Out of curiosity...

[emadmoussa]

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

Ouch - always beat me to it, James

[Pig]

I have read about mirror locks and was told its mainly a problem with Meade SCT's, I dont know if mine has a lock, it doesent mention it in the manual, mind you it doesent mention image shift either

[The Sailor]

James just to clarify, lets take the ST80 out of the equasion and look at the problem with the SCT. I have the OAG on the 6" SCT, attached to that is the Canon 450D, I have the SPC900 on the OAG, now I was just seeing how the OAG works for the first time. On the live view I had Vega nice and clear, but nothing on the cam, I took the cam off and put an EP and again nothing. Taking the EP off I looked down at the prism and seen Vega as a doughnut and slightly off, am not sure what the problem is or how to correct that but as said not a real problem.

Now on guiding I use a dual mount bar with the guide mount holding the ST80, this holds the QHY and is of no problem .

As said, my orignal idea was to see how the OAG worked as it has been sitting around for at least two years, it was not my intention to use it as a guider, it has now been packed off

Jim

[emadmoussa]

James just to clarify, lets take the ST80 out of the equasion and look at the problem with the SCT. I have the OAG on the 6" SCT, attached to that is the Canon 450D, I have the SPC900 on the OAG, now I was just seeing how the OAG works for the first time. On the live view I had Vega nice and clear, but nothing on the cam, I took the cam off and put an EP and again nothing. Taking the EP off I looked down at the prism and seen Vega as a doughnut and slightly off, am not sure what the problem is or how to correct that but as said not a real problem.

Now on guiding I use a dual mount bar with the guide mount holding the ST80, this holds the QHY and is of no problem .

As said, my orignal idea was to see how the OAG worked as it has been sitting around for at least two years, it was not my intention to use it as a guider, it has now been packed off

Jim

Sorry, by live view...do you mean directly into the telescope or the Canon live view functionality? I'm a bit confused here P)

[The Sailor]

"Liveview" is as used with the DSLR and the image on the computer screen.

Jim

[Pig]

I almost feel clever being involved in this thread

[JamesF]

I have read about mirror locks and was told its mainly a problem with Meade SCT's, I dont know if mine has a lock, it doesent mention it in the manual, mind you it doesent mention image shift either

It's a problem inherent in the design of SCTs and Maks generally I think. I'm sure it would be possible to design such a telescope without the problem, but it would probably be far more expensive

Basically the primary mirror is mounted on a carriage through its centre. The carriage slides up and down the outside of the baffle tube that runs from the back of the OTA towards the secondary mirror, pushed or pulled along by the focuser mechanism. The carriage can't be an interference fit on the baffle tube otherwise it would never move, and if it isn't an interference fit then it's free to rock slightly along its length because there's space between the inside of the carriage and the outside of the baffle tube.

This causes two problems. The first is that when the focuser changes direction, before actually changing the focus it rocks the carriage the other way, shifting the image across the camera or eyepiece. On my 127 Mak there's enough movement there that with a 4x ImageMate a target can move completely across an SPC900 sensor.

The second is that as the OTA moves from the east to west side of an EQ mount the weight of the mirror acts in a different direction and the mirror can rock again. I've also read that the mirror can actually flex, but I can't say that I've noticed that myself.

I've had my Mak apart once already, so I'm not worried about doing it again and once I get the hang of my lathe I might well try to make up a more accurate carriage. It won't totally eliminate the problem, but it might well reduce it. I imagine if the carriage ran on roller bearings rather than just sliding up and down that might help too, but that might well be gilding the lily

James

[Pig]

lol @gilding the lily

Thank you James, I guess mine has built in image shift. I will remember not mention that if i ever sell it

[emadmoussa]

lol @gilding the lily

Thank you James, I guess mine has built in image shift. I will remember not mention that if i ever sell it

we know.

[JamesF]

James just to clarify, lets take the ST80 out of the equasion and look at the problem with the SCT. I have the OAG on the 6" SCT, attached to that is the Canon 450D, I have the SPC900 on the OAG, now I was just seeing how the OAG works for the first time. On the live view I had Vega nice and clear, but nothing on the cam, I took the cam off and put an EP and again nothing. Taking the EP off I looked down at the prism and seen Vega as a doughnut and slightly off, am not sure what the problem is or how to correct that but as said not a real problem.

I wonder if it's just a problem matching the distances of the cameras to get both in focus? Obviously to be in focus the OAG camera must have exactly the same length optical path as the imaging camera. If the OAG camera isn't close enough to right you get that problem that many of us are all too familiar with where the target becomes a larger and larger doughnut until it disappears altogether.

I can't help also wondering whether it's reasonable or not to expect the OAG to pick up a star for guiding that is centred in the imaging camera's field of view (if I've understood your description properly). If not perhaps you need to tweak the prism around a bit to see if you can find another guide star (though obviously if you've already given the OAG to someone else that's fairly academic

James

[JamesF]

Thank you James, I guess mine has built in image shift. I will remember not mention that if i ever sell it

All scopes have their design compromises. That's why it's necessary to own so many

James

[michael.h.f.wilkinson]

I may have got this wrong, but I assume the Cannon EOS is on the T2 port of the OAG, and the SPC900 on the EP port. If Vega is visible in the EOS LiveView, then the EP port should not show it. It is after all an off-axis guider. This means it picks up an image off the optical axis (i.e. outside the FOV of the camera). You do need to see if you can get focus simultaneously. I have used my OAG with an EP for manual guiding in the distant past. if the weather clears, I could check whether the SPC900 will reach focus properly with the my EOS 450 in good focus.

[The Sailor]

James yes I am giving it away but that will not solve the problem. I can see the star in liveview or even in the viewfinder but I have a doughnut in the prism, which to my way of thinking means the prism is out of kilter and has to be moved up or down to bring it into focus, there is a small screw (allen key) on the outside to hold the prism but don't know if it allows it to be adjusted, even giving it away no sence giving him a dummy .

Jim

[The Sailor]

I may have got this wrong, but I assume the Cannon EOS is on the T2 port of the OAG, and the SPC900 on the EP port. If Vega is visible in the EOS LiveView, then the EP port should not show it. It is after all an off-axis guider. This means it picks up an image off the optical axis (i.e. outside the FOV of the camera). You do need to see if you can get focus simultaneously. I have used my OAG with an EP for manual guiding in the distant past. if the weather clears, I could check whether the SPC900 will reach focus properly with the my EOS 450 in good focus.

Yep, have both DSLR and EP in the correct holders, as I understand the prism is seening what the DSLR sees. For instance, back in my range days if I had a problem with someone shooting badly I would put a prism on his rear sight and lie at an angle to him and look throught the prism, I am seeing the target as he see it and I can see his aiming problem. So same with the OAG, the EP holder should see the same target as the DSLR but in this case the DSLR is focused wher the prism is not so needs moving.

Jim

[michael.h.f.wilkinson]

Sorry, no, the little off-axis mirror or prism should pick up something just outside the FOV of the camera. This is important, as otherwise the prism would cast a shadow on the image you are shooting.

[Pig]

Jim,

I think you should aim it in my direction by 1st class post

I am prepared to beg

[The Sailor]

Well took it out of the box and played with the prism, moved it up and down, probably that is the answer, focus the DSLR and them move the prisim, certainly do not understand having to get a seperate target for the prism as in the case of Vega there was nothing beside it to see in the prism. Anyway not a problem for me, I just wanted to see how it worked.

Shaun, have already promised it to someone so sorry, if you do want it off him I will let him know, he is on this site .

Jim

[michael.h.f.wilkinson]

This actually is a problem using an off-axis guider: the choice of the guide star. If there are few stars in the direct vicinity of your target, how do you guide. With two separate scopes you can guide on the target itself (assuming it is bright enough).

[JamesF]

I do wonder if you have such tight framing on a star that there's no other suitable guide star in the entire field of view of the telescope, whether you need to guide at all?

James

[The Sailor]

Thanks all, I will leave it have hijacked this thread enough, will let someone else take the problem, am happy with the guiding I already have.

Jim

[emadmoussa]

Thanks all, I will leave it have hijacked this thread enough, will let someone else take the problem, am happy with the guiding I already have.

Jim

Yep, it'd would £15 please Gotta rush home now and set up the gear...promised clear skies tonight. Hopefully I'll get Saturn if I manage to stay up..

[Pig]

Well took it out of the box and played with the prism, moved it up and down, probably that is the answer, focus the DSLR and them move the prisim, certainly do not understand having to get a seperate target for the prism as in the case of Vega there was nothing beside it to see in the prism. Anyway not a problem for me, I just wanted to see how it worked.

Shaun, have already promised it to someone so sorry, if you do want it off him I will let him know, he is on this site .

Jim

I wouldnt even know where to start with it Jim - I was only fooling