focusing to be a frustration, trying to find the sweet spot without going past it, all the more difficult in poor seeing conditions. Already have a Revelation focuser on its way which I hope will help a lot! Kinda wondered if it was worth going for something more expensive, but I'm not going to overload this one with imaging kit...

[DrRobin]

Hi,

I have a C11 and wanted a motorised focuser, so bought a Celestron one, but it wasn't very good, you still get image shift. I went the Crayford route and bought a second hand Baader steeltrack to which I mounted a stepper motor and connected to Dave Trewen's excellent Sharpsky USB focuser. I set the crayford for the optimum distance behind the back of the SCT and then use the SCT's focuser for coarse adjustment with the motorised crayford for everything else. It works really well.

Like Glasswalker, I considered a Revelation crayford, but the Baader came along at just the right time, possibly a good thing as my C11 has the bigger SCT thread on the back and the Baader fits both. I am not sure if the Revelation would have fitted my scope.

I have attached a pic of the motorised Baader. Sorry no pics of it on the scope.

[Leebert]

Thank you for the replies. Thank you IanL for the very comprehensive and detailed reply. It is much appreciated. I've not had an SCT prior to this. It's good to know what I am experiencing is normal.

I seem to have picked up the focusing method quite by accident by backing off and winding in the focus slowly to take up any play.

I have downloaded metaguide and will give it a try next clear night. The views of clusters and double stars have impressed me but I was a little concerned with the resolution I was seeing with Jupiter at high magnification.

A star test revealed what I think is a small misalignment but reading the replies here and searching on the web has meant I will be trying to get this perfectly collimated so that I get the best performance.

At this stage, I think I'll persevere with the standard focusser for imaging the planets. I will be getting a focal reducer in the not too distant future though to use with a DSLR.

I have thought about using a clothes peg on the focussing knob for more control but I'm pretty sure a Crayford style focusser will appear at sometime in the future.

I look forward to trying for a clearer image of Jupiter soon.

Many thanks and clear skies.

[Merlin66]

I used Meade Lx200 SCT's (8", 10" and 12") for many years and found you needed to run the mirror up and down the baffle regularly to spread the grease and remove/ minimise any slop.

I've now changed over to Celestrons (C9.25 and C11). The focusing arrangement is different from the Meade design and IMHO much better than the older Meades...

The slop is minimal and I find just using a Feathertouch 10:1 focusing knob ( which replaces the std knob) to be an excellent solution.

[ollypenrice]

Regarding collimation the standard SCTs still have considerable coma. A visiting optical engineer (who has made several SCTs from scratch) showed me how important it is to collimate using a star well centred in the eyepiece. If you don't you risk getting a false collimation. I was surprised by the level of coma in my Meade, I must say. In focus it doesn't show at all but it certainly does in star testing.

Olly

[GlassWalker]

Just got the focuser... it's one serious lump of metal! Hope there's plenty of back focus distance from the Meade 8"... just need a clear night to try it!

[IanL]

Just got the focuser... it's one serious lump of metal! Hope there's plenty of back focus distance from the Meade 8"... just need a clear night to try it!

You'll have no problem with back focus on an SCT. Despite the many issues of mass-produced SCTs, one of the benefits is that the moving mirror design allows you to move the focus point practically in to next door's garden if you need to. In a Newtonian, the mirrors are fixed and inward focus travel is the problem, so a longer imaging train requires a change to a low profile focuser or even physically shortening the OTA to decrease the distance between the primary and secondary, thus moving the focus point further out (which in turn may require a larger secondary, more obstruction, etc.)

The ideal focus point for the SCT will be in the vicinity of the rear cell/standard visual back and pushing it further out is not ideal, but frankly it is a compromise worth making in my view and a lot easier to achieve than for any other type of scope.

[Hairycamel]

I have a Celestron CPC 925 with a WO Crayford focuser and there is no problem with clearance at the bottom of the forks.

[Leebert]

Thanks for the additional replies.

I have another question that I hope someone can help me with. Once on the mount - a HEQ5 Pro - I could gently rock the OTA a tiny bit. Not loose but more like something flexing. I traced this down to the rear dovetail fixing. It appears to be 2 small screws holding the dovetail to the rear of the tube. These screws are set such that the dovetail is not quite touching the scope body. I guess this is to correct cone error (I haven't checked it yet) but I would have thought it would be more secure? The dovetail has an additional threaded hole in between the small fixing screws that could be used to security more firmly I guess but I don't want to do anything wrong if it's supposed to be like that.

Apologies for the mobile phone quality photos.

Many thanks.

Dovetail1 by MrLeebert, on Flickr

Dovetail2 by MrLeebert, on Flickr

Dovetail3 by MrLeebert, on Flickr

[Merlin66]

Hmmm

On my original dovetail these screws were done up tightly and clamped hard to the outer surface of the cell. Are you sure they are fully tightened??

There shouldn't be a gap. (The other hole shown was for a "push" screw to, as you say, correct cone error. I only make use of this when using tube rings to hold the scope ie smaller refractors etc.)

[Leebert]

Thanks for the reply.

The screws are done up tightly but, with the gap, it feels like they are rubber mounted if that makes sense. I'll drop FLO an email on Monday and see what they say as it came from them.

[Andymod]

Hi Leebert just had a look at mine and as previously stated by merlin my dovetail is clamped tightly to the outer end.

[Andymod]

Just photographed mine, hope you can see it.

[Leebert]

Thanks very much. I'll have another look at mine and see if the dovetail can be tightened to the rear cell.

[Leebert]

After finding an imperial hex key which took a while, the OTA is now a lot secure on the mount. I had hoped to give it a test run tonight but the clouds have come and stopped play before it started