SCT Collimation

[DavidH]

I have been trying to collimate my Meade 8" f10 SCT, as the planetary views I have been able to get with it are somewhat dissapointing. i.e. Jupiter shows the main two cloud bands and very little else. I can see as much with my ED80.

Got some Bob's knobs to make the job a bit easier, but am unsure as to how accurately SCT's have to be collimated, and what I should expect in terms or performance from this scope. I bought it second hand, so did not get to look through it as new. So far, the seeing up here (Mackay) has only allowed me to collimate with magnification to about 300x, and this has not improved the planetary images.

Any input or suggestions on this subject would be appreciated.

Regards,
David.

[merlin8r]

Hi David,
What magnification are you looking at Jupiter with? It may be there is nothing at all wrong with your collimation. If you are trying to pull too much magnification out of the scope, it's going to disappoint. While in theory you should be able to get 400X out of it, this theory does seem to have a limiting factor. On all but the best of nights I find it hard to beat around 250-300X. The atmosphere is the limiting factor, not your telescope.

Clear skies,
Shane

[rumples riot]

Seeing and mirror temperature play a huge part in looking at the planets. Mackay has had the Jet Stream over it for so long I doubt that anyone would know what the planets look like. (sorry) The Jet can remove fine detail from viewing and also ruins imaging attempts.

The only way to know what temperature your mirror is in relation to the ambient is to install a temperature sensor. Easy enough to do if you are game and can accurately mark your corrector.

Without the mirror being at the right temperature you will never know what the seeing is really like. I left a thread here on cooling a C14, you might like to read it. It documents how to do it and gives some ideas. You need to know what the mirror temp is so that you can know then actively cool it so that you can get it down to see what the seeing is like (hope that made sense). Its a circular thing.

SCT's will hold mirror temperatures for longer than a newt, you need to have it outside for at least 3-4 hours before your 8" would reach equilibrium. My C14 will never reach ambient unless I actively cool it. It is not 2 hours as the popular theory goes. Try double and maybe more times with an SCT.

Collimation on Meades has to be very accurate too. Because they use an f2 for the primary this means that the collimation angle is very narrow and less forgiving than the Celestrons. The 9.25 and C14 have wider collimation angles and allow easier collimation. So you will need to be very accurate with your collimation to get superb views. However, even moderate collimation should get some detail in the belts and the GRS. Also don't forget that SCT have trouble with contrast because fo the large secondary. So while you have your SCT apart do the flocking at the same time. It does improve contrast.

Let us know how you go.

Paul

[beren]

Here's a link to a page that has some instructions on SCT collimation http://home.comcast.net/~astrophoto/Articles/Collimation.htm , have attached a diagram from the page that aids the process immensely but take special note of this rule amongst the text "All that matters is that the screw that is lowest to the ground be called “A” in the drawing and that the eyepiece be pointing up away from “A”.
This info and the use of Bobs knobs makes SCT collimation a lot easier :)

[Mick]

Hi David,

I'm rarely able to use magnifications over 200x with my 8" SCT as well. In my case the mirror temp and conditions seem to be the problem as Paul has stated. I have seen Saturn and Jupiter using a barlowed 9.7mm ep in amazing detail on rare occasions from home this rules out a collimation problem for me. The hardest part is snagging a good night in the tropics for collimation. Good luck mate see you soon.

[DavidH]

Thanks for your reply Shane,

I have been using 11mm and 6 mm eyepieces giving me 182x and 333x. Neither view is great, but as you say the atmosphere is a limiting factor, and I may be expecting too much.

Regards,
David.

[DavidH]

Thanks for the input Paul,



Yes, as long as I have been looking at the jet stream map, it has been blue or yellow over Mackay. Can I expect this to change or is it a fact of life in this area?



Might have a go at this.




Yes, I appear to have a relatively large bright area in the FOV when viewing Jupiter. Moving the planet closer to the edge of the FOV seems to increase the contrast, so flocking may be a good idea. Can you suggest what I should use to do this?

Regards,
David.

[DavidH]

Thanks Beren,

I tried using an Allen key for a couple of nights, but the knobs do make the job much less frustrating.

Regards,
David.

[DavidH]

Hey Mick,

Don't tell me that I'm going to move house. Would like a chance to compare our scopes some night.

Regards,
David.

[jase]

So far this thread has had a visual focus, however I'd like to add a twist. Don't automatically assume that a SCT (or for that matter any scope that requires collimation) which is well collimated visually will perform well for astrophotography (either planetary or DSO).

To get the most out of a scope for imaging you should be collimating with the imaging train in place. Failing to do so can result in significant FWHM and HFD variations as I found out... I used CCDInspector on the 11" SCT I had around 13 months ago. Visually, the collimation was good. Plenty of detail resolvable with focus snapping into sharp contasty views, but when I attached an imaging train results were less than optimal. As it turned out, not only was the collimation out, but so was optical axis squareness with the camera. As camera chips are flat any form of optical curvature has a negative effect, hence the reason why you pay high prices for scopes that have native flat fields. Unfortunately I didn't keep my pre/post CCDInspector graphs to show the results. Still got them for the FSQ (which I'm still battling with curvature probs, but that's another story).

I have simply raised this as for awareness. I wouldn't bother going to the extent of camera orientated collimation if a) you're only doing visual work b) not worried about achieving the highest image resolution possible.

[rumples riot]

The Jet is supposed to move south in the summer months so you might get a reprieve from the onslaught.

I used adhesive backed velvet from Spot light (haberdashery store) and it cost 25 dollars for a 1.3m x0.9 metre piece.

Jase, as someone who is doing planetary imaging all the time, I can say I tried collimation with the imaging train in place early on (3years ago) and never went back to it. It assumes that the chip is square to optical line and you need to use stars that are very bright. You also need to defocus too far to get reasonable results. Tight collimation needs to be performed on low mag stars and only slightly defocusing and then looking at the airy disk and I can thoroughly attest that using the optical method works better than using a chip method. My results speak for themselves.

The chip method is fine for DSO imaging but you need to be more accurate for planetary imaging.

[wolfman]

David, I went through this as well, thinking I had problems with my scope, I had clear night after clear night and could not get a sharp view, I took it to Astro and Bintel to get it checked still the same.
1 week later clear night, WOW! perfect views, seems not all nights are the same, lesson learned.
But I rarely ever go over my 14mm for anything 150X is usually allways good, I have a 8.8mm giving 230X and its a real rare night I would even bother using it.

[MarkN]

G'day there Dave.

The most satisfying planetary views with my 8" LX90 are with a TV 11 mm Plossl. As I don't have a 10 mm EP the next step is a 9 mm Nagler but the 222X it gives is ultimately unsatisfying in that 'scope. Different story in a 10" Newtonian.

As others have indicated, good viewing nights can be few and far between and the mere absence of clouds is not an indicator of such.

Mark.

[DavidH]

Thanks for all your replies,

I guess the consensus is that I am trying to push the scope too far under mediocre skies. Am going to try flocking the scope as I suspect that it is suffering from too much internal reflection. Will let you know how it goes.

Regards,
David.

[acropolite]

I rarely use anything less than 13mm, my best views are often with a 26mm, I suggest you try a lower magnification, unless the seeing and transparency are good even 11mm will be too much magnification.

[DavidH]

Hmm, I'm used to my 8" f7 dob home made with a Murnigan instruments main mirror. The 6mm ep in this scope gives 233x, and is about the "sweet spot" for planetary use. The 11mm ep in my 8" f10 SCT is lower mag than this (182x), so I suspect that I should be able to at least as good a view (albeit smaller) given the same sky conditions. Will have to get both scopes out under the same sky and try to compare apples and apples.

Regards,
David.