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SCT's & off-axis autoguiding - solution ?


Tim

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A week or two back I managed to use the celestron Off axis guider (OAG) in conjunction with my C9.25, but instead of using an eyepiece and manually guiding, I attached an autoguiding camera. I had to use all manner of extension pieces and stuff to get the two cameras to come to focus together, but got there in the end.

However, this was only at F10. Trying to get DSO images at F10 is pretty soul destroying, especially when I have an 8" F4 OTA to compare it to.

So the question is, can the celestron autoguider work in conjunction with the F6.3 focal reducer? The key thing to take into account, is that the imaging camera has to be a certain distance (110mm) from the FR lens, which makes adjusting the imaging camera not really possible. The problem then, is with the guidecam. If you just drop it into the eyepiece holder on the OAG, chances are that it will be waaaaay off focus, and you may find that the guidecam chip needs to move closer to the OAG prism.

I found a very simple way to do this with my Meade DSI Pro, by removing the filter/nosepiece housing, and gluing a 1.25" tube to the face of the camera. This now drops into the eyepiece holder on the OAG and provides enough movement at F6.3 to bring the stars into sharp focus.

The main question people have been asking about this, is "are there enough stars to guide with?".

If you were to put an eyepiece in, chances are you would hardly see a star. Enter the Exview HAD chips.

The really sensitive guidecams can pick out stars much fainter than the eye will see, which massively increases the usefulness of an OAG. Originally I had totally lost my rag with the OAG, it was impossible to use with my skies, visually. But pop a sensitive camera in, and wahey!

The benefits? I have done this to eliminate the flex issues I have when trying to guide a c9.25 with a WO72. Subs over 180 secs started to trail, 300 secs was impossible. As I type, the QHY8 is merrily racking up 400 secs subs with lovely round stars, and in a moment I will be trying 900 secs for a bit.

Also there is the weight issue. I have no need of a guide scope now, which saves me several Kg, makes the mount easier to balance, and means the mount has to do less work.

When I've finished this run, and get round to it, I'll post the test pic (M81 again). The skies are pretty murky, but there's enough to show the benefits of OA-autoG.

The next step is to get the same setup working with the WO72, and with the GSO 8" newt. I have just ordered one of the new TS OAG's to try and figure that into the setup. With 10 days or so away in dark skies for xmas i'm hoping to get that nailed too.

:(

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Steve, when the little one arrives from Bern, i'll have a play, and then you are welcome to borrow it to see if it will play. I dont know for sure that it's going to work well.

The main problem on the celestron OAG is that there is no way to finely adjust the focus, it's just push or pull the camera until its in focus and tighten the thumb screws. This gets to be a pita on an SCT as sometimes you need to refocus after slewing, which of course alters focus on both cameras. The TS OAG apparently has a means of adjusting focus, but on the newt (and hopefully WO72) it shouldnt be as much of a problem.

I ended up going to bed at 06:30, and got up at 07:15. Two targets M81 and M51, just to test the pair together. The seeing was terrible, and the transparency could best be described as 'opaque', but I did get some signal, and the 15 minute subs were devoid of an trace of trailing or oval stars, something which would have been nigh impossible before. I'll maybe process them later.

TJ

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Depending on the required back focus the Baader micro focuser ( #2458125) might be a go. It has a 4mm travel with a T2 connection and 1.25" outlet. I use it on my spectroscope for "fine" tuning the camera position. Only takes up about 28mm.

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I agree Tim, the key to getting the OAG to work at F10 is having a decent guidecam. Are you using a Lodestar?

With my active optics set up once the guider is par focal with the main chip there is no more messing around. The 2 stay par focal regardless of what happens to the scope focus. Provided you introduce the focal reducer infront of both cameras the 2 stay matched to each other.

Not sure an OAG would work with a Mak Newt though, aren't they a bit pushed for back focus or have Skywatcher overcome this.

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I'm going to try it on my GSO when it arrives

http://www.telescope-service.com/accessories/start/accessoriesstart.html#TSOAG9

The mak-newt is a fairly hefty beast by all accounts and would definetly benefit from losing a guidescope.

My guidecam is a meade DSI Pro, with sony exview HAD chip. Although the resolution is limited, the sensitivity is brilliant, and its not even the most sensitive out there.

Geoff, I do my imaging through Nebulosity, and guiding with PHD. Maxim users can use that for both. I have a neximage. I havent tried it for guiding, but wonder if its sensitive enough when used with an OAG? Maybe with the gain turned up a bit.

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Here is the couple of pics, given a quick process. The poor visibility really shows up as noise, but they show the potential. I dont yet have the bahtinov mask for the c9.25, so focus may have been slightly soft too, the poor seeing made it difficult to use the HFR function in Nebulosity.

11097_normal.jpeg

(click to enlarge)

11098_normal.jpeg

(click to enlarge)

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I'm going to try it on my GSO when it arrives

http://www.telescope-service.com/accessories/start/accessoriesstart.html#TSOAG9

The mak-newt is a fairly hefty beast by all accounts and would definetly benefit from losing a guidescope.

My guidecam is a meade DSI Pro, with sony exview HAD chip. Although the resolution is limited, the sensitivity is brilliant, and its not even the most sensitive out there.

Geoff, I do my imaging through Nebulosity, and guiding with PHD. Maxim users can use that for both. I have a neximage. I havent tried it for guiding, but wonder if its sensitive enough when used with an OAG? Maybe with the gain turned up a bit.

Thanks for that, I'll give it a try and let you know the outcome assuming I ever get any clear sky round here, it's been overcast all this week :(

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Here is the couple of pics, given a quick process. The poor visibility really shows up as noise, but they show the potential. I dont yet have the bahtinov mask for the c9.25, so focus may have been slightly soft too, the poor seeing made it difficult to use the HFR function in Nebulosity.

Get me some accurate dims and ideally some pics of the front of the OTA and i'll get you a 3rd order template drawn up also let me know what effectice FL you want ti to work at :(

Billy...

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Righto, when I uncover it all again i'll get some pics. The sizes should be the same as the template you already sent, but the focal length is different. Now that I seem to have cracked the OAG, I will probably use the SCT more, and from what people have said, there is little to gain, and possibly a lot to lose by imaging at F10 with it on a big chip camera, so I will likely stick to F6.3.

Just wish I could use my Moonlite focusser in the combo too!

Thanks Peter

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Righto, when I uncover it all again i'll get some pics. The sizes should be the same as the template you already sent, but the focal length is different. Now that I seem to have cracked the OAG, I will probably use the SCT more, and from what people have said, there is little to gain, and possibly a lot to lose by imaging at F10 with it on a big chip camera, so I will likely stick to F6.3.

Just wish I could use my Moonlite focusser in the combo too!

Thanks Peter

I got a bit confused between an CPC925 and C 9.25 thats why I wanted to recheck the dims TJ Where the other masks clip onto the front of the OTA I had designed the CPC ones to fit inside the FCP cell - can only do this for scopes which I can "trust" the dims on - which means ones taht I have or can get to to measure myslef.. as there is little room for error with this method...

The reason I want to have a lokk and soem dims of the OAG imagign setup is there very little clearance between the back of the scoep and the mount through the forks -althoughthe 800 is better than the 925 in this respect... should have gone EQ....

Billy...

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Here you go Peter.

The stuff attached to the back of the OTA are the following, from left to right;

Skylight filter, F6.3 FR, Celestron Radial guider, Baader adjustable spacer, 9mm spacer, T2 CLS filter, QHY8 Nosepiece/IR cut, QHY8.

Meade DSI pro in the eyepiece holder of the OAG.

The ruler in the shot is in cm. Sorry about quality, it was taken with my phone.

11128_normal.jpeg

(click to enlarge)

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You could try being clever and put a 2" diagonal in there, but its still gonna be pretty tight. The TS OAG that I am trying at the moment may be an option though Peter. I need a couple of different adaptors for the SCT thread and then i'll try the TS guider on the C9.25.

At any rate, I think possibly the fork mounts are less likely to be affected by flex???

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WOW!

This is awesome work - I gave up with my meade OAG for this very reason - I could never get the DSLR and DSI parfocal with each other - even @ F/10 - the focus point for the DSI seemed to be deep into the OAG tube somewhere - probably another indication of my lack of patience :(

Well done - those images are stellar btw.

Cheers,

Richie

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You could try being clever and put a 2" diagonal in there, but its still gonna be pretty tight. The TS OAG that I am trying at the moment may be an option though Peter. I need a couple of different adaptors for the SCT thread and then i'll try the TS guider on the C9.25.

At any rate, I think possibly the fork mounts are less likely to be affected by flex???

I'm only being greeedy TJ...

If i go the OAG route then I wont need to add a third OTA to SCT OTA for my latest hairbrain scheme... want to take a piggyback DSLR shot, Megrez72 DSLR shot and 8" DSLR shot all at the same tiem whilst autoguiding as well for good measure... so would have constellation, 3x1.5 degree and closeup shots all at the same time......

Merrlin66..

I suppose as long as i setup the slew limits properly then the CPC "firmware" should take care avoiding collisions...?

Billy...

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