66 replies to this topic

[Dave Bush]

Seems that EdZ's write up concerns back focus and what ends up with moving the primary to reach focus--and that's ultimately causing aperture reduction. 

 

But that still seems different from having an eyepiece located at the focal plane operating at full aperture, but the field stop exceeds the size of the baffle tube.

 

I don't know if my guess is right or David's regarding full illumination, however. 

 

While it might be true that all of the light that can reach the eyepiece is there, if that is still getting vignetted by the aperture of the eyepiece, then I suspect we are losing light.

 

I thought vignetting would only reduce FoV, not light/aperture.

 

Quoting EdZ here regarding soft and hard vignetting...

 

"We just had this discuission perhaps a month or so ago. At the time I showed proof that the baffle tube DOES NOT limit the field of view. Yet i still see this claim made. Well it just aint so. What troubles me though is that people, even though they've received accurate information, keep repeating the same incorrect information over and over. It can be tiresome to keep correcting this misinformation.

The baffel tube, if smaller than the field stop of the eyepiece, will vignette the "amount of light" delivered to that outer portion of the field of view. For the baffel to actually cut off the field of view, that baffel would have to be so close to the field stop as to actually take the place of the field stop. Unless you are sticking your eyepiece right into the visual back, that will never be the case.

It has also been shown the light drop off caused by that vignette of the outer edges of the field of view is so gradual that in many cases, it cannot be seen at all. Isn't it ironinc that here in the SCT forum people raise false alarms about the outer fov vignette, and yet for years in the binocular foum we have been trying to explain to people the extent of the outer fov vignette, but they just can't see it.
 

...

 

The vignette will appear worse generally when it is also accompanied by hard vignette cut off. That is, if something is actually cutting off the aperture, then whatever is in the way is reacting as a stop and the field stop may be no longer acting as intended, therefore the outer fov might appear as significantly darkened."

[Dave Bush]

Quoting EdZ here regarding soft and hard vignetting...

 

"We just had this discuission perhaps a month or so ago. At the time I showed proof that the baffle tube DOES NOT limit the field of view. Yet i still see this claim made. Well it just aint so. What troubles me though is that people, even though they've received accurate information, keep repeating the same incorrect information over and over. It can be tiresome to keep correcting this misinformation.

The baffel tube, if smaller than the field stop of the eyepiece, will vignette the "amount of light" delivered to that outer portion of the field of view. For the baffel to actually cut off the field of view, that baffel would have to be so close to the field stop as to actually take the place of the field stop. Unless you are sticking your eyepiece right into the visual back, that will never be the case.

It has also been shown the light drop off caused by that vignette of the outer edges of the field of view is so gradual that in many cases, it cannot be seen at all. Isn't it ironinc that here in the SCT forum people raise false alarms about the outer fov vignette, and yet for years in the binocular foum we have been trying to explain to people the extent of the outer fov vignette, but they just can't see it.
 

...

 

The vignette will appear worse generally when it is also accompanied by hard vignette cut off. That is, if something is actually cutting off the aperture, then whatever is in the way is reacting as a stop and the field stop may be no longer acting as intended, therefore the outer fov might appear as significantly darkened."

And that hard vignette occurs when you have too great a back focus as EdZ explains in that link I provided earlier.

[areyoukiddingme]

Interesting. When I have used a 2" diagonal with my C6, my recollection is that I start seeing light drop off with my 27 Panoptic (I don't recall if that was using the reducer, however).

 

About 1.5 degrees seems to be about the maximum reasonably illuminated field given the aperture of the baffle tube. After that point, I think the light drop off gets sufficiently obvious that it start interfering with my enjoyment of the view. It's my impression that I am not alone in this view.

 

I take EdZ's point above to be that you could stick a 41 Panoptic back there and you'd still get light at the edges. But my guess is that you'd be hard pressed to find anyone who would use that twice in a C6.

[Dave Bush]

One more from good ol' EdZ...

 

See post #44 from this thread...

 

https://www.cloudyni...-3#entry5197635

 

He gives his reasons for recommending that for the C6, one should stick to 1.25" eyepieces and a 1.25" diagonal.

[areyoukiddingme]

I agree with his 1.25" view. I use a Takahashi 1.25" prism diagonal and max out with a 24/68 and .63 reducer.

 

I also find that mine works well with binoviewers on the planets--surprisingly so actually. But again, that's with 1.25". 

[wachuko]

Well... made a mistake ordering the polarizing filters.  I ordered two 1.25.  One to place on the diagonal and one on the eye piece.  The new diagonal is not 1.25.  In fact, I can't see any inside threads on it to place a filter.  I need to figure out how I would go about installing one.  When I do, I will order another one the correct size for the diagonal. 

 

This is to have a setup of double polarizing filter where I just turn the eyepiece to reduce the glare.  Something I read here in the forum.

 

As to the spare 1.25 polarizing filter, nothing goes to waste, already installed on another eyepiece. 

 

Edited by wachuko, 07 November 2023 - 09:26 AM.

[Dave Bush]

Well... made a mistake ordering the polarizing filters.  I ordered two 1.25.  One to place on the diagonal and one on the eye piece.  The new diagonal is not 1.25.  In fact, I can't see any inside threads on it to place a filter.  I need to figure out how I would go about installing one.  When I do, I will order another one the correct size for the diagonal. 

 

This is to have a setup of double polarizing filter where I just turn the eyepiece to reduce the glare.  Something I read here in the forum.

 

As to the spare 1.25 polarizing filter, nothing goes to waste, already installed on another eyepiece. 

 

Polarizing filters on eyepieces.jpeg

There is no way to attach a filter to the diagonal as it is with the SCT connector.  You would need to replace the SCT nose piece with a traditional one that is 1.25" and has the filter threads.  You would then need to use the Celestron visual back that came with the scope.  

 

This is what you would need from Baader...

 

https://agenaastro.c...14-2458105.html

 

The distance (back focus) would be a bit longer but visually I don't think it would matter much. 

Edited by Dave Bush, 07 November 2023 - 10:41 AM.

[wachuko]

There is no way to attach a filter to the diagonal as it is with the SCT connector.  You would need to replace the SCT nose piece with a traditional one that is 1.25" and has the filter threads.  You would then need to use the Celestron visual back that came with the scope.  

 

This is what you would need from Baader...

 

https://agenaastro.c...14-2458105.html

 

The distance (back focus) would be a bit longer but visually I don't think it would matter much. 

So best just to leave it alone as I have it now... 

[Dave Bush]

So best just to leave it alone as I have it now... 

Well, if you want the variable polarizing capability you will have to change out that nosepiece.  You can of course have both filters together on the eyepiece but that means pulling it out every time you want to change the filter rotation.  But for other filters you'll just have to put them in each eyepiece.  That's what I do.  Mostly I'm using filters for deep sky objects (UHC, OIII, etc.)

[wachuko]

Well, if you want the variable polarizing capability you will have to change out that nosepiece.  You can of course have both filters together on the eyepiece but that means pulling it out every time you want to change the filter rotation.  But for other filters you'll just have to put them in each eyepiece.  That's what I do.  Mostly I'm using filters for deep sky objects (UHC, OIII, etc.)

I just do not know if doing that would be a step back to the current setup you have recommended. 

 

I will leave it as is for now.  If I really see the need to use a double polarizing filter setup, then I might make the change.  Or just get the double polarizing filter even if it is more work.  So that I can keep the current setup.

[wachuko]

I might not be visualizing how it all comes together.  Let me order that adapter, it is only 35.00, and see how it all works together... 

[Dave Bush]

I might not be visualizing how it all comes together.  Let me order that adapter, it is only 35.00, and see how it all works together... 

Pictures are worth thousands of words right?  So below we have...

 

1. The diagonal in its "normal" configuration with the SCT nosepiece.
2. The diagonal with the SCT nosepiece removed.
3. The diagonal with the 1.25" nosepiece adapter installed.
4. The diagonal inserted into the Celestron visual back (but not of course, attached to the scope)

 

 

 

 

[wachuko]

Crystal clear now.  Thank you for taking the time to explain and share the photos!  

[wachuko]

Got it all... I have not had the chance to try out either combination.  But just looking at it.  Don't I lose a heck of a lot of light with the second setup?

 

 

I mean, what is the benefit of this setup (outside of being able to rotate the polarizing filter)?  Am I missing better views doing this?  Also, this is not as secured as the setup suggested in this thread.

 

This setup has not way to come lose or anything falling off by mistake...

 

 

Where the one I just installed to test, could work itself lose and fall... I mean, my wife just walked by it and accidently hit the eyepiece and rotated the whole setup... (yes, I am moving it from where it is not just because of this... )

 

[Dave Bush]

Got it all... I have not had the chance to try out either combination.  But just looking at it.  Don't I lose a heck of a lot of light with the second setup?

 

Eyepiece.jpeg

 

I mean, what is the benefit of this setup (outside of being able to rotate the polarizing filter)?  Am I missing better views doing this?  Also, this is not as secured as the setup suggested in this thread.

 

This setup has not way to come lose or anything falling off by mistake...

 

Eyepiece1.jpeg

 

Where the one I just installed to test, could work itself lose and fall... I mean, my wife just walked by it and accidently hit the eyepiece and rotated the whole setup... (yes, I am moving it from where it is not just because of this... )

When using the visual back your back focus in longer than when using the SCT attachment.  You could compensate (almost entirely) by removing the 15mm spacer from the eyepiece end and attaching the twist lock holder directly to the diagonal (as I have it in my photos from my earlier post above).  But I'm assuming that your use of the visual back and 1.25" nose piece is for the ability to have a variable polarizer function which, you would be using only on the moon correct?  The moon is pretty bright so any slight light loss would not really be noticeable.

 

As for stability, yeah, don't bump it.

Edited by Dave Bush, 15 November 2023 - 02:45 PM.

[SeattleScott]

Typically you only lose aperture in extreme situations that the scope was not designed for. Like using a reducer with a 2” diagonal. With my 6” Mak and Antares reducer and 2” refractor diagonal, I do get some hard vignetting but no aperture loss. Others have tested certain models and found you can lose half an inch or maybe an inch of aperture using a reducer with 2” diagonal on SCT. It is easy to test lost aperture. Set the scope next to a flat surface like a wall. Put reducer on and diagonal in. Instead of inserting eyepiece, shine a flashlight into diagonal. Look at the light coming out of the scope and shining against the wall. Measure the diameter and see if it matches telescope aperture.

[SeattleScott]

Technically I believe back focus is a contributing factor, so one should probably put reducer on, diagonal in, eyepiece in, reach focus at infinity, and then do the flashlight test.