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Guidance on guiding


Guest ecopley
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Guest ecopley

Hi all.

I wonder if any of you have any experience with off axis guiders? I read lots about them and decided that an oag would be a better guiding solution than a separate scope setup so I bought one.

 

I have a 6" SCT on a celestron advance mount. I'm using the celestron OAG which is fully adjustable in pretty much every way you could imagine - it rotates freely about the imaging axis and the prism (which seems very large compared to others I have seen) is adjustable in and out of the frame as well as having a screwthread focussing mechanism to focus the guide camera separately from the imaging camera. At the moment I have the prism fully in as my imaging camera (canon 450d) doesn't seem to mind. I can sometimes find a guidestar visually but I am struggling to get my guide camera (i'm using a zwo asi 120MC) to pick up a star strongly enough for PHD to guide on it.

 

 

Also, much more irritatingly, I'm struggling to find guide stars that will allow me to frame the object I actually want to image. Ive tried using longer exposures - sharp cap will allow very long, upto 1000 seconds. PHD will allow upto 10s. I've tried altering the gain and tinkering the drivers. I just can't seem to find suitable stars to guide on that the camera will pick up - even on the occasions when I can see them visually.

 

 

It took me a while to realise that the eyepeice and the camera aren't parfocal. It's really very difficult to focus the image on the camera because, even in sharpcap with long exposures, the camera just doesn't pick up very much. So I use my illuminated reticle eyepeice to centre the guidestar (unframing my picture in the process) and then take a long time to focus the guide camera. Even after focussing it as best I can it still struggles to guide.

 

 

Clearly i am doingsomething wrong!!

 

I think the camera is at least part of the problem - there seems to be a more successful outcome using the lodestar camera.

 

Does anyone have an opinion before I dive in?

 

Also, lots of people have suggested that a separate guide scope is, in fact, the way to go. I can see lots of advantages to this setup but I can't see how I would fix a guide scope to my sct. Does anyone have a picture and/or a description of such a set up.

 

Many thanks,


Ed.

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Guest ecopley

I've been having a play today and have explored binning with the zwo. This might make a big difference. I've also realised that I can alter the focus of the camera by adding an extension ring and the eye piece by not inserting it fully so that might make the focusing easier. The oag yields good images during the day so there's nothing wrong with it optically. I just need to find me some stars.

Still thinking about the lodestar but will have another go tonight and see where I get to.

I'd be very interested if anyone had any comments (helpful ones I mean, James ;))

Cheers,

Ed.

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Guest Kheldar

Par focal (or very damn close) is best achieved on a distant daytime target.

Your guide camera is probably going to need a good extension to get it par focal ... However it's equally possible it needs more in focus which could be a problem ...

Guide scope on an SCT is not something I would recommend - the focal ratios are too far apart and your guiding won't be accurate enough (IMHO)

Edited by Kheldar
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Guest Kheldar

What I didn't mention is that also using an OAG on an SCT you're going to need a good guide camera due to lack of light. The zwo and similar might just not be sensitive enough.

If you have a lodestar - use it :)

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Guest ecopley

OK, that's what I thought and very helpful. Thank you very much!

I don't have a lodestar but I'm thinking of getting one. I'll have a tinker tonight and see how I get on with the binning and what not but I'm already more than half way to hitting the buy now button.

Cheers!

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I had some success guiding my 8" sct with a st 80 on a dual mount bar, I had a 6.3 reducer on the back of the sct with a canon 1100d imaging, the st 80 was guided with a atik titan using PHD, the PHD graph was one of the flatest I have had, managed 14 5 minute subd of m51

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Guest ecopley

Interesting to hear that you have done the separate scope guiding setup up with such success but this is where things get tricky.

Firstly, how does one go about mounting two scopes on one mount? As far as I can make out there are two types of dove tail. Vixen and Losmandy. For a start, is this right. I'm assuming the dirty great 11 inch SCTs use Losmandy and my little tiny baby 6 inch uses Vixen. So one way would be a double Vixen (no sniggers, please)?

I have also seen pictures of people's rigs with one scope mounted on top of another, kind of like a finder scope. I gather they have to be extremely rigidly fixed. Do I need to investigate rings? What an I looking for and where would I start with that?

Looking into the lodestar camera it seems that it was superseded by the qhy5l ii but, interestingly, that has the same sensor as the asi 120.

Not really sure what to do other than keep tinkering at the moment. The oag seems to come in and out of favour depending who you read, what they want to do and what other kit they have.

It's all just very frustrating. Surely it isn't really this difficult? Maybe I am being impatient in finding my guide star. The annoying thing is that I can see them. They are definitely there. I just can't get the camera to pick them up.

Hmmmm. Alcohol will help me think clearly. Maybe a beer and then some more tinkering.

Thanks for the advice so far. Keep it coming!

Happy skies!

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dual mount bar is the way to go if your using different scopes, for example i use the sct for moon and planets of course, sometimes dso but i also have a starwave 80 which is good for dso imaging, using the dual mount method allows me to swap scopes while leaving the st80 guide scope where it is.


http://www.firstlightoptics.com/adm-vixen-type-v-series/adm-v-series-side-by-side-mounting-system.html


i`m not saying you need to buy this dual mount bar but it works well for me, i did piggy back the st 80 on the starwave with great effect, but had to keep stripping it down when swapping to the sct which was a pain.


guide cameras come in all shapes and sizes, i`ve used a cheap philips webcam with good effect but takes some setting up as the focus needs to be almost perfect to get a star, but it does work, the atik titan is great,never any trouble with that, just turn on, slightly focus and away you go, but they are expensive for just a guide camera, good are the qhy 5 older versions as well as newer ones.


again all depends on budget. 


but webcam £65 ish


dual mount bar £80 ish second hand,


st80 guide scope £65 second hand.


FREE SOFTWARE ! phd, and astro photograpy tool for me.


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Guest Kheldar

Looking into the lodestar camera it seems that it was superseded by the qhy5l ii but, interestingly, that has the same sensor as the asi 120.

 

No it didn't :) Where did you get that info from?

 

The QHY5 II only replaced the QHY5 I - both are respectable guide cameras, I owned one myself for a good couple of years when I used a dual scope setup (though I did not have the focal length of the SCT - I was imaging on a 200P with a similar focal ratio to the guide scope which was an ST80)

 

But I will never go back to one now having used a Lodestar :D

Edited by Kheldar
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Duel scope guiding with an SCT will not compensate for any mirror flop or movement which can be a problem with an SCT.


 


 


with the SCT the FOV is very narrow so it may still be a bit hit and miss for a guidestar with the OAG- get one you can rotate and lock


 


Steve


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  • 2 weeks later...

I have an ST80 attached to my 8" SCT, which I use to guide:


 


http://www.eastmidlandsstargazers.org.uk/topic/7494-new-observatory-to-house-my-mount/


 


I use a Losmandy dovetail to connect to the mount and a Vixen one to attach the guidescope. The guidescope can be quickly removed by undoing one knob.


 


All seems to work fine from a guiding point of view (at least with the HEQ5 mount I used previously). I have suffered from mirror flop though.


 


I've used the holes that are already there to attach the two dovetails, although I had to make up a small bracket for the Vixen one on top as the hole spacing was wrong at one end.


 


I intend to use the same set up with the C11 I've just bought, not sure if that will track as well.


 


Dave


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