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Shibby

OAG Woes

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Hi, it's been a long time since I checked in on SGL as I haven't been doing a lot of astronomy recently. I have been out a few times for some imaging but without much success to speak of, least of all in the early hours of this morning!

In a (failed) effort to make things a bit easier I recently purchased a QHY5L-II mono to use as a guide camera, to replace the old webcam. Last night, I spend a good 2-3 hours trying to get this up an running off-axis with my SX mini filterwheel & integrated OAG.

Needless to say, I just could not find anything through the guide camera. Eventually, I realised that the pick-off prism barely entered the light path and only a tiny fraction of the sensor was illuminated. Turns out I failed the moron test and had mounted my filters on the front of the FW cartridge instead of the intended back, meaning there was not enough room for OAG prism.

The problem now, unless I'm missing something, it seems to me that the Baader Ha (and possibly other) filters are backwards, i.e. the gold side of the Ha filter should be the front, right?

On another note, why is it so fiddly to focus? Why a tiny allen key socket?? Also, I can't figure out what the second (smaller / top) thumb screw is for - it doesn't seem to do anything, although I'm probably still failing the ongoing moron test here! 

Thanks for any help you good folk can provide. Maybe next time I'll achieve something!

Edit: One more question, how do I add an IR cut filter to the guide cam? Or is this not necessary? (I do use a coma correcter so there is glass involved).

Edited by Shibby

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Thanks for the response Louise. You're probably right about the allen key socket, I'll have to have another fiddle with the thumb screw, it must be for focus adjustment until you lock it.

Do you have an SX filter wheel? Do you find the filters are back-to-front? I feel like I'm missing something!

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Ah Lewis... a double failure!

Assuming you have this http://www.sxccd.com/the-sx-mini-wheel-oag

The top screw is for focusing the camera - loosen, adjust the focus, tighten.

The lower bigger one will be for adjusting the prism - loosen, adjust, tighten.

The set screw locks the prism into position.

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Yeah, always a shame when nothing works out! At least it was a full moon, which is why I chose that night to tinker!

So, I'm still stumped about the filter issue. Surely someone else has encountered this problem with their filter wheel??

In this image, you can see the filters screwed to the back of the cartridge so there's room for the prism. But they're back-to-front!

miniwheel2a.jpg

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can you not just turn the wheel round as with the atik oag?

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Hi Louise, thank you very much, but I've read through it though and there's nothing to help really.

In the image on FLO's page, you can see the filters are screwed into the back of the wheel, meaning they are back-to-front!

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can you not just turn the wheel round as with the atik oag?

This would work with the separate OAG, but with this FW the prism would be behind the wheel!

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without sounding rude.......lets see a pic of yours...lol

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Hi Louise, thank you very much, but I've read through it though and there's nothing to help really.

In the image on FLO's page, you can see the filters are screwed into the back of the wheel, meaning they are back-to-front!

Hi

I don't think it matters which way round the filters are from an optical point of view:

http://www.astrodon.com/Orphan/astrodonfaqnarrowband/

Louise

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Ah now that IS interesting. Astrodon claim:

Other astronomy filter suppliers simply suggest placing the shinier side toward the telescope without providing any rationale.

This is putting my mind at ease and I thank you for it. I had read somewhere that the gold side of an Ha filter must face the telescope or you get nasty reflections, but in that article they argue otherwise.

I'm very grateful you found this - thank you!

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Ah now that IS interesting. Astrodon claim:

This is putting my mind at ease and I thank you for it. I had read somewhere that the gold side of an Ha filter must face the telescope or you get nasty reflections, but in that article they argue otherwise.

I'm very grateful you found this - thank you!

Hi Lewis

I'm sure Astrodon know what they're talking about! I'm in the midst of setting up my own fw so it's of interest to me too though my one doesn't have an oag and filters screw in the 'right' way round :)

Hope your setup works well for you

Louise

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I think it's a no-go on the OAG :(

I've been having a play with my setup in the daytime but I found that I could not achieve focus on the guide camera (the focal plane is too far in).

My setup is fairly standard, I think?...  scope : coma corrector : M48-T adapter : FW : camera

I tried adding a T-extension to my main camera, the idea being to move the focal plane outward a bit, but unfortunately I could then not achieve focus at all on the main camera (I only had about 10-15mm of inward travel to play with). I suppose a shorter extension might *just* get me there, but anyway, this wouldn't be ideal as I'd be outside the ideal corrector separation. This is how it looked with the extension:

med_gallery_5051_1849_2942394.jpg

Any thoughts?

Is the baader corrector T-threaded? This might save me a couple of mm before the filter wheel...

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Hi

Looking at the spacing diagram on FLO's page, the fw gives you a total of 54mm spacing behind the coma corrector but I think you add 0.7mm for a filter. The spacing is to the ccd sensor which will obviously be some distance behind the ccd flange. The fw appears to allow 17mm space between the fw and sensor. It's hard to tell but I would imagine that in that case you wouldn't need a spacer between the fw and the ccd? Do you have some digital calipers to measure the spacing? They are very useful and quite cheap from Amazon.

Louise

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Hi Louise, yeah the spacing was correct before I added the spacer. I think you misunderstood - the only reason I've tried that undesirable spacer is to try and achieve focus with the guide camera.

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Hi Louise, yeah the spacing was correct before I added the spacer. I think you misunderstood - the only reason I've tried that undesirable spacer is to try and achieve focus with the guide camera.

Hi

Oh, ok. I thoght it shouldn't be there. To get focus with the guide camera, you'd need (roughly) the same distance from the cc to the guide cam sensor. It's possible you might need a spacer between the oag and guide cam for that. I've no experience of an oag but they're obviously designed to enable standard guide cams to reach focus.

Hope you can sort it.

Louise

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That'd be an easy solution but unfortunately the focal plane is too far in, meaning I can't move the guide camera far enough inward to achieve focus, even though, as you say, you'd expect this to be a fairly standard setup.  :icon_scratch:

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That'd be an easy solution but unfortunately the focal plane is too far in, meaning I can't move the guide camera far enough inward to achieve focus, even though, as you say, you'd expect this to be a fairly standard setup.  :icon_scratch:

Hi

It must be possible. I imagine you have to fiddle with the thumbwheels and adjust the prism and focus positions. I've heard oag's can be fiddly to adjust....

Louise

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Believe me, I've tried. There is no inward travel left on the OAG focus; and moving the prism would have no affect on focus.

I think all I can hope for is to remove the M48:T-thread adapter, instead adding that spacing behind the FW. This would probably mean getting a new coma corrector... admittedly, this situation does seem odd - surely someone out there has a similar setup??

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Hi

Um, I'd have thought if you were to move the prism up, that would shorten the path length? Can you tell how far out the focus is (with a distant object)?

Louise

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Looks like one option could be to replace the face plate with an M48 version. Something like this: http://www.altairastro.com/product.php?productid=16685&cat=321&page=1

But it's another £30 and I don't know for sure if it'll let me achieve focus. Curses!

Louise, moving the prism doesn't affect the length of the light path, just which part of it your're picking off.

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Maybe a low profile M48 to T-thread adapter would help?

I bougt a OAG with M48 to my corrector and T-thread to my filter wheel, it works very good and I can reach focus, but I had to loosen the allen screw on the guide cameras T-thread so I can get move my guide camera closer to the OAG, then I got perfectly focus and can also have the prism on an optimal range inside the OAG.

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Hi

The oag seems to be optimised for the Lodestar and Superstar guide cams. They both have 1.25" barrels and 12.5mm backfocus. Is your qhy5l-ii a new one? Just wondering what nosepiece it has fitted though I think any should give 12.5mm or less backfocus. Can you do some measurements to see where you might be gaining unwanted distance?

Louise

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