Chips on the edge of OAG prism. Is it an issue?

[oymd]

Hi everyone

I have an OVL OAG. 

I’ve noticed some minor chips on the very edge of the prism. 
 

I have not used it yet. 
 

I looked though the light port in the turret with my naked eye during daytime, and noticed the effect at the very edge at just one point on the circle’s diameter. 
 

Will this be an issue with guiding? Never used an OAG before, so not sure what to expect? Is this a major issue, or it can function fine?

the light port is 5.6mm in diameter. I intend to use a ZWO 120MM mini with my short focal length refractors, and a ZWO 290MM mini with my SCT?

I tried to show how it looks when looking down the light port. 
 

Many thanks

 

Edited April 30, 2021 by oymd

[vlaiv]

Question is - how did it get to be like that?

If you purchased new item - do return it. It should not look like that.

If it was second hand item - my guess is that it was stored in box of surplus bits without protection and over years it suffered damage.

It will work for guiding, that is not much of a problem, but I'm not sure you want to use it like that. I'm much more concerned about light scatter that will happen inside optical path rather than guiding performance.

It can happen that you get all sorts of artifacts on your images - like strange reflections or spikes or whatever. Maybe careful placing of the prism (like moving it a bit further away from optical axis - bad for guiding, good for light scatter) will help if you notice issues.

I would suggest that you return it if it's a new item - or give it a go and see how it performs. Maybe you won't have issues, but in case you notice some issues - well, OAG will be prime suspect for that.

[prusling]

I bought an OVL OAG secondhand and the seller was up-front about a chip off the prism and priced it accordingly. I haven't found it to cause a problem in practice, with guiding or optical artefacts.

As Vlad says, return it if new, otherwise give it a try and I hope your experience is the same as mine. 

By the way this design doesn't have a clamping screw to hold the pillar tight but aluminium foil does the trick to stop any tilt/wobble.

[oymd]

48 minutes ago, vlaiv said:

It can happen that you get all sorts of artifacts on your images - like strange reflections or spikes or whatever. Maybe careful placing of the prism (like moving it a bit further away from optical axis - bad for guiding, good for light scatter) will help if you notice issues.

I

Thanks Vlad

I was under the impression that the OAG will only affect the image being seen by the Guide camera and PHD2.

Do you mean that those chips can actually cause artifacts in the light going to the MAIN camera sensor?

Also, this is the first time I actually handle an OAG. The light path to the guide camera is only 5.6mm in diameter!? That is TINY. Is that how OAGs are designed?

I had a look on FLO and saw the ZWO OAG v2, ZWO OAG M68 and aslo the Celestron OAG, but there is no mention anywhere of the actual size of the light path size to the guide camera chip. Just that all are 8mmx8mm prisms, and the COAG is 12.1mm.

I am confused. If the prism of the OVL OAG is 8x8mm, why restrict the light path to 5.6mm?

On SCTs, I've read many threads advising to use at least the 174MM Mini for its bigger sensor size. Those sensors are 11mm on the longer side, and would be a complete waste with the OVL OAG then? Regardless of prism size, if the light path to the guide camera is 5.6mm diameter, that limits OVL OAGs to the 120MM mini and BARELY JUST with the 290MM mini? (290MM mini is 5.6mm on the loner side)

[oymd]

11 minutes ago, prusling said:

I bought an OVL OAG secondhand and the seller was up-front about a chip off the prism and priced it accordingly. I haven't found it to cause a problem in practice, with guiding or optical artefacts.

As Vlad says, return it if new, otherwise give it a try and I hope your experience is the same as mine. 

By the way this design doesn't have a clamping screw to hold the pillar tight but aluminium foil does the trick to stop any tilt/wobble.

I have tried to move the turret in and out, and it is a very very tight fit.

I've read about the aluminum foil fix but to me, I do not think its an issue

[vlaiv]

2 minutes ago, oymd said:

Do you mean that those chips can actually cause artifacts in the light going to the MAIN camera sensor?

Yes - that might be actual problem.

With normal prism - you have straight edges and light reflects where it is supposed to. With chipped glass - you have edges / surfaces pointing in random directions - all of these cause scatter and reflections - and some of the light that scatters of edges and reflects of surfaces where prism is chipped - can land on your main imaging sensor (or it may not - depends on actual angles involved).

Even regular straight edges of prism can cause diffraction spike if you put it in direct

light path hitting main sensor. I had this with my OAG:

It only affects edge stars and not all stars in the image.

8 minutes ago, oymd said:

Also, this is the first time I actually handle an OAG. The light path to the guide camera is only 5.6mm in diameter!? That is TINY. Is that how OAGs are designed?

Yes, that is how it works and yes you are right - it is small opening. Using large sensor with OAG simply does not make sense. You also must be careful to place prism as close to main imaging sensor as you can - although it will depend on speed of the system.

Fast systems are likely to stop down OAG due to prism size. Prisms are 8mm and if you for example have F/5 scope - then you must place guide camera sensor closer than 40mm (8 * F/5) if you want to avoid stopping down by prism. Stalk opening is another "choke point".

Slower systems are better with respect to this.

Using main aperture helps here as we are still imaging point sources and large aperture helps even if we have somewhat stopped down light reaching guide sensor.

12 minutes ago, oymd said:

I am confused. If the prism of the OVL OAG is 8x8mm, why restrict the light path to 5.6mm?

Designers of OAG did not really considered all things?

Maybe they just used available prism size - as 8mm seems to be standard size for most OAGs out there.

13 minutes ago, oymd said:

On SCTs, I've read many threads advising to use at least the 174MM Mini for its bigger sensor size. Those sensors are 11mm on the longer side, and would be a complete waste with the OVL OAG then? Regardless of prism size, if the light path to the guide camera is 5.6mm diameter, that limits OVL OAGs to the 120MM mini and BARELY JUST with the 290MM mini? (290MM mini is 5.6mm on the loner side)

Yes, don't use 174 unless you want to use it for something else as well (like Ha solar, planetary or whatever).

I guide with ASI185 which is 1/1.9" sensor or 8.9mm diagonal. I'm on F/8 system and have TS OAG (16mm thick version - not ultra thin 9mm).

You can clearly see that there is vignetting over sensor and that central part is the brightest.

On faster systems - this will be even more pronounced.

[oymd]

3 hours ago, vlaiv said:

Yes - that might be actual problem.

With normal prism - you have straight edges and light reflects where it is supposed to. With chipped glass - you have edges / surfaces pointing in random directions - all of these cause scatter and reflections - and some of the light that scatters of edges and reflects of surfaces where prism is chipped - can land on your main imaging sensor (or it may not - depends on actual angles involved).

Even regular straight edges of prism can cause diffraction spike if you put it in direct

light path hitting main sensor. I had this with my OAG:

It only affects edge stars and not all stars in the image.

Yes, that is how it works and yes you are right - it is small opening. Using large sensor with OAG simply does not make sense. You also must be careful to place prism as close to main imaging sensor as you can - although it will depend on speed of the system.

Fast systems are likely to stop down OAG due to prism size. Prisms are 8mm and if you for example have F/5 scope - then you must place guide camera sensor closer than 40mm (8 * F/5) if you want to avoid stopping down by prism. Stalk opening is another "choke point".

Slower systems are better with respect to this.

Using main aperture helps here as we are still imaging point sources and large aperture helps even if we have somewhat stopped down light reaching guide sensor.

Designers of OAG did not really considered all things?

Maybe they just used available prism size - as 8mm seems to be standard size for most OAGs out there.

Yes, don't use 174 unless you want to use it for something else as well (like Ha solar, planetary or whatever).

I guide with ASI185 which is 1/1.9" sensor or 8.9mm diagonal. I'm on F/8 system and have TS OAG (16mm thick version - not ultra thin 9mm).

You can clearly see that there is vignetting over sensor and that central part is the brightest.

On faster systems - this will be even more pronounced.

Amazing. Thanks Vlaiv. I’ll start experimenting