Baader Hyperion Zoom – thoughts from a newbie

[Spile]

Thanks to everyone for updates and links. I've edited by post above.

I measured the difference at 5mm so even less than the 6mm found with the MK III.

 

 

Edited September 22, 2021 by Spile

[Franklin]

It can be used in any of these configurations. The reason I use the barlow in 2" mode without the 1.25" nosepiece is because it reduces the length and does not foul the diagonal. If you have the barlow attached to the 1.25" nosepiece and then use the 2" skirt, be sure to check the barlow doesn't come into contact with the mirror/prism in your diagonal. Fitting the barlow to either the 1.25" nosepiece or the zooms field length snout, results in a slightly different distance that the barlow element is from the zoom and thus gives a slightly different amplification. With the zoom @ 8mm the difference in either configuration amounts to 3.4mm or 3.5mm effective focal length. If you put the barlow on the zooms field lens snout and try using it in 1.25" it will not work, the barlow will unscrew when you turn the zoom.

[Spile]

7 hours ago, Franklin said:

If you put the barlow on the zooms field lens snout and try using it in 1.25" it will not work, the barlow will unscrew when you turn the zoom.

As discussed earlier in this thread.

 

[Don Pensack]

10 hours ago, Spile said:

Thanks to everyone for updates and links. I've edited by post above.

I measured the difference at 5mm so even less than the 6mm found with the MK III.

 

 

I still don't get it.

In neither case does the section with the name Hyperion on it rotate.

In both cases, the section with the rubber gripper does rotate.

 

[Spile]

3 minutes ago, Don Pensack said:

I still don't get it.

In neither case does the section with the name Hyperion on it rotate.

In both cases, the section with the rubber gripper does rotate.

 

I am not sure how to convince you that it does other than taking a video which seems overkill. Do you have a Mark IV ?

[Tiny Clanger]

9 minutes ago, Spile said:

I am not sure how to convince you that it does other than taking a video which seems overkill. Do you have a Mark IV ?

Just checked his online shop, looks like he has 19 in stock ...

[Spile]

4 minutes ago, Tiny Clanger said:

Just checked his online shop, looks like he has 19 in stock ...

I am looking forward to the explanation!

[Pixies]

Here you go: 

 

[Don Pensack]

OK, got it.

The inner field lenses turn no matter how the Barlow is configured.

So if the Barlow is attached to the 1.25" tube that is attached to the lower section, it doesn't turn, though the field lenses in the zoom are turning above it.

But if it's connected to the field lens section itself it will turn with the upper section.

 

In some Zooms, the field lenses (or interior lenses) move up and down but do not turn.  In the Baader, obviously the field lenses move up and down by turning.

 

Thank you, by the way, for posting the short video.  I appreciate it.  I guess I'd always attached the barlow to the 1.25" tube.

[Spile]

Thanks for saving me a job @Pixies ! 

[PeterC65]

I was observing the almost full Moon last night with the BHZ and took the opportunity to check the AFOV that I was getting for a 24mm zoom setting. I found I could only just fit the Moon’s disc into the field of view of the EP and I’d say I was getting an AFOV of about 0.6°. With my Skymax 127 that suggests the BHZ is giving a FOV of 38° for 24mm. The Baader spec says it should be 48° but @Louis D mentioned in this post that it may be less and this review of the BHZ measures the FOV at 42° for 24mm. I haven’t checked the FOV at 8mm but the Baader spec is 68° and that doesn’t seem to be disputed.

In a previous post I’d had a discussion about wider AFOV and considered upgrading to 2” eyepieces, but since the AFOV of the Skymax 127 is always going to be limited, and given the cost involved in switching to 2”, I’ve decided to stick with 1.25”. Having made this decision, I looked around for the longest FL EP I could get with a 1.25” barrel and came upon the Celestron Omni Plossl 40mm which I now have. This only has a FOV of 43° but at this FL it gives me the maximum AFOV that I can get with a 1.25” barrel and the Skymax 127 which is about 1.1°.

The other reason for going for a 40mm Plossl was to maximise the exit pupil size which people say is helpful when using a UHC filter (since it produces a brighter image). With the Moon so bright I’ve not had a chance to try the UHC filter, or more correctly, I tried it and saw nothing which I’m hoping is down to the brightness of the Moon! It was quite hard to keep the image in view with the 40mm Plossl last night but I assume this was because the exit pupil, at 3.4mm, wasn’t much smaller than my eye pupil in the moonlight? I just needed to stay very still to keep the image fully in view.

The Moon looked good through the Celestron EP and easily fitted into its field of view. It also looked good through the BHZ and I found it helpful to be able to adjust the zoom to see just the right amount of cratered arc (zoomed to 12mm seemed about right). The craters in shadow looked amazing by the way!!

[Louis D]

1 hour ago, PeterC65 said:

I was observing the almost full Moon last night with the BHZ and took the opportunity to check the AFOV that I was getting for a 24mm zoom setting. I found I could only just fit the Moon’s disc into the field of view of the EP and I’d say I was getting an AFOV of about 0.6°. With my Skymax 127 that suggests the BHZ is giving a FOV of 38° for 24mm. The Baader spec says it should be 48° but @Louis D mentioned in this post that it may be less and this review of the BHZ measures the FOV at 42° for 24mm. I haven’t checked the FOV at 8mm but the Baader spec is 68° and that doesn’t seem to be disputed.

Remember, the field stop diameter actually determines the TFOV visible, not the angular size of the AFOV.  There can be, and generally is, quite a bit of edge distortion that decreases the TFOV from what an AFOV calculation would predict.  Thus, I'm sure the actual AFOV of the BHZ is actually around 40° to 42°, but due to distortion, the effective AFOV (eAFOV) is actually 38° as you found.  The AFOV of 68° at 8mm may actually translate to an eAFOV of 66° due to distortion.  Only detailed measurements can resolve this issue.

The 24mm ES-68 has quite a bit of distortion, being modeled after the Panoptic line.  My 27mm Panoptic has 68° to 69° of AFOV, but has an eAFOV of only 65° to 66° due to distortion.  Thus, using AFOV figures to calculate TFOV is fraught with error unless you know the eAFOV based on field stop measurements.

[Don Pensack]

OK, the 27.0mm field stop in the 24mm Panoptic is consistent with an eyepiece with a 64.5° field and zero distortion.

I don't see what value that information has, because the eyepiece has a 68° apparent field.

Yes, it's due to pincushion distortion, but all eyepieces of that focal length and widest field for 1.25" have distortion.

The APM 24mm UFF has a 27.3mm field stop (determined by star timing).  That is consistent with a 65.2° apparent field.

Is it orthoscopic?  No, because people who have measured the apparent field get 63° +/-, so the eyepiece's distortion characteristics modify the apparent field.

Of what value is knowing the calculated eAFOV is 65.2°?  None.

At best, such a measurement might give you a clue as to the amount and type of distortion, but, even knowing that, the eAFOV figure doesn't have any value, because that is not what you see.

You see the AFOV and you see the TFOV indicated by the Field diameter.

Trying to adjust the AFOV so it can derive a TFOV simply doesn't make sense, because you have to know the actual field stop before you can derive the eAFOV,

and if you know that, what's the point of eAFOV?

 

[Don Pensack]

In essence, if you want to know the field stop diameter that simply ignores distortion, do a star timing and you'll be very accurate.

If you want to know the apparent field, use the flashlight test (or a similarly-measured test) and get a figure that is +/- 0.5°.

AFoV won't allow you to calculate TFoV, but the field diameter will.  And be a LOT more accurate than TF = AF/M

 

Example: 24mm Panoptic, using the TF = AF/M in my scope = 0.894°

24mm Panoptic, using field stop diameter and telescope focal length TF = (FS/TFL) x 57.2958 = 0.847°, which is 5.3% smaller.

[this is the reason the Astronomy Tools field comparator is a bogus tool--it uses the TF=AF/M formula to calculate the field]

Neither formula is easier or harder than the other, so starting with field stop is just easier than using the field stop diameter to calculate a fictitious effective apparent field to fit the first formula.

 

 

[PeterC65]

On 24/09/2021 at 14:29, Louis D said:

Remember, the field stop diameter actually determines the TFOV visible, not the angular size of the AFOV.

I assume this is only the case when the field stop diameter is the limiting factor, so when (FS/TFL) x 57.2958 is smaller than AF/M to use the formulae mentioned by @Don Pensack. Generally then, the field stop diameter will only be the limiting factor for higher FL EPs and lower magnifications.

Also, am I right in thinking that the exit pupil size is not affected by the field stop diameter? The formula I have for it is [telescope aperture] / ( [telescope focal length] / [eyepiece focal length] ).

[PeterC65]

Thinking some more about the field stop diameter, I've taken measurements of the BHZ. The inner diameter of the 1.25" barrel measures 28mm and the inner diameter of the snout that we spoke about earlier in this thread measures 23.1mm. Inside the snout there is a field lens carrier which moves in and out when the zoom control turns (and as the snout rotates, as we have discussed). The inner diameter of the field lens carrier measures 17.6mm and so I guess this is the field stop diameter of the BHZ at all zoom settings. Interestingly this corresponds to the calculated TFOV (AF/M) of the BHZ for 24mm assuming the AFOV is 42° as measured in the article that I quoted. At higher zoom settings (smaller FL) the field stop diameter is not the limiting factor.

The inner diameter of the 1.25" barrel for the Celestron 40mm is 28.1mm and so this field stop is the limiting factor for the TFOV at 1.07° rather that the calculated value (AF/M) of 1.15°.

On 24/09/2021 at 22:02, Don Pensack said:

[this is the reason the Astronomy Tools field comparator is a bogus tool--it uses the TF=AF/M formula to calculate the field]

I'd noticed that this tool doesn't allow the field stop parameter to be entered. The Oculars add-in for Stellarium does allow this parameter to be entered but when it is non-zero the field stop diameter is always used to calculate the TFOV rather than using the AF/M formula and this seems to me to give incorrect results when the field stop diameter is not the limiting factor (i.e. most of the time!).

 

[Louis D]

16 hours ago, Don Pensack said:

Of what value is knowing the calculated eAFOV is 65.2°?  None.

OMG, it allows you to use bogus online tools like Astronomy Tools field comparator more accurately.  That's certainly one valuable usage I would think.  Too many newbies take the AFOV as gospel when it comes to calculating TFOV.  They end up passing over eyepieces with smaller AFOVs and lower distortion in favor of eyepieces with larger AFOVs and higher distortion thinking they're getting more TFOV.  I'm thinking specifically of the 24mm ES-68 vs 24mm APM UFF.

Why don't you go out and write a corrected ATFC webpage that uses field stop instead of AFOV for TFOV comparisons?  Until then eAFOV is very useful, at least for that webpage.

I also find eAFOV valuable when comparing eyepieces of like focal length.  Sure, I know the field stop values, but how does that translate at a particular focal length?  eAFOV has a much more intuitive feel to it than field stop diameter when comparing eyepieces at a particular focal length to better understand what will actually be seen if distortion was nonexistent.  I do use field stop values to compare eyepieces of different focal lengths' TFOVs, if that makes you feel any better.

[Don Pensack]

On 25/09/2021 at 08:43, PeterC65 said:

I assume this is only the case when the field stop diameter is the limiting factor, so when (FS/TFL) x 57.2958 is smaller than AF/M to use the formulae mentioned by @Don Pensack. Generally then, the field stop diameter will only be the limiting factor for higher FL EPs and lower magnifications.

Also, am I right in thinking that the exit pupil size is not affected by the field stop diameter? The formula I have for it is [telescope aperture] / ( [telescope focal length] / [eyepiece focal length] ).

The field stop diameter is the limiting factor for true field at all magnifications/eyepiece focal lengths.

Think of field stop as a circular opening in an index card, laid down on a map (the focal plane of your scope).  The opening determines how much map you see.

The scale of the image on the telescope's focal plane determines how much sky that opening sees.

Image scale of a telescope in degrees is (1/telescope focal length) x 57.296.  Multiply by 60 to get the image scale in minutes of arc.

Exit pupil is not affected by field stop diameter.

[Don Pensack]

On 25/09/2021 at 13:51, PeterC65 said:

Thinking some more about the field stop diameter, I've taken measurements of the BHZ. The inner diameter of the 1.25" barrel measures 28mm and the inner diameter of the snout that we spoke about earlier in this thread measures 23.1mm. Inside the snout there is a field lens carrier which moves in and out when the zoom control turns (and as the snout rotates, as we have discussed). The inner diameter of the field lens carrier measures 17.6mm and so I guess this is the field stop diameter of the BHZ at all zoom settings. Interestingly this corresponds to the calculated TFOV (AF/M) of the BHZ for 24mm assuming the AFOV is 42° as measured in the article that I quoted. At higher zoom settings (smaller FL) the field stop diameter is not the limiting factor.

The inner diameter of the 1.25" barrel for the Celestron 40mm is 28.1mm and so this field stop is the limiting factor for the TFOV at 1.07° rather that the calculated value (AF/M) of 1.15°.

I'd noticed that this tool doesn't allow the field stop parameter to be entered. The Oculars add-in for Stellarium does allow this parameter to be entered but when it is non-zero the field stop diameter is always used to calculate the TFOV rather than using the AF/M formula and this seems to me to give incorrect results when the field stop diameter is not the limiting factor (i.e. most of the time!).

 

It is a rare eyepiece where the field stop is the inside diameter of the eyepiece.  Usually, there is a small iris in there which makes the field stop smaller than the barrel.

The "usual" limits are 46mm for 2" eyepieces and 27mm for 1.25" eyepieces, though a few eyepieces sneak another 0.2-0.3mm onto that.

Your Celestron 40mm has a 27mm field stop for true field calculations.  Field stop is ALWAYS the limiting factor for true field of view because it ignores all distortion.

What IS true is that field stop won't tell you the apparent field.

 

The field stop of the Zoom changes as the internal lenses move.  that is a result of the field lens moving.  So there will not be a constant field stop diameter at all focal lengths.

If it was constant, the field at 8mm would be 3x as wide as at 24mm instead of ~1.5x.  But, the field stop is still the limiting factor for true field, even in a zoom.

[Don Pensack]

1 hour ago, Louis D said:

OMG, it allows you to use bogus online tools like Astronomy Tools field comparator more accurately.  That's certainly one valuable usage I would think.  Too many newbies take the AFOV as gospel when it comes to calculating TFOV.  They end up passing over eyepieces with smaller AFOVs and lower distortion in favor of eyepieces with larger AFOVs and higher distortion thinking they're getting more TFOV.  I'm thinking specifically of the 24mm ES-68 vs 24mm APM UFF.

Why don't you go out and write a corrected ATFC webpage that uses field stop instead of AFOV for TFOV comparisons?  Until then eAFOV is very useful, at least for that webpage.

I also find eAFOV valuable when comparing eyepieces of like focal length.  Sure, I know the field stop values, but how does that translate at a particular focal length?  eAFOV has a much more intuitive feel to it than field stop diameter when comparing eyepieces at a particular focal length to better understand what will actually be seen if distortion was nonexistent.  I do use field stop values to compare eyepieces of different focal lengths' TFOVs, if that makes you feel any better.

Distortion does not determine the true field seen, it only determines the apparent field.

So comparing field stop diameters will definitely tell you which eyepiece has the wider true field.

eAFOV is a calculation based on a poor formula, TF = AF/M, while field stop is a physical characteristic of the eyepiece.  eAFOV is not a physical characteristic of the eyepiece, unlike AFoV.

So in my Eyepieces Buyer's Guide, here is what the true field calculation is based on:

IF the mfr's field stop diameter is listed (known), the true field is calculated by TF = (EPFS/TFL) x 57.296, where EPFS is eyepiece field stop and TFL is telescope focal length

If the mfr's field stop is not known, it defaults to the calculated field stop using the formula (AF/57.296) x EPFL where AF is apparent field and EPFL is eyepiece focal length

If the field stop cannot be calculated (like in a zoom), the true field shows as N/A.

Using the spreadsheet, you can directly compare true fields of one eyepiece to another.

A link to the spreadsheet (current as of April.  I have a much more updated version but it is not posted anywhere):

https://www.cloudynights.com/topic/758306-2021-eyepieces-buyers-guide/?p=10917573

[PeterC65]

I guess that the issue, particularly for newbies, is that most EP manufacturers don't specify the field stop diameter but all do specify AFOV (from your spreadsheet @Don Pensack only 30% of EPs have a manufacturers field stop listed). Comparing the manufacturers specified field stop diameter, where one is specified, against the calculated field stop diameter in the spreadsheet shows a median difference of just over 3%, although this can be as much as 35%. This small median difference probably explains why people use the TFOV = AFOV / M formula. The problem I think is when this formula fails badly which it does when a 1.25" barrel diameter gets in the way for a high FL EP.

Edited September 25, 2021 by PeterC65

[Don Pensack]

To see whether your hypothesis is true, let's take a 13mm Ethos.

The field stop is 22.3mm.

Using the apparent field to calculate true field in my scope, TF = 0.712°

Using the field stop, TF = 0.70°, a difference of 0.012° or only 1.7%, or 0.7' of arc.

That's nothing.

Using a 31mm Nagler, the AF calculation yields 1.392°, while the field stop calculation yields 1.318°, a difference of 0.074°, or 5.3%, and 4.44'

So, you might be right--the smaller the field stop, the less the discrepancy between the true field calculated by apparent field versus field stop.

[Louis D]

@Don Pensack, how can you instantly get a feel for the amount of stretching or compression at the edge from AFOV and FS diameter without further calculations?  I happen to like comparing AFOV to eAFOV because I instantly get a feeling for which kind of distortion the eyepiece has and how much of it.  When the AFOV is greater than the eAFOV, I know the edge will appear stretched.  When the AFOV is smaller than the eAFOV, I know the edge will appear compressed.  When they equal, there is little to no noticeable stretching or compression.  The relative difference between the two gives me an indication of how strong the effect will appear.  I'm at a loss to understand this relationship from merely looking at the AFOV and the FS diameter without further calculations.  Please describe how these two values describe the edge distortion present in an eyepiece without further calculation.  Perhaps I'm missing something obvious.

[Don Pensack]

Louis, you're right.

But I can't tolerate eyepieces with edge compression, or barrel distortion.

It makes me feel the field is rolling over a ball or that I'm looking at the surface of a globe.

It's why I don't own the 12.5mm Docter or the Nikon NAV-SWs.

 

The good news is that eyepieces with edge compression are very rare among astronomy-oriented eyepieces.

There are no Pentax, TeleVue, Explore Scientific, Stellarvue, or Baader eyepieces with it.  The APM XWAs don't, but I'm not sure about the UFFs.  It looks like the AFoVs claimed for the UFFs might be calculated eAFoVs.

And since almost all eyepieces have varying degrees of radial stretching at the edge, I just don't find it objectionable or even noticeable, regardless of percentage.

Judging from comments on this and many other forums, I think my point of view is a common one.

 

 

Edited September 26, 2021 by Don Pensack

[Louis D]

For the 24mm APM UFF, I measured the AFOV to be 63° and the eAFOV to be 66° (27.5mm field stop diameter).

For the 30mm APM UFF, I measured the AFOV to be 72°/73° (projection/photographic) and the eAFOV to be 70° (36.4mm field stop diameter).

Thus, one must have edge compression while the other edge expansion.  In use, neither is all that noticeable.  The 24mm's edge vignetting/fuzziness right at the field stop is noticeable, though.