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Beginner Question on Eyepiece TFOV

Eyepieces Beginner
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#1 RodgerDodger008

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Posted 25 October 2021 - 09:06 AM

Hi Cloudy Nighters smile.gif

 

My first post / question but have been non registered reader for about 2 months and just recently bought my first scope a Skywatcher 200p flexitube and having lots of fun with it. 

 

I was looking at Starman's list of eyepieces for 2021 and particularly comparing two eyepieces TFOV as calculated for 1200mm focal length and F6 scope on his spreadsheet. Amazing info btw!

 

Celestron Luminos 15mm 82degree AFOV

Celestron X-Cel LX 25mm 60 degree AFOV 

 

I have always calculated TFOV as AFOV/MAG but then I noticed on Starman's sheet the calculation uses the MFR's field stop. I did a bit of research and I understand how this calculation works. 

 

My question is based on the MFR's field stop is the 15mm Luminos really going to give me a wider field of view in the eyepiece (1.24 TFOV) than the 25mm Xcel (1.19 TFOV)?

 

If I do my old calculation of 82/80 = 15mm Luminos is 1.025 TFOV vs the X-Cel 25mm 60/48 = 1.25 TFOV.

 

I have the Luminos 15mm and I want to buy the 25mm Xcel to get a wider TFOV for DSO observing but now I'm not sure it will actually give me a wider field of view for my F6 scope.

 

Your help is much appreciated.

 

Clear skies!

 

 



#2 Echolight

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Posted 25 October 2021 - 09:40 AM

If you want the widest field of view the scope can offer, you’ll need to start looking at 2 inch eyepieces in the 30 to 40mm range. Usually with 68 to 70 degree apparent field of view: But sometimes 82 degree AFOV in the 28mm to 32mm range.

Something like this will offer a true field of view in the 1.7 to 2.2 degree range, depending on which eyepiece you choose.


Edited by Echolight, 25 October 2021 - 09:49 AM.

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#3 pregulla

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Posted 25 October 2021 - 09:51 AM

No, the 25mm Xcel won't give you wider field.

The true filed calculated from filed stop is the accurate one, value you get from AFoV is an approximate one, as it doesn't take distortion into account. 

15mm Luminous is already has nearly maximum field possible in 1.25" format (you can squeeze about 4 percent more by using something like ES68 24mm or 32mm plossl), so you indeed need to switch to 2" eyepieces to get larger TFoV


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#4 TOMDEY

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Posted 25 October 2021 - 10:09 AM

This graph also relates >>> I concocted it from existing data a few years ago. Overtly indicates how the draw-tube inner diameter ultimately limits the possibly-achievable field. The exact upper limits are subject to a bit of debate, and many telescopes have other limiters that constrict unvignetted field even more... but the affect is nevertheless inexorable. It becomes clear why people tend to upgrade ~graduate up~ from 0.965 to 1.25 to 2.00 to 3.00 inch draw tubes.   Tom

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  • 75 EYEPIECE GRAPH absolute field drawtube Tom Dey.jpg

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#5 RodgerDodger008

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Posted 25 October 2021 - 07:34 PM

If you want the widest field of view the scope can offer, you’ll need to start looking at 2 inch eyepieces in the 30 to 40mm range. Usually with 68 to 70 degree apparent field of view: But sometimes 82 degree AFOV in the 28mm to 32mm range.

Something like this will offer a true field of view in the 1.7 to 2.2 degree range, depending on which eyepiece you choose.

Thanks for the reply. I was trying to avoid swapping out to the 2 inch adaptor for essentially 1 eyepiece but I see now this will be the only way.



#6 RodgerDodger008

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Posted 25 October 2021 - 07:36 PM

No, the 25mm Xcel won't give you wider field.

The true filed calculated from filed stop is the accurate one, value you get from AFoV is an approximate one, as it doesn't take distortion into account. 

15mm Luminous is already has nearly maximum field possible in 1.25" format (you can squeeze about 4 percent more by using something like ES68 24mm or 32mm plossl), so you indeed need to switch to 2" eyepieces to get larger TFoV

Thanks this is really interesting, makes sense now that I think about my 52 degree plossl 25mm I got with the scope, it was odd to me that my 15mm Luminos felt so close in terms of TFOV.



#7 RodgerDodger008

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Posted 25 October 2021 - 07:38 PM

This graph also relates >>> I concocted it from existing data a few years ago. Overtly indicates how the draw-tube inner diameter ultimately limits the possibly-achievable field. The exact upper limits are subject to a bit of debate, and many telescopes have other limiters that constrict unvignetted field even more... but the affect is nevertheless inexorable. It becomes clear why people tend to upgrade ~graduate up~ from 0.965 to 1.25 to 2.00 to 3.00 inch draw tubes.   Tom

Thanks Tom, this clearly shows me the only way wider is 2 inch. much appreciated.



#8 RodgerDodger008

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Posted 25 October 2021 - 07:39 PM

Thanks all, this info has saved me from buying essentially a useless eypiece in the context of what I already have . will start looking at 2 inch for my starfield and DSO eyepiece.


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#9 Jon Isaacs

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Posted 25 October 2021 - 08:28 PM

Hi Cloudy Nighters smile.gif

 

My first post / question but have been non registered reader for about 2 months and just recently bought my first scope a Skywatcher 200p flexitube and having lots of fun with it. 

 

I was looking at Starman's list of eyepieces for 2021 and particularly comparing two eyepieces TFOV as calculated for 1200mm focal length and F6 scope on his spreadsheet. Amazing info btw!

 

Celestron Luminos 15mm 82degree AFOV

Celestron X-Cel LX 25mm 60 degree AFOV 

 

I have always calculated TFOV as AFOV/MAG but then I noticed on Starman's sheet the calculation uses the MFR's field stop. I did a bit of research and I understand how this calculation works. 

 

My question is based on the MFR's field stop is the 15mm Luminos really going to give me a wider field of view in the eyepiece (1.24 TFOV) than the 25mm Xcel (1.19 TFOV)?

 

If I do my old calculation of 82/80 = 15mm Luminos is 1.025 TFOV vs the X-Cel 25mm 60/48 = 1.25 TFOV.

 

I have the Luminos 15mm and I want to buy the 25mm Xcel to get a wider TFOV for DSO observing but now I'm not sure it will actually give me a wider field of view for my F6 scope.

 

Your help is much appreciated.

 

Clear skies!

 

The question in my mind, is the the field stop of the 15 mm Luminos accurate? It's listed at 26 mm, that's very large for a 15 mm 82 degree eyepiece. For comparison, the 16 mm Type 5 Nagler has a 22.1 mm field stop and the 13 mm Ethos has a field stop of 22.3 mm.

 

A 26 mm field stop would be consistent with a 99 degree AFoV..

 

Here's the potential issue:

 

An eyepiece like the 15 mm Luminos is a negative-positive eyepiece. That means it has a Barlow-like negative lens in the barrel which extends the effective focal length of the telescope. The positive lens in the body actually magnifies the image.

 

The field stop of a negative-positive eyepiece lies between the two sections. So normally what is reported for a negative-positive eyepiece is the "effective" field stop diameter rather than the actual field stop diameter. The lengthing effect of the negative lens must be accounted for when calculating the effective field stop diameter.

 

The effective field stop diameter provides the correct TFoV, the actual field stop diameter will normally over estimate the TFoV of a negative-positive eyepiece. 

 

In this case, I'd try to measure the actual TFoV. This can be done by "drift timing", measuring the time it takes a star to cross the field of view.  Ideally the star has a declination of near zero.  Then you divide by 239.34 (seconds/degree) to get the TFoV 

 

TFoV = Drift time (seconds) / 239.34 (seconds) x cos (declination) degrees.

 

The 239.34 seconds comes from the fact that the sideral day is slightly shorter than the solar day. The solar day is 24 hours so the sky rotates 15 degrees per hour or 4 minutes per degree.

 

Jon


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#10 sevenofnine

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Posted 25 October 2021 - 09:45 PM

Welcome to C/N! welcome.gif

The way I avoid adapters is by mostly using a Baader 8-24mm Mark IV zoom eyepiece with it's own 2" threaded skirt. That way I only switch from my 2" wide angle eyepiece to the 2" zoom for higher powers. It's nice to have on those nights when you just don't feel like juggling expensive black glass in the dark ohmy.gif


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#11 gnowellsct

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Posted 25 October 2021 - 11:51 PM

The way I avoid changing adapters is for every 1.25 inch I piece I have in use I buy a 2 in adapter and I leave it on. That way every eyepiece is 2 in and I don't have to think about it.
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#12 RodgerDodger008

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Posted 26 October 2021 - 12:01 AM

 

An eyepiece like the 15 mm Luminos is a negative-positive eyepiece. That means it has a Barlow-like negative lens in the barrel which extends the effective focal length of the telescope. The positive lens in the body actually magnifies the image.

 

The field stop of a negative-positive eyepiece lies between the two sections. So normally what is reported for a negative-positive eyepiece is the "effective" field stop diameter rather than the actual field stop diameter.

This is a excellent point, I was wondering what impact impact the accuracy of the Mfr's fieldstop could have.

 

I still think on balance the 25mm X-cel LX 60 degree will be too close in tfov to the 15mm Luminos to be worth it for me. 

 

I will do a bit of a search online to see if anyone has measured the effective field stop for the 15mm Luminos.

 

Thanks heaps for your insighful comment.



#13 RodgerDodger008

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Posted 26 October 2021 - 12:03 AM

Welcome to C/N! welcome.gif

The way I avoid adapters is by mostly using a Baader 8-24mm Mark IV zoom eyepiece with it's own 2" threaded skirt. That way I only switch from my 2" wide angle eyepiece to the 2" zoom for higher powers. It's nice to have on those nights when you just don't feel like juggling expensive black glass in the dark ohmy.gif

thanks great to be a member and amazed already how supportive this community is :)

 

Just saw the baader ones today and realised some work on 1.25 and 2 inch focuser, very cool!



#14 RodgerDodger008

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Posted 26 October 2021 - 12:09 AM

The way I avoid changing adapters is for every 1.25 inch I piece I have in use I buy a 2 in adapter and I leave it on. That way every eyepiece is 2 in and I don't have to think about it.

 

is this a 2inch to 1.25 adapter? I got a 1.25 inch and 2 inch adaptor with the scope. I think to get really wide views im ok now after reading all this to swap out.

 

I think if viewing wide field targets I probably wont switch out to high power eye pieces.

 

right now I have the 7mm & 15mm Luminos. they are ok, could not afford ES. But I am worried a little about getting 2inch in Luminos, many report very bad edge of field brightening in the 2inch luminos eyepiece especially the 19mm. 

 

wont need the 19mm or 23mm but a 31mm would be nice. could always buy it on amazon test and send back of the eof brightening is too bad for my liking.



#15 Jon Isaacs

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Posted 26 October 2021 - 05:00 AM

is this a 2inch to 1.25 adapter? I got a 1.25 inch and 2 inch adaptor with the scope. I think to get really wide views im ok now after reading all this to swap out.

 

I think if viewing wide field targets I probably wont switch out to high power eye pieces.

 

right now I have the 7mm & 15mm Luminos. they are ok, could not afford ES. But I am worried a little about getting 2inch in Luminos, many report very bad edge of field brightening in the 2inch luminos eyepiece especially the 19mm. 

 

wont need the 19mm or 23mm but a 31mm would be nice. could always buy it on amazon test and send back of the eof brightening is too bad for my liking.

 

I have quite a few eyepieces, both 1.25 inch and 2 inch.  I generally just use the adapter when swapping eyepieces. In your situation, you would swap one time, when going from the 2 inch to the 1.25 inch... It's no big deal.  

 

Regarding the 31mm Luminos.. My suggestion is the 30mm 70 degree APM UFF.  It's a high quality, sharp to the edge eyepiece.  I am very spoiled in terms of long focal length 2 inch eyepieces, my collection includes the 31mm Nagler, the 35mm and 41mm Panoptics plus the 32mm TV Wide Field and the 30mm APM UFF. 

 

I purchased the 30mm UFF in February, the others I have had for a number of years.  What I was looking for was a high quality eyepiece, comparable to the TeleVues in terms of sharpness but something that was lighter in weight and more compact. The 31mm Nagler and the 41mm Panoptic are close to a 1000 grams and they are bulky.  The 31mm Luminos is even heavier, 1134 grams and bulkier.   These are big eyepieces and the weight can create balance problems as well as handling problems. 

 

The 30mm APM only weighs 550 grams.. And I am using it a great deal.. The field of view is wide enough, in your scope it would provide a 1.73 degree TFoV. 

 

(I measured the TFoV by the above mentioned drift timing procedure, the field stop is 36.3mm)

 

It's a $250.. 

 

https://www.eyepiece..._p/17102030.htm

 

I tend to think of eyepieces as decent and as "keepers."  A decent eyepiece is one that is enjoyable to use but in the back of your mind, there is always the thought that some day, I will get sometime better.  A keeper, that's an eyepiece that a lifetime eyepiece.. 

 

I owned the 23mm Luminos for a while, it was OK but the EFoB and the overall sharpness, not a keeper.  The 30mm APM UFF, it's a keeper.  

 

Jon


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#16 Echolight

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Posted 26 October 2021 - 05:20 AM

I agree that the weight can be a significant factor for ease of use. Balance.

 

And from all accounts, the 30 UFF is enough better in image quality than the typical 32mm SWA to justify the higher price.
Wider and less expensive than a 27 Panoptic. And for most, easier to look through.


Edited by Echolight, 26 October 2021 - 05:21 AM.

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#17 gnowellsct

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Posted 26 October 2021 - 06:51 AM

is this a 2inch to 1.25 adapter? I got a 1.25 inch and 2 inch adaptor with the scope. I think to get really wide views im ok now after reading all this to swap out.

I think if viewing wide field targets I probably wont switch out to high power eye pieces.

I think that's probably not true that you won't want to switch eye pieces. You'll see something in the wider field that you think bears a closer look.

Yes I have 2 inch to 1.25 in adapters on every eyepiece. For me the adapter issue has all but disappeared. The only time it comes up is if I lend an eyepiece to someone in the field. I use just one adapter for two or three years and there was enough aggro involved that I made the switch.

Martin that company 7 mentioned that was what he did. At first the very idea of it struck me as a wild extravagance. But once the idea was planted it never let go. At some point about 18 years ago I bought a bunch of stellar view two to 1.25 inch eye pieces and then moved on to other things. I've never regretted it.

I also never regretted building an eyepiece box and getting rid of those plastic bolt containers. I was spending too much time at the end of every night putting everything back together. And they effectively double a volume that you have to carry.

For beginners I recommend that you get what you can afford. When I was acquiring my first telescopes my view of eye pieces was that the best way to improve the view through an eyepiece was to give it bigger telescope to sit in. I made gradual eyepiece improvements until I had a c14 and then I turned my attention to eyepieces.

In the matter of widefield eye pieces acquiring a great refractor drove the need for quality much faster than I had anticipated. My first refractor was an FS 128. It is f/8.1 but even so it's going to teach you a thing or two about why you don't want cheap eye pieces.

I started at the bottom most budget eyepieces, which in those days were Riny and Russell eyepieces and gradually worked my way up to midrange. There was a long pause there because so many people had helpfully but grossly incorrectly explained to me that eyepieces don't matter on an SCT. They very much do.

Eventually what happens is your observing program and accumulating experience will drive your decisions. You will have an ahhh moment. It will probably be with a Pentax or televue option.

At the time I was building my collection the Meade Japanese 14 mm uhw was highly prized and recommended. CN did not even exist. Another one was the nagler 17 mm 82°. Those were my first truly luxury high-end eye pieces.

When I tried Pentax I couldn't get rid of the Meade fast enough. It took longer to get rid of the nagler 17 t4 but eventually it went too. Since then televue has introduced the delos and ethos lines. I have tried them but they're not for me.

Anyhow your observing program and budget will let you know when it's time to upgrade. For me it was when I was looking for one arc second planetary nebulae in Cygnus and I realized that every Star looked like a one arc second planetary nebula at the edge. And then the refractor entered my life and the 2-inch 40 mm University Optics had to go. That was a mid-range Japanese eyepiece that had a great reputation. I lent it to a guy once who had never used a 2-in eyepiece in his dob and he was blown away I was too when I first started using it. He went out and got one. A you or so later I got rid of mine. He was ready for uo. And later on I was ready to get rid of it.

An eyepiece collection is a gradual process. An eyepiece you think is fine gradually gets to be annoying. You have made your other big ticket acquisitions and finally decide to try this or that ocular. Eventually you find the one that is a very good match and you stick with it and you don't see any possible reason to change. And so on for your entire collection.

Edited by gnowellsct, 26 October 2021 - 06:57 AM.

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#18 RodgerDodger008

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Posted 26 October 2021 - 08:10 AM

 

Regarding the 31mm Luminos.. My suggestion is the 30mm 70 degree APM UFF.  It's a high quality, sharp to the edge eyepiece.

 

Jon

Thanks Jon, immediately googled that one. I think my problem is in Australia the variety of easily available choice does not seem that great. I'm going to have alook if I secure that APM here but will need a bit more than a quick google it seems. Thanks for the advice.



#19 RodgerDodger008

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Posted 26 October 2021 - 08:16 AM

I think that's probably not true that you won't want to switch eye pieces. You'll see something in the wider field that you think bears a closer look.


An eyepiece collection is a gradual process. An eyepiece you think is fine gradually gets to be annoying. You have made your other big ticket acquisitions and finally decide to try this or that ocular. Eventually you find the one that is a very good match and you stick with it and you don't see any possible reason to change. And so on for your entire collection.

wow man this was a cool read, thanks for sharing



#20 RodgerDodger008

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Posted 26 October 2021 - 08:18 AM

I agree that the weight can be a significant factor for ease of use. Balance.

 

And from all accounts, the 30 UFF is enough better in image quality than the typical 32mm SWA to justify the higher price.
Wider and less expensive than a 27 Panoptic. And for most, easier to look through.

Weight is defo a factor but my dob is super sturdy, yet saying that if (when) i get another scope I need to consider the eyepieces I buy now so you are right



#21 Echolight

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Posted 26 October 2021 - 08:26 AM

Weight is defo a factor but my dob is super sturdy, yet saying that if (when) i get another scope I need to consider the eyepieces I buy now so you are right

It’s about balance. When you switch from a 10 ounce eyepiece to a 2 pound eyepiece, there tends to be a shift in balance that could lead to the front of your dob tipping down a bit. With a 10 ounce to 20 ounce swap there is less chance of this being an issue.


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#22 RodgerDodger008

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Posted 01 November 2021 - 08:48 AM

ok I can confidently say the 15mm Luminos does not have a TFOV of more than my 52 degree 25mm plossl. I counted the stars I could see in each and the 25mm plossl was slightly wider.

This is telling me the mfr field stop on the luminos is giving a false estimate bof the TFOV.

I think I will get that 60 degree xcel lx 25mm after all


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