113 replies to this topic

[turtle86]

After putting the mask directly on the primary, there was no change from the previous attempt at putting it over the front aperture. The edges were still significantly more vignetted than without the mask in place.

 

I thought maybe this was because the vignetting didn't actually change, just the view got dimmer, and thus if the vignetting stayed the same, it would look proportionally darker.

But to counter this, I simply increased the brightness of the headlamp and still saw considerably more aggressive vignetting when masked down to 12".

 

I may need to revisit my tests of my 12" F/5.

 

I'm starting to wonder if this Nikon + P2 simply doesn't like longer focal ratios for whatever reason.

 

It occurred to me that most of the people who have NOT reported issues have scopes at ~F/4 and below, while those of us who have reported issues have scopes F/4.5 and above.

 

I'd recommend calling Televue if you can't get to the bottom of this.

[CrazyPanda]

I actually have all the specs on the scope, as I designed it to suit my specific requirements; and then Peter Read at SDM Telescopes built it to my specs.

 

The primary is a 14"/F4.465 Zambuto.  Diameter = 355mm, Focal Length = 1585 mm.

Focuser is a 1.5" drawtube Feathertouch.

The inside diameter of the secondary cage is 370mm.

The Intercept distance to the focal plane is 250mm

The secondary is a 2.6" (66mm) Protostar Quartz secondary

 

The fully Illuminated Field is 12mm.

 

Hope this helps.

 

Cheers

Thanks for the info. 
 

Are you able to test your Nikon and your scope with the stock Paracorr tunable top and not with Hank’s custom adapter? 

[Starman1]

I suspect it could be due to a couple issues or maybe both at the same time:

1) a too-small secondary for the intercept distance resulting in significant edge of field light loss.

The test for this is to see that vignetting reduces with shorter focal length eyepieces until it disappears at some point and no shorter focal lengths vignette.

2) an intercept distance too short for the entire light cone to enter the bottom lens of the Paracorr.  This is far more likely in short f/ratio scopes than longer ones.

 

 

A test you can do is to focus on the Moon, see where the light cone hits the bottom of the Paracorr.  If not all the light cone enters the bottom lens when the light cone is centered, the secondary is too small.

The Moon only represents a portion of the entire light cone entering the Paracorr, and it is mandatory that 100% of that light cone enter the bottom lens.

Then, move the scope back and forth to see how much of the light cone misses entering the bottom of the Paracorr when the Moon is off axis. You'll see the light cone angle off to the side.

If the Moon is still in the field of the scope (you can still see the bright light cone hitting the bottom of the Paracorr), and you can see the light cone on the bottom of the Paracorr

but moving the Moon off axis causes the bright light cone to mostly miss the Paracorr,  this is the source of your vignetting.  The Moon is only a portion of the field of the scope, and its light

should enter the bottom lens of the Paracorr when it is off axis until the Moon is occulted and the light cone disappears.  

 

If you graph the light cone from the primary, you'll see that to 100% illuminate the 42mm field in the 31mm Nagler would require a huge secondary.

If the focuser drawtube sticks out a lot beyond the Paracorr this could also be a source of vignetting.  Remember the Paracorr required you to move the focuser in by about 14mm,

placing the leading end of the drawtube closer to the secondary.  Since the P2 parfocalizes your eyepieces, essentially, I utilized a 1.5" drawtube travel (the shortest) to reduce this possibility.

My scope is f/5, though, instead of f/4.5, and my intercept distance is 17% of the focal length.  If your intercept distance is shorter, or you drawtube longer, this could be a source of vignetting.

To know for sure, you'd have to remove the Paracorr and insert the eyepiece and NOT refocus and just check for vignetting only (possible with an out of focus eyepiece).  If vignetting disappears,

then the Paracorr is definitely to blame, and not the focuser or secondary.

 

I don't fully understand why or how, I admit.  I can only say I see none in a 21mm or 17mm Ethos in my scope with the Paracorr in place.

 

In practice, we allow the illumination at the edge to be ~70%.

[areyoukiddingme]

I suspect it could be due to a couple issues or maybe both at the same time:

1) a too-small secondary for the intercept distance resulting in significant edge of field light loss.

The test for this is to see that vignetting reduces with shorter focal length eyepieces until it disappears at some point and no shorter focal lengths vignette.

2) an intercept distance too short for the entire light cone to enter the bottom lens of the Paracorr.  This is far more likely in short f/ratio scopes than longer ones.

 

 

A test you can do is to focus on the Moon, see where the light cone hits the bottom of the Paracorr.  If not all the light cone enters the bottom lens when the light cone is centered, the secondary is too small.

The Moon only represents a portion of the entire light cone entering the Paracorr, and it is mandatory that 100% of that light cone enter the bottom lens.

Then, move the scope back and forth to see how much of the light cone misses entering the bottom of the Paracorr when the Moon is off axis. You'll see the light cone angle off to the side.

If the Moon is still in the field of the scope (you can still see the bright light cone hitting the bottom of the Paracorr), and you can see the light cone on the bottom of the Paracorr

but moving the Moon off axis causes the bright light cone to mostly miss the Paracorr,  this is the source of your vignetting.  The Moon is only a portion of the field of the scope, and its light

should enter the bottom lens of the Paracorr when it is off axis until the Moon is occulted and the light cone disappears.  

 

If you graph the light cone from the primary, you'll see that to 100% illuminate the 42mm field in the 31mm Nagler would require a huge secondary.

If the focuser drawtube sticks out a lot beyond the Paracorr this could also be a source of vignetting.  Remember the Paracorr required you to move the focuser in by about 14mm,

placing the leading end of the drawtube closer to the secondary.  Since the P2 parfocalizes your eyepieces, essentially, I utilized a 1.5" drawtube travel (the shortest) to reduce this possibility.

My scope is f/5, though, instead of f/4.5, and my intercept distance is 17% of the focal length.  If your intercept distance is shorter, or you drawtube longer, this could be a source of vignetting.

To know for sure, you'd have to remove the Paracorr and insert the eyepiece and NOT refocus and just check for vignetting only (possible with an out of focus eyepiece).  If vignetting disappears,

then the Paracorr is definitely to blame, and not the focuser or secondary.

 

I don't fully understand why or how, I admit.  I can only say I see none in a 21mm or 17mm Ethos in my scope with the Paracorr in place.

 

In practice, we allow the illumination at the edge to be ~70%.

Actually, you do see some vignetting in your 17 ethos in p2. At least you did when we did a comparison! It was subtle, but it was there.

 

The fact that your scope (also a 12.5" F5) seemed to show a lesser effect than mine is consistent with the possibility that there's something going on with the secondary, so well worth investigating futher. But we didn't do back-and-forth A-B tests, so my recollection may be biased.

[CrazyPanda]

Thanks Don.

 

Last night I had clear skies again and I did some testing. I observed something, and I don't know what it is, so I will only report what I saw.

 

I put the 17 HW + Paracorr into the focuser defocused the image (I can't remember if I was inside or ouside focus)

 

I then centered on Polaris.

As I moved Polaris to the right of the field, I observed it starting to get cut off by some edge before it reached the field stop. This started at approximately 35 degrees from center. By the it reached the field stop, over 90% of the defocused star had been cut off by this "edge". So this means the center 70 degrees of the 102 degree field showed the full defocused star. Beyond that, the star got increasingly cut off, to the point that it had almost been completely cut off before it even reached the field stop. Only a sliver of light was left, at which point the field stop swallowed the rest.

When I switched focus, the curve of this "edge" switched orientation. It wen to the other side of the defocused star, and was curved away the curve of the field stop.

 

I'm assuming what I was observing was the illumination of the secondary mirror since defocusing it flipped the orientation of the cut off edge.

 

I repeated this test in the 17 HW, but without the Paracorr. Again, the star started getting cut off about 35 degrees from center (at the 70 degree field mark), but as I moved it towards the edge, nowhere near as much of the star got cut off. By the time the defocused star hit the field stop, roughly 50% of it remained, rather than a tiny sliver. At that point the field stop ate the rest of it. For consistency, I defocused by the same amount.

 

I then repeated this test with a variety of other eyepieces to see when the cut off started, and how much was cut off by the time it hit the field stop. That included eyepieces with large field stops (21E, 31 Nag), and eyepieces with small ones (10 E). The 21 Ethos, despite having a larger field stop than the 17 HW, only started showing a start being cut off about 40% from center, and with the Paracorr in place, was still greater than 50% visible by the time it hit the field stop (if I recall).

 

But oddly, the 10 Ethos showed a cutoff starting at around the same point (approximately 80% of the field). This was unexpected as the much smaller field stop should have shown the star being nearly totally preserved edge-to-edge.

 

Even more strangely, when I tried this experiment with the Docter, I noticed a slight protrusion in the shape of this cut-off edge. I then rotated the Docter and the protrusion rotated with it. This cut-off edge appeared to coming from the eyepiece, but it was NOT the field stop that was directly visible. It was still a few degrees away from the real field stop.

 

I couldn't reconcile this. If what I was seeing was the edge of the secondary mirror, then I should have not have seen that defect rotate when I rotated the Docter.

 

It was also strange given there appeared to be little difference in how stars cut off in the 21 Ethos and 10 Ethos despite very different field stops.

 

Either way, the 17 HW was the only eyepiece where off-axis defocused stars got more aggressively cut off with the presence of the P2. But whatever it is that's cutting off the stars, it certainly lines up with the vignetting I'm seeing. Over 90% of the star is gone by the time it reaches the field stop, which makes sense why it appears so dim when in focus and gets moved to the field stop.

 

I need to do more observing to better understand what I was really seeing, and next time I have clear skies, I'm going to do science and actually record values of what I'm seeing, along with taking photos or sketches.

 

Mel also got back to me with testing he's been doing and he said by stopping his scopes down to F/4.5, he was able to replicate what I was seeing. But he phrased it in a way that I needed some clarification on what he meant, so I'm waiting to hear back. He did say that the presence of the P2 with the Nikon made it much harder to get eye placement just right, which is also what I experience. It's far more comfortable without the P2 than with. But I don't know if that's necessarily related to the vignetting issue. I have also experienced the same thing with my other eyepieces. They're all more comfortable to look through without the P2 in place (it's like the P2 slightly reduces eye relief and reduces the "sweet spot" of the exit pupil). Again, separate issue from the vignetting, but possibly related in some way?

Edited by CrazyPanda, 12 October 2021 - 01:21 PM.

[Starman1]

Actually, you do see some vignetting in your 17 ethos in p2. At least you did when we did a comparison! It was subtle, but it was there.

 

The fact that your scope (also a 12.5" F5) seemed to show a lesser effect than mine is consistent with the possibility that there's something going on with the secondary, so well worth investigating futher. But we didn't do back-and-forth A-B tests, so my recollection may be biased.

Yes, but not severe.  I see it in the 21mm too.

Crazy Panda's description is that of a much more severe vignetting.  He is describing more than 2 magnitudes of darkening, which is getting into nebula filter territory.

[25585]

Which was designed first the P2 or 17E? Also is the HW more than 100° AFOV? As vignetting is said above to occur with the 21E, perhaps 17mm & longer 100° AFOV optics go beyond the easy capability of the P2?

 

As 17mm ES92 does not vignette, that 8° less AFOV might be making the difference.

[Starman1]

Well, I suspect there is a design issue with the eyepiece as it interfaces with the Paracorr that causes this.

I'd be interested in hearing an explanation from TeleVue as to why some eyepieces with larger field diameters don't visibly vignette while others with smaller field diameters do.

[25585]

Perhaps an interface lens is needed, as TV once developed one for their Big Barlow 2". Its interesting that the 17 HW with its EiC putting it in 14mm FL does not vignette, obviously something in the Barlow is putting things right enough for the P2 to cope with. 

 

Do the Ethos 17, 21 & 17 HW vignette with Barlows (any make) and Powermates at all? 

[ausastronomer]

Thanks for the info. 
 

Are you able to test your Nikon and your scope with the stock Paracorr tunable top and not with Hank’s custom adapter? 

I don't have the Nikon.  I used Hanks Nikon and his adapter, in my scope.   I have a PC2008, which is more than adequate for an F4.5 scope and a 17mm Nagler T4.

 

Cheers,

[ausastronomer]

Not related to OP's topic , but what thickness is your primary ? Fine scope .

If you were addressing that question at me, the primary is 33mm thick at the edge.  At the time Carl did the mirror (2007) it was the thinnest primary he had done to that time (at my request) and he wasn't prepared to go any thinner.  Carl used one of his 14.5" blanks and machined the diameter down from 14.5" to 14" and thinned it down to 33mm.  He did this for no extra charge.  My requirements were pretty specific in that I wanted a scope no faster than F4.5 and 14" aperture gave me the eyepiece height I wanted for no ladder use.

 

Cheers  

[ausastronomer]

I suspect it could be due to a couple issues or maybe both at the same time:

1) a too-small secondary for the intercept distance resulting in significant edge of field light loss.

The test for this is to see that vignetting reduces with shorter focal length eyepieces until it disappears at some point and no shorter focal lengths vignette.

2) an intercept distance too short for the entire light cone to enter the bottom lens of the Paracorr.  This is far more likely in short f/ratio scopes than longer ones.

 

I just recalculated all of the numbers on your secondary and FIF from what you posted earlier; and I noticed that your Intercept distance is quite long.  I know your UCA has a bit more clearance than mine (which only has 1cm)  but a general rule of thumb is that your intercept distance will roughly be the radius of your primary plus 3", or 75mm.  In your case that would be 10.5".  Add another 1/2" for your wider UCA and that gives you 11".  The Intercept distance you used in your newt calculation was 12 3/8ths, or 311mm.

 

I don't know if this has anything to do with your issue, but I thought I should point it out.

 

Cheers

[faackanders2]

I just recalculated all of the numbers on your secondary and FIF from what you posted earlier; and I noticed that your Intercept distance is quite long.  I know your UCA has a bit more clearance than mine (which only has 1cm)  but a general rule of thumb is that your intercept distance will roughly be the radius of your primary plus 3", or 75mm.  In your case that would be 10.5".  Add another 1/2" for your wider UCA and that gives you 11".  The Intercept distance you used in your newt calculation was 12 3/8ths, or 311mm.

 

I don't know if this has anything to do with your issue, but I thought I should point it out.

 

Cheers

Why did you have him machine a 14.5 inch clylinder blank to 14", since aperture is King!

[faackanders2]

Which was designed first the P2 or 17E? Also is the HW more than 100° AFOV? As vignetting is said above to occur with the 21E, perhaps 17mm & longer 100° AFOV optics go beyond the easy capability of the P2?

 

As 17mm ES92 does not vignette, that 8° less AFOV might be making the difference.

Nikon HW are 102 deg and I believe came out after TV's Paracorr II.

I have no issue with both in my 17.5" f4.1 Dob with 4" minor axis secondary.

 

My 20mm 100 AFOV ES also does not vignette in my scope (nor 30mm 82 AFOV, nor 40mm 70 AFOV).

I do not have 24mm 100 AFOV.

 

P.S Neither 9mm 120 AFOV nor 10mm 100 AFOV vignette either.

Edited by faackanders2, 12 October 2021 - 06:05 PM.

[ausastronomer]

Why did you have him machine a 14.5 inch clylinder blank to 14", since aperture is King!

Aperture had nothing to do with it, as I already had a 10"/F5 scope and an 18"/F4.5 scope and unlimited access to 20"/F5,25"/F5 and 30"/F4.6 scopes.  At the time I designed and had the scope built, I wanted a mid sized scope that was easily transportable, that I could use without a ladder. That meant a maximum eyepiece height of 63".  We worked out the maximum mirror focal length could be 1600mm to achieve this, and that required turning the primary mirror cell upside down. As the PII wasn't available in 2007 when I designed the scope, one of the other main criteria was that it had to be no faster than F4.5. To fit those criteria it had to be 14" and not 14.5".  At 14.5" and not 14" an F4.5 scope would have stood 2.25" taller.

 

Cheers

[areyoukiddingme]

Nikon HW are 102 deg and I believe came out after TV's Paracorr II.

I have no issue with both in my 17.5" f4.1 Dob with 4" minor axis secondary.

 

My 20mm 100 AFOV ES also does not vignette in my scope (nor 30mm 82 AFOV, nor 40mm 70 AFOV).

I do not have 24mm 100 AFOV.

 

P.S Neither 9mm 120 AFOV nor 10mm 100 AFOV vignette either.

Is your 40/70 the Pentax XW? I find that in Paracorr 2 that vignettes very badly indeed, but not in the paracorr 1.

[CrazyPanda]

To me, I think the biggest outlier of all the scopes from those who have reported in, is my scope's diagonal to focal plane distance.

 

It's 12.25" inches. I designed it that way to have a lot of spare in-travel to accommodate my binoviewer, and do native FL planetary imaging with my 224MC.

 

It also accommodates my 1.125" thick focuser board, which I designed to handle the weight of my binoviewer without flex.

 

My *hunch* is moving the focal plane closer to the secondary, and thus moving the P2 closer to the secondary, perhaps in combination with a slightly larger secondary, would resolve the issue.

 

With my new Lockwood mirror on the way, I will have an opportunity to test the first part of that hunch. I'm going to aim for a 10.5" to 11" focal plane distance and see what that gets me.

Edited by CrazyPanda, 12 October 2021 - 06:29 PM.

[Starman1]

I just recalculated all of the numbers on your secondary and FIF from what you posted earlier; and I noticed that your Intercept distance is quite long.  I know your UCA has a bit more clearance than mine (which only has 1cm)  but a general rule of thumb is that your intercept distance will roughly be the radius of your primary plus 3", or 75mm.  In your case that would be 10.5".  Add another 1/2" for your wider UCA and that gives you 11".  The Intercept distance you used in your newt calculation was 12 3/8ths, or 311mm.

 

I don't know if this has anything to do with your issue, but I thought I should point it out.

 

Cheers

I may have made an error.

For my own 12.5", the 

Radius of the UTA is 184mm

Distance to top of focuser board is 25mm

Height of focuser is 45mm

distance to focal plane is 25mm.

Total is 279mm, or 11", rounded off.  That is 17.6% of the 1587.5mm focal length, not the 17% I mentioned earlier.

Since the radius of the mirror is 158.8mm, that means there is an extra 120.2mm added to the mirror radius to the focal plane. 

 

75mm is impossible. The focuser board to focal plane distance is 70mm by itself.

 

If Crazy Panda's scope were set up exactly the same, his intercept distance would be 311mm, or 12-1/4", which is what he has, it seems.

If I subtract 1/4" from the tube clearance dimension, and 1/4" from the focal plane-to-focuser distance, and put the focuser board right at the inside of the tube ring (subtracting 1/2"),

I can pare 1" from the intercept distance on his 15", yielding a 11.75" (298.5mm) intercept distance.  Any shorter than this, the UTA inside diameter begins to vignette the field of the low power eyepiece,

or there is insufficient in travel to accommodate a Paracorr.  I don't see how you could go smaller than that without adding vignetting and having the focuser drawtube extend well inside the UTA when in focus.

Edited by Starman1, 12 October 2021 - 07:11 PM.

[ausastronomer]

I may have made an error.

For my own 12.5", the 

Radius of the UTA is 184mm

Distance to top of focuser board is 25mm

Height of focuser is 45mm

distance to focal plane is 25mm.

Total is 279mm, or 11", rounded off.  That is 17.6% of the 1587.5mm focal length, not the 17% I mentioned earlier.

Since the radius of the mirror is 158.8mm, that means there is an extra 120.2mm added to the mirror radius to the focal plane. 

 

75mm is impossible. The focuser board to focal plane distance is 70mm by itself.

 

If Crazy Panda's scope were set up exactly the same, his intercept distance would be 311mm, or 12-1/4", which is what he has, it seems.

If I subtract 1/4" from the tube clearance dimension, and 1/4" from the focal plane-to-focuser distance, and put the focuser board right at the inside of the tube ring (subtracting 1/2"),

I can pare 1" from the intercept distance on his 15", yielding a 11.75" (298.5mm) intercept distance.  Any shorter than this, the UTA inside diameter begins to vignette the field of the low power eyepiece,

or there is insufficient in travel to accommodate a Paracorr.  I don't see how you could go smaller than that without adding vignetting and having the focuser drawtube extend well inside the UTA when in focus.

 

Hi Don,

 

Those numbers all make sense now that I know Crazypandas focuser board is 1.25" (32mm) thick.  Mine is 12mm thick.

 

 

 

It also accommodates my 1.125" thick focuser board, which I designed to handle the weight of my binoviewer without flex.

 

 

I don't think your focuser board will flex.  That's somewhat over engineered IMO.  22mm hardwood plywood has a 32" span rating of 295lbs.  Over the size of your focuser board 1.25" (32mm) hardwood plywood would have a load rating well over a tonne. You could safely use a pair of your 32mm focuser boards as a pair of ramps to drive the pickup up the kerbing.

 

Cheers

[faackanders2]

Is your 40/70 the Pentax XW? I find that in Paracorr 2 that vignettes very badly indeed, but not in the paracorr 1.

University Optics 40mm 70AFOV MK-70 Koenig (widest TFOV in a 2" and love it for large, and  multiple objects).  Use it as a finder eyepiece, but not for very small nor very dim objects.

[faackanders2]

Aperture had nothing to do with it, as I already had a 10"/F5 scope and an 18"/F4.5 scope and unlimited access to 20"/F5,25"/F5 and 30"/F4.6 scopes.  At the time I designed and had the scope built, I wanted a mid sized scope that was easily transportable, that I could use without a ladder. That meant a maximum eyepiece height of 63".  We worked out the maximum mirror focal length could be 1600mm to achieve this, and that required turning the primary mirror cell upside down. As the PII wasn't available in 2007 when I designed the scope, one of the other main criteria was that it had to be no faster than F4.5. To fit those criteria it had to be 14" and not 14.5".  At 14.5" and not 14" an F4.5 scope would have stood 2.25" taller.

 

Cheers

My scope 17.5" f4.1 requires me (6'1") to stand on first step of ladder when looking at zenith.  Once I had a 6'6" person look through my scope at zenith with his feet on the ground and I told him this would be the perfect scope for him!.  At public observing parties toddlers have to be picked up by their parents to look through my scope if near zenith (and that is usually the best part of the sky, unless they need to see specific types of objects for their sky tour certificates).

Edited by faackanders2, 12 October 2021 - 09:40 PM.

[CrazyPanda]

Don, some extra information about my scope:

 

1. I have the 2.5" FT focuser. This is WAY overkill, especially when using the Paracorr. As my scope is designed right now, I have a 1/2" riser plate sitting above the focuser board for the express purpose of keeping the comically long draw tube out of the light path as much as possible.

2. When in focus, the draw tube is not in the light path, and it only punches into the light path by 1/8" when fully racked in.

3. Before adding the riser, I deliberately set the focal plane right in the middle of the focuser's travel. That gave me 1.25" in, and 1.25" out. Add a 1/2" riser, and I have spare 0.75" in and 1.75" out.

4. When using my binoviewer, I still have ~3/8" spare in-travel. That is, I only needed 3/8" of in-travel from the focal plane to reach focus with my binoviewer (I need to measure this, but that's what it looks like to my eye).

5. The Paracorr doesn't require as much in-travel as my binoviewer, even with the 17 HW (which requires a smidge of in-travel) and the nature of how it parfocalizes eyepieces means I need very little out-travel.

6. In fact, I can reach focus with ALL of my gear, using my 0.8" travel FT. I don't even need a 1.5" travel focuser.

 

Right now, with my current gear, I could make two simple adjustments:

 

1. Move the UTA up 1/2" with some shims between the lower UTA ring and the truss connector blocks, and mirror back 1/4" (I have the collimation travel to do this)

2. Remove the 1/2" riser on the focuser board.

3. This gives me a 0.75" reduction in diagonal to focal plane distance, while sacrificing 1/4" of in-travel that I simply have no need for.

4. Replace the 2.5" FT with the 0.8" FT

5. This gets me to 11.5" diagonal to focal plane distance, and nothing would even come remotely close to protruding into the light path even with the focuser racked all the way in, let alone at minimum focus for my binoviewer.

 

When the new mirror comes in:

 

1. The focal plane will be brought inward another ~1/2" due to how thin the mirror is (2" to 0.85", minus some height difference for the support triangles and a focal length that is 0.2" longer than my current one)

2. Replace the 1.125" thick focuser board with one that is only 0.5" (I would have to figure out a way to stiffen it, but I have some ideas).

3. Now I'm down to an 11" diagonal to focal plane distance, and at this point only the very bottom of the Paracorr would protrude into the light path very slightly when racked all the way in. But I don't need to be racked all the way in to reach focus with the Paracorr, so likely nothing would be in the light path. I don't use the Paracorr when binoviewing, which is what has the closest in-travel.

 

That's all without doing anything exotic or crazy.

 

So I think an 11" intercept distance is a realistic goal for my setup.

 

Will that fix my vignetting problem? Not sure.

 

To find out, I'm going to do an experiment whereby I take the focuser off the board entirely. This will let me get the Paracorr ~1.5" closer to the diagonal. The hole in the focuser board is 3" in diameter. I'm going to fit it with three shims to reduce the effective opening size to 2". I'm then going to slip the Paracorr in and bottom it out on the focuser board.

 

I'm not worried about achieving focus at this point, because I just want to see if it clears up the vignetting issue, which is readily visible as a darkening of the whole field, not just as a mag drop in stars. If it does, then I can proceed with all the adjustments mentioned before.

If it doesn't, then I would make the argument there's very little left as far as the scope is concerned to explain the vignetting, and I might have to live with the fact that some combination of my P2 sample, my 17 HW sample, and my eyesight (somehow) is responsible for what I see.

[CrazyPanda]

I don't think your focuser board will flex.  That's somewhat over engineered IMO.  22mm hardwood plywood has a 32" span rating of 295lbs.  Over the size of your focuser board 1.25" (32mm) hardwood plywood would have a load rating well over a tonne. You could safely use a pair of your 32mm focuser boards as a pair of ramps to drive the pickup up the kerbing.

 

I tend to agree, though I wanted to make sure the heavy Zeiss binoviewer I have wouldn't introduce any FAE after collimation.

 

I have since added an auto collimator to my collimation tools. That thing is stupidly sensitive to changes in focuser axial alignment. Even the wear marks in my Feathertouch's tool steel is enough to throw it off, that's literally thousandths of an inch variation, if that.

 

At the F/4.5 my dob is now (F/4.6 when I get the new mirror), auto-collimation is probably overkill accuracy, so I might not fret deflection in a 1/2" board that much, but I'd rather take basic steps to ensure stiffness where I can, just in case. The last couple of months have shown me incredibly sharp, crisp views of the planets and I definitely want to keep it that way. I've found it doesn't take that much FAE to blur ultra-fine details on Jupiter.

 

I'd rather be safe than sorry, so taking some extra steps to ensure stiffness will give me peace of mind that I'm not leaving performance on the table.

Edited by CrazyPanda, 12 October 2021 - 10:13 PM.

[Starman1]

Don, some extra information about my scope:

1. I have the 2.5" FT focuser. This is WAY overkill, especially when using the Paracorr. As my scope is designed right now, I have a 1/2" riser plate sitting above the focuser board for the express purpose of keeping the comically long draw tube out of the light path as much as possible.
2. When in focus, the draw tube is not in the light path, and it only punches into the light path by 1/8" when fully racked in.
3. Before adding the riser, I deliberately set the focal plane right in the middle of the focuser's travel. That gave me 1.25" in, and 1.25" out. Add a 1/2" riser, and I have spare 0.75" in and 1.75" out.
4. When using my binoviewer, I still have ~3/8" spare in-travel. That is, I only needed 3/8" of in-travel from the focal plane to reach focus with my binoviewer (I need to measure this, but that's what it looks like to my eye).
5. The Paracorr doesn't require as much in-travel as my binoviewer, even with the 17 HW (which requires a smidge of in-travel) and the nature of how it parfocalizes eyepieces means I need very little out-travel.
6. In fact, I can reach focus with ALL of my gear, using my 0.8" travel FT. I don't even need a 1.5" travel focuser.

Right now, with my current gear, I could make two simple adjustments:

1. Move the UTA up 1/2" with some shims between the lower UTA ring and the truss connector blocks, and mirror back 1/4" (I have the collimation travel to do this)
2. Remove the 1/2" riser on the focuser board.
3. This gives me a 0.75" reduction in diagonal to focal plane distance, while sacrificing 1/4" of in-travel that I simply have no need for.
4. Replace the 2.5" FT with the 0.8" FT
5. This gets me to 11.5" diagonal to focal plane distance, and nothing would even come remotely close to protruding into the light path even with the focuser racked all the way in, let alone at minimum focus for my binoviewer.

When the new mirror comes in:

1. The focal plane will be brought inward another ~1/2" due to how thin the mirror is (2" to 0.85", minus some height difference for the support triangles and a focal length that is 0.2" longer than my current one)
2. Replace the 1.125" thick focuser board with one that is only 0.5" (I would have to figure out a way to stiffen it, but I have some ideas).
3. Now I'm down to an 11" diagonal to focal plane distance, and at this point only the very bottom of the Paracorr would protrude into the light path very slightly when racked all the way in. But I don't need to be racked all the way in to reach focus with the Paracorr, so likely nothing would be in the light path. I don't use the Paracorr when binoviewing, which is what has the closest in-travel.

That's all without doing anything exotic or crazy.

So I think an 11" intercept distance is a realistic goal for my setup.

Will that fix my vignetting problem? Not sure.

To find out, I'm going to do an experiment whereby I take the focuser off the board entirely. This will let me get the Paracorr ~1.5" closer to the diagonal. The hole in the focuser board is 3" in diameter. I'm going to fit it with three shims to reduce the effective opening size to 2". I'm then going to slip the Paracorr in and bottom it out on the focuser board.

I'm not worried about achieving focus at this point, because I just want to see if it clears up the vignetting issue, which is readily visible as a darkening of the whole field, not just as a mag drop in stars. If it does, then I can proceed with all the adjustments mentioned before.
If it doesn't, then I would make the argument there's very little left as far as the scope is concerned to explain the vignetting, and I might have to live with the fact that some combination of my P2 sample, my 17 HW sample, and my eyesight (somehow) is responsible for what I see.

All good tests.
You do know the 17mm NAV HW requires a Paracorr position closer than setting A? Due to its long lower barrel, its optimum position would be a setting below A. As a result, you use setting A for the 17 but you cannot use the 17 for setting the Paracorr in the light cone from the primary.

Edited by Starman1, 12 October 2021 - 11:59 PM.

[CrazyPanda]

All good tests.
You do know the 17mm NAV HW requires a Paracorr position closer than setting A? Due to its long lower barrel, its optimum position would be a setting below A. As a result, you use setting A for the 17 but you cannot use the 17 for setting the Paracorr in the light cone from the primary.

Yeah I do use it in setting A. In that setting, I have to rack the focuser in a bit to reach focus since the P2 cannot make the 17HW parfocal with my other eyepieces. That does change the P2 position in the light cone a bit, but it's a very minor adjustment to the focus position.

 

I just very much hope there is a solution to the 17HW and the P2. Not everyone reports the same problem I'm seeing, so I'm hoping that some tweaks to the telescope will resolve it.

 

If not, maybe a call to Tele Vue for some insight, but as helpful as the folks at TV are, I'm not sure what they will say to an issue with a competitor's eyepiece.

 

Else, I will try to pick up a P1 on the market somewhere, as that appears to be less problematic for the 17HW.

Edited by CrazyPanda, 13 October 2021 - 12:10 AM.