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Using an OAG with a Newtonian


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I'm experimenting with using a ZWO OAG (mkII) with my SW 200dps + Baader CC (mk III) & a ZWO Mini 120MM as guidescope. So far I've got it all connected & have set up PHD2 with the calibration assistant.

The only way I could connect everything was with a 21mm M42 extension tube between the camera (ZWO 2600MC PRO) and OAG; & using the M48 OAG adapter to fit the Baader CC, which then inserts into the focus tube.

In order for the OAG prism to be clear of the camera sensor the camera orientation is limited, so I suspect it'll be a case of keeping it one position in relation to the guidescope. The first question then is, if I want to rotate the whole assembly to either obtain better target framing, or to find a guide star, will I have to redo the guide calibration? As I've previously used a finderscope for guiding I always redo calibration each night when setting up, but with the OAG will I have to re-do it a second time if I have to rotate the assembled camera/oag/guidescope in the focus tube?

Also - last night after focusing the main camera/scope, the guide camera distance didn't seem to make much difference to the view of the star in the guide camera or star profile image in PHD2. I understand that it should be set parfocal with the main camera, but how critical is the distance & how do you tell if I can't see it very well in PHD2?

Many thanks!
Ivor

PS: I'm aware that the 120MM may not be the best, but current budget didn't extend beyond the OAG & Helical focuser.

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1 hour ago, Aramcheck said:

if I want to rotate the whole assembly to either obtain better target framing, or to find a guide star, will I have to redo the guide calibration?

Yes you will have to re calibrate guiding because the angle of the camera will change and that will impact on what angle PHD2 sees as north/south and east west... 

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1 hour ago, Aramcheck said:

The first question then is, if I want to rotate the whole assembly to either obtain better target framing, or to find a guide star, will I have to redo the guide calibration? As I've previously used a finderscope for guiding I always redo calibration each night when setting up, but with the OAG will I have to re-do it a second time if I have to rotate the assembled camera/oag/guidescope in the focus tube?

Yes.

It is best to Find/Frame/Focus the target and then move scope to meridian / dec 0 to calibrate just before starting imaging run.

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1 hour ago, Aramcheck said:

Also - last night after focusing the main camera/scope, the guide camera distance didn't seem to make much difference to the view of the star in the guide camera or star profile image in PHD2. I understand that it should be set parfocal with the main camera, but how critical is the distance & how do you tell if I can't see it very well in PHD2?

I find that there is not much tolerance at all... I find that a guide star can go from a round point to a oval within a guide cam movement of 1-2mm... 

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1 hour ago, Aramcheck said:

 

In order for the OAG prism to be clear of the camera sensor the camera orientation is limited, so I suspect it'll be a case of keeping it one position in relation to the guidescope.

If you're using an OAG you don't need a guidescope do you? Keeping the OAG prism aligned with the long side of the ASI2600 sensor allows it to pushed into the light beam further, which may be helpful.

1 hour ago, Aramcheck said:

The first question then is, if I want to rotate the whole assembly to either obtain better target framing, or to find a guide star, will I have to redo the guide calibration?

Afraid so, each time the camera guider assembly rotates the orientation changes so a recal is necessary.

1 hour ago, Aramcheck said:

Also - last night after focusing the main camera/scope, the guide camera distance didn't seem to make much difference to the view of the star in the guide camera or star profile image in PHD2. I understand that it should be set parfocal with the main camera, but how critical is the distance & how do you tell if I can't see it very well in PHD2?

A trick I use which may help is after focussing the main camera, switch to the PHD2 view and select a star then carefully rack the focusser in or out and see what this does to the FWHM or HFD figures.  Keep a careful note of which way and how far you moved the focus.  Then return to 'imaging camera focus point' and adjust the guide camera in or out by the amount the focuser moved.  It should be possible to get pretty good stars.

1 hour ago, Aramcheck said:

PS: I'm aware that the 120MM may not be the best, but current budget didn't extend beyond the OAG & Helical focuser.

I have been using an ASI 120 mini mono for a while now on an OAG and it has never failed to find guide stars, usually a full suite of 12 are available.

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1 hour ago, Aramcheck said:

PS: I'm aware that the 120MM may not be the best, but current budget didn't extend beyond the OAG & Helical focuser.

Theres nothing wrong with the 120MM.. I use the 120MM mini and I'm very happy with it.

 

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Are you really sure you want to rotate to frame? Rarely, if ever, do I find it necessary. I'd much rather shoot in one of two orientations, 'landscape' (long side along RA) or 'portrait' (long side along declination. That way I can easily add more data at a future date. Recreating a random camera angle takes forever.

Olly

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just to add that I'm using a Baader MPCC too and the guide stars in my OAG are awful- much worse a shape than when I was using an RCC1- but it doesn't vignette like the RCC1. It isn't a focus issue but rather the CC not correcting very well that far out I think. PHD2 doesn't seem to mind though- it seems to be guiding pretty well still. I'm also using the 120mm and it seems to work fine. I could use something with a bigger sensor as my OAG prism is quite large but the extra cost hasn't seemed worth it yet. If I can't find a suitable guide star (rare) I just nudge the scope a bit till I do.

Mark

Edited by markse68
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I used to used the ZWO OAG with my 200p, the guide stars were always banana shaped but PHD2 would guide just fine. It can help to have the guide cam slightly defocused to help PHD2 calculate the centroid a bit better. 

On my set up I have the OAG prism intruding into the light path somewhat, not horrendously but it was definitely slightly in front of the sensor. It didn’t affect the images and flats should take care of any dimming of the image due to the prism.  So don’t be hesitant to push the prism in a bit more if it helps with getting nicer guide stars or better focus.


As for rotation I always had the long axis of the camera aligned with RA, whatever the target… it just made everything easier; from aligning multi nights imaging to diagnosing tracking errors. 

Edited by CraigT82
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22 minutes ago, ollypenrice said:

Are you really sure you want to rotate to frame? Rarely, if ever, do I find it necessary. I'd much rather shoot in one of two orientations, 'landscape' (long side along RA) or 'portrait' (long side along declination. That way I can easily add more data at a future date. Recreating a random camera angle takes forever.

Olly

Recreating any angle takes as long as recreating 0 or 90 degrees, but it needs platesolving (yes i know you dont like it😅).

In NINA just hit a button in the platesolving tab with slew to target turned off and it reports the orientation down to 2 desimal degrees. Couple minutes at most to recreate an orientation to within a degree.

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11 minutes ago, ONIKKINEN said:

Recreating any angle takes as long as recreating 0 or 90 degrees, but it needs platesolving (yes i know you dont like it😅).

In NINA just hit a button in the platesolving tab with slew to target turned off and it reports the orientation down to 2 desimal degrees. Couple minutes at most to recreate an orientation to within a degree.

Having standard orientation of sensor allows for markers on camera / ota for quick alignment.

0/90 can also be easily aligned without plate solving - with drift exposures (drift is in RA direction and bright star that drifts in exposure will leave a line - if that line is either horizontal or vertical in frame - you are aligned).

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5 minutes ago, vlaiv said:

Having standard orientation of sensor allows for markers on camera / ota for quick alignment.

0/90 can also be easily aligned without plate solving - with drift exposures (drift is in RA direction and bright star that drifts in exposure will leave a line - if that line is either horizontal or vertical in frame - you are aligned).

Slewing during a short exposure or taking a long enough exposure to have obvious trails takes the same amount of time as platesolving a short exposure. I have used both and really its not a big deal and i wouldnt want to discourage someone from using whatever framing they want.

That said, 95% of targets are fine at either 0 or 90 degrees so marking those on the focuser takes the least amount of time. But also no reason not to mark 75 degrees if one has a project that needs it.

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2 minutes ago, ONIKKINEN said:

That said, 95% of targets are fine at either 0 or 90 degrees so marking those on the focuser takes the least amount of time. But also no reason not to mark 75 degrees if one has a project that needs it.

That is the best solution - having proper markings on rotator (does not have to be automatic). That way, even after plate solve - one can easily rotate correct amount to match needed angle, but that all adds $$$ to setup.

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4 hours ago, Aramcheck said:

how do you tell if I can't see it very well

Hi

Unlike a guide telescope, the focus of the guide camera on an OAG benefits from being as close to perfect as you can get it.

Focus the main camera on e.g. a clump of trees or buildings (it doesn't need to be at infinity) and clamp the focuser. You'll need very low gain on both cameras with the 200p tube cover in place, but with the cap on the removable smaller aperture removed.

Without touching the focus position you just set, bring the guide camera to focus upon the same object.

ZWO provide a nice bin2 in firmware, before the image is downloaded. Recommended, but don't forget to create a new guide profile and dark frame for the bin.

Repeat the process at infinity on a star field knowing that you're now very close to the exact parfocal distance. Anywhere in Cygnus is good ATM.

HTH

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Thanks for all the advice - much appreciated. Last night I figured out the hard way why the guide camera focus didn't appear to change... When I'd set up PHD2 it had, of course, defaulted to the 2600 MCPRO instead of the 120MM.... Took me awhile to figure out why I couldn't save any images with APT.

Haven't managed to get any images yet, as cloud rolled in shortly after, but hopefully I'm on the right track now.

One thing that's worrying me a bit is all the weight hanging off the focus tube. I have a compression ring fitted but even though it's tightened as much as I can, it always seems to have loosened slightly. I think I'll be saving up for a SteelTrack & Clicklock clamp next...

Thanks again
Ivor

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On 15/06/2022 at 19:46, ONIKKINEN said:

Recreating any angle takes as long as recreating 0 or 90 degrees, but it needs platesolving (yes i know you dont like it😅).

In NINA just hit a button in the platesolving tab with slew to target turned off and it reports the orientation down to 2 desimal degrees. Couple minutes at most to recreate an orientation to within a degree.

And platesolving always works and never acts the goat? My robotic clients will be delighted to hear this. When did this miracle occur? 😁

Olly

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12 minutes ago, ollypenrice said:

And platesolving always works and never acts the goat? My robotic clients will be delighted to hear this. When did this miracle occur? 😁

Olly

Definitely doesnt always work, but what does? None of my kit works all the time but then again its not a remote setup so doesnt have to.

Isnt goat short for greatest of all time? In that case we agree 🤣. Just joking, wouldnt try to convince you that platesolving is good even if i was paid to do that!

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Platesolving fanboi that I am, I admit that it can work flawlessly all night or be more...catlike. Sometimes I resort to dodges like bumping the RA half a FOV's worth (literally a dodge in that case). Sometimes sufficiently pungent language works. And at least for Ekos and its solver, the presence of, say, Arcturus in the FOV is guaranteed to overwhelm the poor thing unless I crank down the gain and exposure time that I get the insufficient-stars message.

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