Which off-axis guider?

[StuartT]

14 hours ago, tooth_dr said:

3) align prism with long edge of camera sensor - I mark the body of camera with sharpie on a printed label to make it easy to locate.  Undo grub screws and move prism stalk in or out to get it close to the camera sensor without blocking it.  I remove whole assembly from focuser and eyeball to do this step.

Ok, I understood everything apart from this bit. Align 'what' of the prism to the long edge of the camera sensor? The prism is square, isn't it? Do you mean the prism should be adjacent to the long edge of the camera sensor? (rather than adjacent to the short edge). See screenshot

11 hours ago, newbie alert said:

I'm using a Celestron version,while the 12mm prism is nice I find the body bulky and takes up so much room.. I found I needed to use the thinnest adapter with m48 opening and then search the internet and buy a m48 to SCT adapter to attach it to the scope, not inc in the kit and at the time wasn't that easy to find.. this had to be fairly small as it incorporates into the backfocus figure, with a hd version I believe you should have more space... The turret isn't the best and has never  worked ( I bought it secondhand , so I couldn't sent it back)

Once it's set and locked down it works all ok, I don't rotate my imaging camera, and as said previously I don't search for guidestars so that's locked down too, the Qhy 290 is more than sensitive enough

I'm not too sure what else is out there as I've not looked but I think the Celestron was made for DSLR use , wasn't really that we'll thought about which I find is usual for the Celestron brand( other than a tube I don't think I'd by another Celestron product) 

Quite a few things I don't understand here. 1. what is a turret? 2. I assume the Celestron OAG is made to fit the Celestron scopes, so why would you need an adapter? 3. how could an OAG be made for a DSLR use? I assumed people who use DSLRs are not guiding at all, since they are just starting out in AP, but I could be wrong about that.

Edited April 17, 2022 by StuartT

[tooth_dr]

11 hours ago, StuartT said:

Ok, I understood everything apart from this bit. Align 'what' of the prism to the long edge of the camera sensor?

Prism should be as below 

 

[newbie alert]

12 hours ago, StuartT said:

Ok, I understood everything apart from this bit. Align 'what' of the prism to the long edge of the camera sensor? The prism is square, isn't it? Do you mean the prism should be adjacent to the long edge of the camera sensor? (rather than adjacent to the short edge). See screenshot

Quite a few things I don't understand here. 1. what is a turret? 2. I assume the Celestron OAG is made to fit the Celestron scopes, so why would you need an adapter? 3. how could an OAG be made for a DSLR use? I assumed people who use DSLRs are not guiding at all, since they are just starting out in AP, but I could be wrong about that.

The turret part is the adjustment stem where the guidecam sits.. maybe that's my terminology for it

Celestron products should fit and be able to use with other Celestron products, yeah you would think wouldn't you..I think the Celestron oag was designed for DSLR and their nightscape cameras... Both osc.... So wasn't designed to use with filterwheels etc.. so even with the shortest adapter in the box I was too long for my setup so I needed a SCT to m48 adapter so I could thread it onto the reducer.. scanned the internet and either they was too long or was M42.. finally found 2 which happened to be the same adapter... £5 odd on AliExpress or £29 from that well known German supplier with a waiting time of a few months...

If you align the long side of you guidecam sensor to the long side of the prism and I also have the long side of of the imaging camera also aligned.. it all sounds complicated and is far easier done that said..

[StuartT]

14 hours ago, tooth_dr said:

Prism should be as below 

 

What do you mean? You seem to have 1) moved the prism by 180 degrees (which makes no difference, surely, as it is still adjacent to the long side of the imaging sensor); and 2) moved it inside the circle (I don't understand how that is actually possible) 

13 hours ago, newbie alert said:

 

If you align the long side of you guidecam sensor to the long side of the prism and I also have the long side of of the imaging camera also aligned.. it all sounds complicated and is far easier done that said..

I don't understand this, I'm sorry. The prism is square - so it doesn't have a long side.

Edited April 18, 2022 by StuartT

[Padraic M]

What I understand tooth_dr to be saying, is that if you don't need the ability to rotate your sensor relative to the OAG, you can butt the prism right up to the long edge of the sensor like below. The usual advice is to push the prism in until you can see its shadow in a flat frame, then retract slightly so that it clears the light cone. The long edge can be the top or bottom edge of the sensor, it doesn't matter. This gives the best illumination of the prism, and lets it pick off guide stars closer to the optical centre line where they are most likely to be well-shaped and bright.

Putting the prism in the outer circle like you have above is only needed if you have a rotator between the OAG and camera, where you could potentially have the prism at one of the sensor corners.

 

[newbie alert]

7 hours ago, StuartT said:

What do you mean? You seem to have 1) moved the prism by 180 degrees (which makes no difference, surely, as it is still adjacent to the long side of the imaging sensor); and 2) moved it inside the circle (I don't understand how that is actually possible) 

I don't understand this, I'm sorry. The prism is square - so it doesn't have a long side.

Sorry I've had a brain fart I think..

The prism and the guide cam can be rotated as one( which is what James Lamb is doing in his search for guidestars)  you can rotate this in any position but it doesn't align with the imaging camera sensor, which you can also rotate.. so I keep it as the above image without the yellow line.. 

You can't put the prism in that  yellow line position, it has no lateral movement, only up and down, down drops it into the light cone more

 

Edited April 19, 2022 by newbie alert
Added info

[Padraic M]

4 hours ago, newbie alert said:

You can't put the prism in that  yellow line position

I wasn't implying that you could... just showing the level to which you should insert the prism stalk.

 

.

[tooth_dr]

13 hours ago, StuartT said:

What do you mean? You seem to have 1) moved the prism by 180 degrees (which makes no difference, surely, as it is still adjacent to the long side of the imaging sensor); and 2) moved it inside the circle (I don't understand how that is actually possible) 

I didn’t understand your diagram. I assumed that square was the prism but wondered why it was so far away.  My diagram is what it looks like visually.  If you want to rotate anything, rotate the camera/OAG as one piece. I can’t really explain this any more, you just need to do it yourself. 

[StuartT]

Ok, I don't really follow this. Probably because I don't yet own one of these gadgets and so I don't really understand all the parts and how they are supposed to work. My incorrect diagram is no doubt a reflection of my poor understanding of what's going on.

I think I am going to have to buy an OAG (prob the Celestron as it has the largest prism) and then see how I get on. I can' probably then ask the right sort of questions!

Thanks

[Elp]

If you've got your main camera screwed onto the oag and look directly at the sensor face-on you'll see the rectangular/square surround of the sensor. You then move the prism stalk up or down until the prism is just outside the sensor rectangle/square, this may be on the long side or the short side of the surround, top or bottom for the OAG it doesn't matter the orientation, if the camera sensor is rectangular it makes sense to align the prism next to one of the longer sides so it's further into the light cone. You'll see on your computer software preview if the prism is too far into the light cone as you'll see it on your images as a shadow/out of focus obstruction, in such instances you then move the prism stalk out a bit and then tighten it down again.

If you wish to use a rotator which I never did, you would have to move the prism stalk further out so no matter how you orient the camera to the OAG the prism is in clearance from your main camera sensor completely (imagine drawing a circle from the centre of your sensor which envelopes all four corners of the sensor, your prism can be positioned outside this line). As a general rule though you want your OAG prism inside your light cone as much as possible without infringing on your main camera sensor view so that it can "see" the most amount of stars for guiding.

[StuartT]

Ok, I am going to order the Celestron OAG. What kind of sensor size do I need for the guide camera? (and image scale). Should the image scale be the same as for the imaging camera? The FL is 2350mm

(some use the ASI290mm, but that would give me an image scale of 0.18 which would be horrifically oversampled. I guess I could bin it 4x4)

Thanks 

Edited April 21, 2022 by StuartT

[michael8554]

Hi Stuart

Oversampling doesn't matter per se in the guiding setup, it might in an imaging setup.

But 0.18arcsec/pixel isn't a sensible image scale to guide with.

Binning x4 would improve it, but really it means you have a camera with too-small pixels.

Also as you have opted for an OAG with a 12x12 prism, you want a sensor large enough to match that.

Otherwise you might as well have bought an OAG with an 8x8 prism.

Michael

 

 

[StuartT]

1 hour ago, michael8554 said:

Hi Stuart

Oversampling doesn't matter per se in the guiding setup, it might in an imaging setup.

But 0.18arcsec/pixel isn't a sensible image scale to guide with.

Binning x4 would improve it, but really it means you have a camera with too-small pixels.

Also as you have opted for an OAG with a 12x12 prism, you want a sensor large enough to match that.

Otherwise you might as well have bought an OAG with an 8x8 prism.

Michael

Hmm.. but I was advised that getting an OAG with a small sensor would limit the choice of stars. But I guess what you're saying is that provided the sensor and the prism are small, you will get much the same FOV as a larger sensor and a larger prism. So it's the matching that counts, right?

(or am I just confused again?... this happens often)

Edited April 21, 2022 by StuartT

[michael8554]

Yes.

To matching counts, not you're confused again 😆

Michael

[dan_adi]

https://www.optecinc.com/astronomy/catalog/oag/19340.htm

I have this one and it's great