Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

Atik OAG Off Axis Guider


FLO

Recommended Posts

Hmm, so will the combo of an Atik 314L on the OAG (that should be able to find a guide star...:-)), and a 4000 for main imaging ...and/or 314L/DSI-C (guide cam) with the EFW2 and the OAG all fit/work, using the 9x1.25" filters there ahould be zero vignetting on the 4000 too with this setup

Link to comment
Share on other sites

  • 2 months later...
  • Replies 100
  • Created
  • Last Reply

Being a complete ignoramus with regard to auto guiding, I was wondering if this could be used with a Skywatcher Synguider Autoguider

Reason being it would make for a very light self contained OAG system, and, once I motorise my mount, ( SynScan PRO GOTO Version 3 Upgrade Kit for EQ5 ) any weight saving would be relevant as I can't afford an EQ6.

A separate guidescope setup would be too much weight.

Link to comment
Share on other sites

The weight reduction is one of the primary benefits of using an off axis guider, and the fact that differential flexing issues are all but eradicated.

Hard to see a reason why a Synguider wouldn't work?

Link to comment
Share on other sites

Has anyone actually tried this approach with a SynGuider and DSLR? (and filter wheel?)

Exactly what I was wondering, push come to shove, could it be paired with something like an X-Y Guidestar Locator Orion USA X-Y Guidestar Locator - Telescopes UK: Telescopes & Telescope Accessories in your only London shop

The synGuider would fit the X-Y guidestar which I presume would fit the OAG, but, would love to know if anyone has had similar working.

The weight saving alone would make this advantageous.

I know the square root of zero when it comes to OAG so have been asking loads of stupid questions all over this forum the past few days. So please forgive me if my questions sound stupid :)

Link to comment
Share on other sites

Hard to see a reason why a Synguider wouldn't work?

I am sure it will be fine. The Atik EFW2 Filter Wheel has been released and is incoming but we still don't have an ETA for the Atik OAG off axis guider. When it does arrive we shall make one available for review, with a Synguider autoguider if that helps. Needless to say, if someone were to buy a Synguider or OAG and find it unsuitable we will happily take it back.

Link to comment
Share on other sites

I recently bought an ST80 and Synguider autoguider from FLO, which is working out well. However when attempting to image some objects (like M45), I cannot get a decent guide star on the ST80 whereas I think I'm right that an OAG would solve this issue as there are clearly stars in the main field of view?

Link to comment
Share on other sites

I recently bought an ST80 and Synguider autoguider from FLO, which is working out well. However when attempting to image some objects (like M45), I cannot get a decent guide star on the ST80 whereas I think I'm right that an OAG would solve this issue as there are clearly stars in the main field of view?

Its more likely to be the opposite way around Mike. With a guidescope you can move the guidescope so that it gets stars. With an OAG you are limited to the outer edge of the field of view of the main scope (the pick-off prism doesn't sit in the main field of view or you'd see it :) )

Helen

Link to comment
Share on other sites

This is interesting, what is the FOV from an OAG (using the TLA's), I've heard some people refer to OAG as black magic but some peeps can't say enough good things about it.

Are there times when it's not possible? or just more difficult?

Link to comment
Share on other sites

I must admit that I've had no problems to date - but then I've only used it twice!!

I used a brightstar OAG, a SXVFH9 and the SX guidehead. I set it up in the light to ensure I was very close to parfocal for the cameras. I didn't have many stars to choose from in the field of the guide cam, but you can rotate my off-axis guider to move it to try to get more stars. And you only need one star to guide on....

Helen

Link to comment
Share on other sites

So, two attempts and two successes, it's a good start!

I believe that the Synguider gives a certain amount of freedom of choice within its FOV and coupled with the rotation I would think that chances are a decent star could be found, either that or move whole thing off center and then crop end image.

Does this sound feasible?

Link to comment
Share on other sites

The original "an' best" guider the ST-4 only had a very small TC-211 chip and it managed to guide some of the biggest scopes on the planet. The chip only had 192 x 164 pixel and they were 13.75 x 16 micron!

Getting Started with the SBIG ST4 Autoguider

Alan Holmes at SBIG says:

Radial off-axis guiders have a severe problem in that a small prism or mirror is used to pick off a tiny portion of the light to direct to the eyepiece. Guide stars tend to be dim, and one is forced to rotate the assembly to find a guide star. When one rotates the assembly, the star motion directions (in response to guiding inputs) also rotate, and one is forced to recalibrate the autoguider quite often. Also, the dim stars force some autoguiders to require very long exposures, negating their ability to compensate for periodic errors and drive hops.

With PHD you can get sub-pixel guiding...I'm sure the standalone guiders must be able to do similar??

The current generation of guide cameras have the potential to do much more than the original!!

Link to comment
Share on other sites

Its more likely to be the opposite way around Mike. With a guidescope you can move the guidescope so that it gets stars. With an OAG you are limited to the outer edge of the field of view of the main scope (the pick-off prism doesn't sit in the main field of view or you'd see it :) )

Helen

Thanks Helen, that's a very good point. Further, I presume that the pick-off prism/OAG approach is perhaps only one that works with 1.25"/42mm imaging setup and not with 2" (as I suspect that you would notice it using a DSLR).

Sometimes (not always - but when shooting under light polluted skies against a target with low contrast against the background) the ST80 cannot deliver a bright enough star for the SynGuider. I expect the OAG may just compound the problem.

But I'm really tempted to try anyway...

Link to comment
Share on other sites

The current OAG's position the pick off prism just outside the frame of the DSLR chip, so it never interferes with the primary image.

If it has to be rotated to find a suitable guide star that's when the x-y correction orientation can cause issues.

Under light pollution the oAG should be better - you're working at a longer focal length so less sky v's stars.

Link to comment
Share on other sites

Stupid question time ...

I was under the impression that all this OAG stuff was best achieved using 2" equipment because thare was more light etc to play with.

Don't think I worded that right but I'm sure you can see what I was trying to get across.

Link to comment
Share on other sites

NO such thing as a stupid question....

The light to the guider is only limited by the size and position of the pick off prism - this is usually about 10mm square to cover the 3 or 4mm CCD in a guide camera.

The amount of sky and hence stars that can be seen again is limited by the prism - think of it as a small diagonal whereas the camera see all (well nearly...) the FOV.

Link to comment
Share on other sites

NO such thing as a stupid question....

The light to the guider is only limited by the size and position of the pick off prism - this is usually about 10mm square to cover the 3 or 4mm CCD in a guide camera.

The amount of sky and hence stars that can be seen again is limited by the prism - think of it as a small diagonal whereas the camera see all (well nearly...) the FOV.

"NO such thing as a stupid question..." cheers for that, fully agree

yeah ... I was sort of thinking that the prism could be slightly larger on a 2" diameter but still take up a smaller percentage of the overall area when compared to 1 1/4" diameter equipment.

Therefore giving a better optical view to the guide camera. :) But obviously this isn't the case.

I'll be honest, I am fishing around in the dark here, but to my line of thinking the 2" component system is used for large CCD cameras to give a larger light path to cover the larger chip, this must therefore give a better image than 1 1/4" setup. because it's not (for want of a better word) compressed.

Still not sure where I'm going with this, but, persevering ...

Therefore, the prism on a 2" OAG should be giving the guide camera a better view than on a 1 1/4" OAG

Hope this makes sense :) to some one!

Link to comment
Share on other sites

Unfortunately all the prisms used on OAG are around 10mm sq. This is based on the distance between the prism and the guide camera chip to give maximum illumination. A large prism wouldn't make any difference.

Think about the image formed at the focus of the telescope as an image 100mm diameter (!!?)... then a 2" aperture system will only "see" part of this image - in a 2" diameter circle; likewise the 1.25" (or 42mm for the T thread type) will only "see" part of the image 1.25" diameter. The scale of the actual image doesn't change...only the amount of it that you see.

Link to comment
Share on other sites

Unfortunately all the prisms used on OAG are around 10mm sq. This is based on the distance between the prism and the guide camera chip to give maximum illumination. A large prism wouldn't make any difference.

Think about the image formed at the focus of the telescope as an image 100mm diameter (!!?)... then a 2" aperture system will only "see" part of this image - in a 2" diameter circle; likewise the 1.25" (or 42mm for the T thread type) will only "see" part of the image 1.25" diameter. The scale of the actual image doesn't change...only the amount of it that you see.

That's helpful, thanks.

So given that the focuser is 2", the OAG's prism will stick out into the optical train and there will be a black blob on the DSLR image where the prism should be.

But this isn't a problem with 1.25" as the prism would need be using an area of the optical train that wouldn't fall on the smaller CCD anyway.

Link to comment
Share on other sites

Okey dokey, now I've got it, the OAG prism is basically scraping off the excess light at the edge of the focal path without intruding into the used light heading into the filters to the CCD.

So it (the prism) won't actually see the centre of the view (the target), what I would see in my eyepiece.

Link to comment
Share on other sites

The prism is usually positioned just off the "frame" of the CCD chip, both in the 1.25" and the 2" sizes.

The trick is remembering where and what the OAG is looking at - the star you see in the OAG will NOT be in the imaging camera FOV - so it's easier to position the OAG above the camera in Dec so that you can position the imaging camera FOV then move the scope slightly in Dec to find a field star to set the focus in the guide camera, then move as necessary to frame the shot around the best guide star image.

Link to comment
Share on other sites

Sooo, it's really a bit of a balancing act then, bit of patience required.

So if I've got this right;

1. Set up on the target with OAG above the camera in dec.

2. Find a guide star, moving the Dec slightly if required.

3. Setup and run guide camera.

4. If target is still acceptable then start taking pictures / video.

OK, thanks for your time and patience, it must have felt like hard work for a while.

But, trust me it was well appreciated and I feel that it has cleared up a lot of unknowns for me.

Cheers.

Link to comment
Share on other sites

The prism is usually positioned just off the "frame" of the CCD chip, both in the 1.25" and the 2" sizes.

The trick is remembering where and what the OAG is looking at - the star you see in the OAG will NOT be in the imaging camera FOV - so it's easier to position the OAG above the camera in Dec so that you can position the imaging camera FOV then move the scope slightly in Dec to find a field star to set the focus in the guide camera, then move as necessary to frame the shot around the best guide star image.

I'm with you. I wonder if the prism is off the frame of the CCD sensor in a full 360 degree rotation or if it only works if the prism protrudes up to the longer side of the chip (i.e. it can go on the top or the bottom with respect to the CCD sensor's width).

My SynGuider is connected to an ST80 which is fixed in place, so I cannot move it around to find that guide star. I'm wondering if scope rings or an OAG is the next best step to make for upgrading the imaging system.

I'm thinking OAG but will await the reviews!

Link to comment
Share on other sites

Each design of OAG is subtley different - Atik/ Starlight/ Orion/ Lumicon/ Baader/ Meade/ Celestron are all different!!!

Orion (us) do a x-y eyepiece translation platform which fits guide scopes for exactly that reason! AE have an excentric 2" to 1.25" adaptor which allows the guide camera to be rotated around the centre of the field; the alternative is adjustable guide rings...IMHO the never seem to work - to much fiddle fart - they get adjusted once then get left there....you never seem to adjust every time!!

There lies the benefit of a QHY5/ ST80 combo with say PHD - you can guide on ANY star image in the FOV....but you need a computer connection.....Hmmm life's never simple!

(I've gone to a 80/20 beamsplitter for my aquisition and guiding with the spectroscope - that way you can guide on ANY star over the whole field of view!!!)

Link to comment
Share on other sites

Each design of OAG is subtley different - Atik/ Starlight/ Orion/ Lumicon/ Baader/ Meade/ Celestron are all different!!!

Orion (us) do a x-y eyepiece translation platform which fits guide scopes for exactly that reason! AE have an excentric 2" to 1.25" adaptor which allows the guide camera to be rotated around the centre of the field;

Interesting info, thanks!

... the alternative is adjustable guide rings...IMHO the never seem to work - to much fiddle fart - they get adjusted once then get left there....you never seem to adjust every time!!

That's what I was concerned about. I didn't want to get into fiddling about constantly between targets.

There lies the benefit of a QHY5/ ST80 combo with say PHD - you can guide on ANY star image in the FOV....but you need a computer connection.....Hmmm life's never simple!

With the Synguider I can guide on any star in the FoV. The Synguider display shows a pretty good representation of the stars in the eyepiece FoV, although you do have to fiddle with brightness and noise variables first. My concern about this new OAG that FLO will be carrying is this: Will there be enough stars, bright enough for the SynGuider to lock and guide and how does it compare to the ST80 for FoV and reliability?

(I've gone to a 80/20 beamsplitter for my aquisition and guiding with the spectroscope - that way you can guide on ANY star over the whole field of view!!!)

The main field of view of the main OTA? That would be quite impressive :)

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.