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Out of curiosity...

emadmoussa

By emadmoussa
in The Astro Lounge

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emadmoussa

emadmoussa

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I've heard too much about the slippery loose 80ED focuser...For me personally, I haven't had much of a problem with it, probably the 1100D I attach to it is relatively light. Anyway, Looking at ASIGN_baz's observatory thread http://stargazerslou...k/page__st__120 I noticed he uses some kind of ''holder''...Now, I'm not sure it's there to lock the focuser, stopping it from slipping especially with that heavy CCD attached to it, or does it have a different function?

Very awesome obsy and kit he's got there, don't you think? :)

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JamesF

JamesF

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It may not be there because the focuser slips (I imagine it would help with that), but to keep the camera central on the optical axis. There's a lot of focuser extension there and a big camera at the end of it. Quite possibly the focuser tube would "droop" without additional support.

Only guessing, mind. You could always ask him :)

James

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The Sailor

The Sailor

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My 80ED gave me no end of problems, slipping when I had the DSLR attached so I stripped it down and found a few loose bit. After a bit of a cleanup and tightening here and there I have not had a problem since, hold the DSLR rigid on the vertical :).

Jim

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JamesF

JamesF

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I'm just guessing, but if it's a 2" focuser tube then the diagonal would normally consume somewhere around 90mm of the optical path, bringing the tube considerably further in. Without the diagonal you've got to provide that additional 90mm somehow to be able to reach focus.

James

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JamesF

JamesF

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Understand that James, did forgot, I understand the problem if he had the diagonal his camera would be akward to fit, would have to sling it low due to the gear above hence the longer tube.

A diagonal is also another optical component that causes (a small amount, granted) of light loss, one more thing to keep clean and another source of potential alignment problems. All in all it's generally easier not to have one for imaging. When I started planetary imaging I left the diagonal in place in my Mak. Life became so much simpler when I realised I could do without it :)

James

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The Sailor

The Sailor

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I can also say the same for the OAG I was using on my SCT last night, had a bad time with it so now being deleted from my inventory,a freebie to a friend. I would have a problem using an extension tube on my setup, as is I am scraping the paint off the window frame on slewing so can't go for any extensions, diagonals it has to be :)

Jim

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The Sailor

The Sailor

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OAGs are supposed to be the way to go for guiding SCTs as far as I'm aware. Why were you having problems with it? Trouble finding a guide star?

James

I had the DSLR attached and an SPC900 above and could not get anything showing but I had a star central in liveview, changed the cam for an EP and still the same. Removed the EP and could see the star in the mirror but it was an doughnut and slightly to the side.

The star was Vega and was perfect in the live view, not quite sure how to fix that. I only put the OAG on there as a tryout, have had it a few year but never used it. As for guiding I have the QHY5 on the ST80 and that works perfect, had both PDH and live view on screen together and Vega was central in both so no problem there.

I am not really bothered bout it so will be letting it go.

Jim

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JamesF

JamesF

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The problem that arises (at least as far as I understand it) is that if you're guiding with the ST80 its short focal length compared to the SCT means that the image on the main camera can move quite significantly before the same change becomes obvious on the guide camera. For instance, assuming your 6" SCT is a C6 with a focal length of 1500mm then the image will appear to shift almost four times as much across the image plane of the SCT than the ST80. If the guide camera also has bigger pixels than your imaging camera it gets even worse. You may find that the SCT image can shift a fair bit before the guide system registers any kind of movement. Even with a focal reducer on the SCT the problem still exists because the differential in focal lengths is so great.

James

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JamesF

JamesF

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What is an OAG ? :huh:

Off-Axis Guider.

A system for getting a guide camera to use the same telescope as the imaging camera rather than needing a separate guide scope, basically by inserting a small prism into the light path where it won't affect the image on the main camera.

James

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emadmoussa

emadmoussa

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Do you really need an OAG?! I'm very happy without it and like the idea of guide scope better :) I guess it reduces the hassle of having 2 scope and more wires...but well...I just love guide scopes - oh, did I just say that? :D

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JamesF

JamesF

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If you have a guide scope that is roughly the same focal length as your imaging scope and the cameras have comparable pixel sizes then I don't think there's any major reason not to stick with guide scopes. The issue is the different in focal lengths of the scopes.

Actually, there's another reason to use an OAG with an SCT or Mak, though it's perhaps more easily avoided. The nature of their design means the primary mirror can shift slightly as it moves from the east side of the mount to the west side. A guide scope will not register that the image has shifted as a result and no correction will be made for it whereas an OAG will. I think some more expensive SCTs have mirror locks to prevent this happening though I'm not sure.

James

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emadmoussa

emadmoussa

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The guide scope is usually used to keep the scope on target. So, basically there's a so-called ''guide camera'' attached to it (be it the QHY5 - which is the cheapest on the market and a replica of the the Orion StarShoot). The guide camera, using the right software (be it PHD "Push Here Dummy") locks on a star and sends pulses to the mount to keep it ''on target''.

I use an ST80 guide scope and QHY5 guide camera mounted on top of the 80ED - all mounted on an NEQ6.

There's a ST4 cable that links the guide camera to the guide port on the mount.

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JamesF

JamesF

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I like the sound of a guide scope - not sure what its for (apart from its guiding capability) is it for the camera ?

It's basically to make sure that the telescope stays pointing exactly where you want it pointing even if the mount mechanics drift a little. Even the likes of a £1000 NEQ6 doesn't track absolutely perfectly.

James

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