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So just as I thought I had the basics sorted out after over a year with two dobs a couple of times a week...

I have a C8 on loan from a friend, and I am back to the beginning with some basic questions:

Image shift - how much is 'normal' ? I estimated last night that the image shifts a few percent of the field when I change focus direction. Is that just the way it is when you move the primary to focus and I need to get used to it? It's a bit disconcerting but more of a distraction than a real problem.

Field curvature - I'm only going to use this instrument for visual, but even so, center to edge sharpness needs what seems to me like a lot of refocus. Would the f/6.3 reducer/flattener be a good item to put on my wish-list?

Vibration sensitivity - there is a certain springiness in the OTA/dovetail itself. Guess that is not going to change. It's just that I never even had the issue before.

By the time I had carried the mount, field tripod, power supply and OTA around, let the whole thing cool for an hour, I was missing my 6 inch 'grab and go' dob...

I do have to say though, it was also a great night once I started looking at the moon rather than the telescope...

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The image shift is pretty marked and a pain for small chip imaging. An aftermarket standard focuser eliminates it. As for the image, I'd rate mine as 'Very good but not excellent.' I never use the reducer visually. A widefield 2 inch EP does far better.

Olly

Edited by ollypenrice

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You can minimise image shift with the standard focuser by always finishing focusing with an anti-clockwise turn.   Anti-clockwise pushes the mirror upwards.  It is a slight pain but works and becomes sort of second nature after a while.

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The C9.25 I have doesn't have image shift but I guess the way to eliminate it is to use a focusser such as the Baader Steeltrack Baader Steeltrack

The reducers aren't of much use visually. They have a restricted aperture and the best way to go wide field is through 2" eyepieces. I can get 1.28° on the C9.25 with minimal aberrations - it's never going to be a come free, flat field without some intervention, but I can live with what I have.

I changed the supplied vixen dovetail for a losmandy type. Far more stable.

 

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4 minutes ago, Mr Spock said:

The C9.25 I have doesn't have image shift but I guess the way to eliminate it is to use a focusser such as the Baader Steeltrack Baader Steeltrack

The reducers aren't of much use visually. They have a restricted aperture and the best way to go wide field is through 2" eyepieces. I can get 1.28° on the C9.25 with minimal aberrations - it's never going to be a come free, flat field without some intervention, but I can live with what I have.

I changed the supplied vixen dovetail for a losmandy type. Far more stable.

 

Interestingly the 8" Edge HD I had had significant image shift. The C9.25 I'm using at the moment has far less and is quite useable without a secondary focuser.

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Interestingly the 8" Edge HD I had had significant image shift. The C9.25 I'm using at the moment has far less and is quite useable without a secondary focuser. 

I wonder if the longer primary focal length of the C9.25 has something to do with it? I can't imagine mechanically they would be much different.

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7 minutes ago, Mr Spock said:

 

 

I wonder if the longer primary focal length of the C9.25 has something to do with it? I can't imagine mechanically they would be much different.

There must be something in it. As you say I doubt they are mechanically different, but the C925 is an odd one out of the line up so perhaps that explains it somehow?

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11 minutes ago, Stu said:

Interestingly the 8" Edge HD I had had significant image shift. The C9.25 I'm using at the moment has far less and is quite useable without a secondary focuser.

This is interesting, it's makes you wonder why there would be any difference? I can only think QC and tolerances - My C8 Edge had less image shift than my C8 xlt, and it was also very minimal with my C6 xlt. The Edge was a truly wonderful piece of kit with hyperwide EP's.

 Depsite their flaws, I always miss SCT's when I sell them!

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3 minutes ago, Chris Lock said:

This is interesting, it's makes you wonder why there would be any difference? I can only think QC and tolerances - My C8 Edge had less image shift than my C8 xlt, and it was also very minimal with my C6 xlt. The Edge was a truly wonderful piece of kit with hyperwide EP's.

 Depsite their flaws, I always miss SCT's when I sell them!

I guess that's probably it Chris. We are working on a pretty small sample here, just the two C925s which have little shift, but your C8 Edge seems to be better than mine was.

I do miss my Edge too. I think I may grab another one in future. The 925 is a cracking scope, but is just that bit bigger and with the narrower fov it's not quite as suited to dark sky DSO hunting (without a GOTO ;))

EDIT Sorry for any thread drift.... 

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18 minutes ago, Stu said:

I do miss my Edge too. I think I may grab another one in future. The 925 is a cracking scope, but is just that bit bigger and with the narrower fov it's not quite as suited to dark sky DSO hunting (without a GOTO ;))

EDIT Sorry for any thread drift.... 

Grass always greener on the side of fence, isn't it:icon_biggrin:

Joking aside, I always try to finish focus with Count-Clock rotations, which seems to minimize the image shift. Also when using the key-pad for centering the objects, always finishing with up or right arrows a few times, which also minimize the image shift.

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5 hours ago, AlentejoSkies said:

So just as I thought I had the basics sorted out after over a year with two dobs a couple of times a week...

I have a C8 on loan from a friend, and I am back to the beginning with some basic questions:

Image shift - how much is 'normal' ? I estimated last night that the image shifts a few percent of the field when I change focus direction. Is that just the way it is when you move the primary to focus and I need to get used to it? It's a bit disconcerting but more of a distraction than a real problem.

Field curvature - I'm only going to use this instrument for visual, but even so, center to edge sharpness needs what seems to me like a lot of refocus. Would the f/6.3 reducer/flattener be a good item to put on my wish-list?

Vibration sensitivity - there is a certain springiness in the OTA/dovetail itself. Guess that is not going to change. It's just that I never even had the issue before.

By the time I had carried the mount, field tripod, power supply and OTA around, let the whole thing cool for an hour, I was missing my 6 inch 'grab and go' dob...

I do have to say though, it was also a great night once I started looking at the moon rather than the telescope...

I have an old C8 (circa 1999) and the image shift when focussing (more specifically changing direction of focus as you say) is very noticeable when imaging on a small chip camera, but not an issue visually (at least not so much for it to cause a problem or distraction). Logically I would have thought that the image shift problem would change dependent on the angle of the scope as the friction between the mirror and the baffle tube would change - I've never tested it myself - might give it a go. 

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21 minutes ago, RobertI said:

 Logically I would have thought that the image shift problem would change dependent on the angle of the scope as the friction between the mirror and the baffle tube would change - I've never tested it myself - might give it a go. 

Good point Rob, this was definitely the case when using the big 16" Meade LX200's way back in my Uni days. I remember siting it as a source of uncertainty for a photometric study I did.

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It's stated in the C8 manual, CCW focusing moves the mirror forwards (against gravitation), therefore less chance the mirror sliding downwards, which happens easier if your last focusing move was Clock-wise. Similar to focusing a refractor, if you point it at zenith, quick rotating the focus knob, when you ends with downwards movement and let go your fingers, the focuser is more likely to slide a bit more.

Edited by YKSE

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Sounds like there is a fair variation between instruments in any case. I haven't got a feel for this one yet but I guess that will come with practice. This was the first time I had it out on my own.

From now on, I will be finishing with a CCW focus adjustment. Thanks for the tip.

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On a new SCT I remember someone suggesting focusing back and forth the whole range of focus a few times. This redistributes the grease and reduces the shift a little.

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With my old C-11 (1990) I used Dow Corning high vacuum grease to improve the mirror movement. That made a lot of difference in mirror shift.

The importance of high vacuum grease is that it does not produce any vapours because of evaporating solvents, so no film on the inside of the corrector or the mirror.
Besides that it holds its lubricating capacities a lot longer then other greases because of that same feature. (except PTFE)

Waldemar

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I've never been bothered by image-shift in the newer model SCT's I've used. Older one's have this problem though. The cure is using a regular Crayord-focuser to do the focusing - after essentially setting the onboard-focuser to the basic area of focus - and then just leave it there. I let the Crayford take it from there. Check to see if the SCT you've got to use has a problem, before dropping $£¢ into a Crayford though.

Enjoy your CAT!

Dave

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I have the 9.25SCT and I have noticed only minimal image shift.

 

I also mod'd the original focuser with a 10:1 type Featherlight, made a huge difference.

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So many great suggestions - which from my side will have to wait a couple of weeks until I get back from the summer holidays...

Two weeks without any optics... has its advantages and disadvantages.

And then I will go for the grease redistribution idea first and put the result here for posterity.  Any modifications will be only once I have a feel for the instrument as it is. It is still such an unknown to me compared to my other equipment. It's so new I am still getting to grips with collimation.

I'd better stop now, I'm starting to miss it...

Thanks for all the answers and support.

Douglas

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So, for completion, now I got back home and tried @beka's grease redistribution tip, as the scope is quite new.

Ran the focuser through its complete range and back 5 times. Image shift when changing focus direction is unchanged. I still don't know how much shift is 'normal' but finishing with a ccw turn is indeed becoming second nature as it seems it should when using an SCT, so that's that for now.

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21 hours ago, AlentejoSkies said:

So, for completion, now I got back home and tried @beka's grease redistribution tip, as the scope is quite new.

Ran the focuser through its complete range and back 5 times. Image shift when changing focus direction is unchanged. I still don't know how much shift is 'normal' but finishing with a ccw turn is indeed becoming second nature as it seems it should when using an SCT, so that's that for now.

With my own CPC 1100 a very rough estimate  of image shift would be about 0.5 arcminutes. 

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