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Practical angle of view for dobsonian

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#1 CFM

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Posted 10 August 2022 - 11:22 AM

I am a beginner observer and I am in the process of learning about the pros and cons about high magnification eyepieces, particularly when used with a dobsonian telescope.

I own a Celestron starhopper 8 inch telescope and I have used it with its 25mm eyepiece it came with and a 2x Barlow. Although I have been able to see Saturn and the details of the moon, I noticed that constant adjustment of the scope is necessary or I need to place the scope so the object moves into the eyepiece frame.

I am not sure if this applies to all dobsonians telescopes, but with mine adjusting the telescope between the mount stops takes some time. I use a wedge to make this work. I assume this task will become more tedious as I move to a 9/7/5mm eyepiece since the angle of view will be smaller at higher magnifications.

If you can share your experience with the same type or similar telescopes with high magnification eyepieces and any tricks to work around this, I will really appreciate it.

Is this a limiting factor on what eyepiece you have chosen for your dobsonian telescope? If it was, what was the max magnification you use on your dob telescope?


Edited by CFM, 10 August 2022 - 11:24 AM.

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#2 SoCalPaul

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Posted 10 August 2022 - 11:31 AM

I am not sure if this applies to all dobsonians telescopes, but with mine adjusting the telescope between the mount stops takes some time. I use a wedge to make this work. I assume this task will become more tedious as I move to a 9/7/5mm eyepiece since the angle of view will be smaller at higher magnifications.

Hey CFM, can you please explain what you mean by, "I use a wedge to make this work."?

 

The issue that you identify is why I LOVE a telescope that tracks the sky. :-)

 

Having said that, I also love my 16" Dob.

 

But bottom line, unless you have either 1) an alt-az drive system for your dob, or 2) an equatorial platform, objects will always drift out of the field of view. And this will happen faster and faster at higher magnifications with smaller true fields of view.

 

One way to deal with this, to a degree, is to invest in wider field eyepieces such as Naglers, Delos, etc. The object will remain in the FOV a little longer.

 

But bottom line, without a drive system, you will always have to adjust the scope and yes, as the magnification increases and true field of view decreases, you will have to do this more and more often.

 

This is one reason why people invest so much thought, time and money in the smoothness of their dob bearings and movements. Too much stiction or herky-jerky movements can completely ruin the pleasure of an observing session.

 

If your dob's movements are not smooth or require too much force to overcome the stiction (and therefore jerks too far in one direction), you might want to investigate ways to clean and/or improve your bearings.

 

Hope this helps.

 

Clear skies,

Paul


Edited by SoCalPaul, 10 August 2022 - 11:32 AM.


#3 rhetfield

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Posted 10 August 2022 - 11:40 AM

Generally, people get better at tweaking the scope aim as they get more practice.  They also put a lot of time and effort into getting the scope motions adjusted with the right amount of friction to move smoothly.  Wide angle eyepieces help, but they are pricey.

 

On a scope that size, atmospheric conditions are as likely as anything to limit magnification.  Near as I can tell, your scope is somewhere around an F6.  Therefore, the 5mm will likely about as short as you will want to go.  More often, the atmosphere will limit you to using the 7 or 9mm.


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#4 bbasiaga

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Posted 10 August 2022 - 11:52 AM

Its really a limit in any scope that doesn't have a tracking drive.   Higher power views = lower field of view.  Which, as Paul said, can be offset to some degree by getting 68, 70, 82 or 100 degree eyepieces (and why they are so expensive and popular).  

 

Did your current eyepiece come with the scope/  If its a simple 1.25" 25mm eyepiece its probably a plossl or kellner type, which has around a 50degree AFOV.  If you replaced that with a 68 or 82 AFOV eyepiece you'll increase your drift time by quite a bit.  That will be a 2" eyepiece, so make sure your focuser can handle that.  For higher magnification, you'll find 68-100 degree eyepieces in the 17mm and less range.  This also makes higher power viewing much better.  WIth the simple designs eye relief shrinks with focal length, making them hard to use from that perspective as well as from the tiny FOV perspective.  

 

Since you mention you are learning, I'll add this.   Magnification isn't always the main goal.  Though for planets it kind of is, to be fair.  For many deep sky objects, brightness and contrast are it, and both can be had at more moderate powers with good eyepieces.  It would be good for you to run an eyepiece calculator and find an eyepiece that will give you a 1 degree true field of view, or close to that.  That will become your finder/general use eyepiece.  Then another one at half that FOV/roughly twice the magnification/half the focal length.  Two good eyepieces with wider AFOV will give you good views that also have a wider TFOV (true field of view), and improve your viewing experience. 

 

Your scope shouldn't be too hard to move along.  Maybe there is a clutch or something overtightened.  Share some pics if you can.  For coping with small field of views, it is common to allow the object to drift across the field as you watch it, then reposition and repeat.  So you've already figured that out. 

 

 

-Brian

 

TFOV = true field of view.  This is how much of the sky you actually see through the combination of your scope and eyepiece.  

 

AFOV = apparent field of view.  This is how wide of an angle you appear to look through to see the field stop of your eyepiece.  With the same scope, a wider AFOV eyepiece will provide a wider TFOV than another eyepiece of the same focal length with a lower AFOV.  i.e a 20mm nagler at 82 AFOV will provide a wider true field than a 20mm plossl with a 55 degree AFOV, but both images will have the same magnification.   


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#5 ButterFly

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Posted 10 August 2022 - 01:45 PM

It's practice for the most part.  The scope's physical limitations will be reached eventually.

 

Start with objects that are transiting meridian.  As they pass through meridian, objects are mostly moving in AZ only.  There is very little ALT motion.  That reduces your tracking to one dimension.  See how much power you can use smoothly, without noticing jitters or motions.  See how much power you can use by using shift and drift.  Push those limits as you get comfortable - that's what practice is.  Now you know your AZ axis.

 

As objects get further from meridian, their ALT motions grow larger and larger.  Then you are tracking on both axes.  For objects due East or West, near the horizon, the motion is nearly all ALT.  Views are generally terrible near the horizon, but they make for good practice.  Practice just like with AZ, but for ALT this time, when the seeing isn't great anywhere else anyway.

 

Anywhere else in the sky is some mix of the two.  That will take practice as well.  Fortunately, you are looking at objects, so there is incentive to practice!  The immediate feedback will get you good in no time at all.  Once you have maxed out your scope's physical abilities, you can start asking questions such as: "how do I make the axes smoother;"  "will soaping the bearings help"; should I add some teflon to the bearings" ... .  Find the limits first.

 

A "wedge" can equatorialize your dob.  It's incline is the same as ninety degree minus your latitude.  You stick it under the dob so that what would have been zenith is now the pole.  Some dobs can't use this either because there are hard stops in alt built into it, or the mirror would go crashing out of its support at some angles.  If it can work, then motion is mostly on one axis, depending on how well you polar align.

 

An equatorial platform isn't hard to build.  It tracks for you, essentially tipping the dob by just the right amount so that its alt and az change without having to move the scope.  About an hour of tracking is normal, then you reset the platform again.


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#6 Neanderthal

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Posted 10 August 2022 - 01:53 PM

CFM,

It's easier to get better at judging the nudging if you practice with lower-mag EP's first. Nudging with a 30mm EP will be a lot easier to get the hang of than a 9mm EP.

 

Is nudging really a word?  lol



#7 SoCalPaul

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Posted 10 August 2022 - 02:15 PM

Is nudging really a word?  lol

Not just a word, but a very expressive word!

 

Paul



#8 MellonLake

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Posted 10 August 2022 - 04:05 PM

You may want to look at your bearings and ensure they are clean.  I can easily track at 250X and it takes about 30 seconds for the planet to go through the 82 degree filed of view.  With a little effort I can track at 400X plus but the planet is only in the filed of view for a short time before I have to move the scope again.  If my bearings are sticky, when I try to move the scope, it jumps too far and I loose the planet.  If the bearings are too smooth it is hard to get the telescope to stop in the right place.  You need just the right amount of friction and stiction (force to move it from its current position).  I have used soap and silicone oil on my bearings to keep them running smoother. 

 

FYI... I have tracked the ISS and seen the shape of the ISS in a DOB... The ISS moves pretty fast and tracking it is difficult but doable (I do lose it a few times in a pass).  My 18 year old daughter tracked the ISS 2 weeks ago in my 16".   If we can track the ISS... you can track planets.   It just takes practice and making sure your bearings are running smoothly (you may also want to consider tube balancing weights so you need less friction on the altitude bearing).

 

If you really want to solve the tracking problem, you can always get and equatorial platform which will track for you.   

 

Rob


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#9 CFM

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Posted 10 August 2022 - 10:01 PM

Hey CFM, can you please explain what you mean by, "I use a wedge to make this work."?

 

The issue that you identify is why I LOVE a telescope that tracks the sky. :-)

 

...

 

One way to deal with this, to a degree, is to invest in wider field eyepieces such as Naglers, Delos, etc. The object will remain in the FOV a little longer.

 

...

 

This is one reason why people invest so much thought, time and money in the smoothness of their dob bearings and movements. Too much stiction or herky-jerky movements can completely ruin the pleasure of an observing session.

 

If your dob's movements are not smooth or require too much force to overcome the stiction (and therefore jerks too far in one direction), you might want to investigate ways to clean and/or improve your bearings.

Paul: I use a wedge between lazy susan platform to stop it at the right location. Otherwise moving the scope in the horizontal axis feels like going over a speed bump. I will work on lubricating the bearing and see if this helps. Wider eyepiece for higher magnification eyepiece noted.

 

Generally, people get better at tweaking the scope aim as they get more practice.  They also put a lot of time and effort into getting the scope motions adjusted with the right amount of friction to move smoothly.  Wide angle eyepieces help, but they are pricey.

 

On a scope that size, atmospheric conditions are as likely as anything to limit magnification.  Near as I can tell, your scope is somewhere around an F6.  Therefore, the 5mm will likely about as short as you will want to go.  More often, the atmosphere will limit you to using the 7 or 9mm.

rhetfield: 2x for working on smoothing bearings. 2x on wider eyepiece. So far I have found the celestron luminos 7mm 82degree AFOV 1.25" eyepiece for about $50CAN more than the 62 degree AFOV equivalent. At this time I am not planning on getting anything smaller than a 7mm eyepiece (or a x171 magnification)  

 

Did your current eyepiece come with the scope/  If its a simple 1.25" 25mm eyepiece its probably a plossl or kellner type, which has around a 50degree AFOV.  If you replaced that with a 68 or 82 AFOV eyepiece you'll increase your drift time by quite a bit.  That will be a 2" eyepiece, so make sure your focuser can handle that.   ... 

 

It would be good for you to run an eyepiece calculator and find an eyepiece that will give you a 1 degree true field of view, or close to that.  That will become your finder/general use eyepiece.  Then another one at half that FOV/roughly twice the magnification/half the focal length.  Two good eyepieces with wider AFOV will give you good views that also have a wider TFOV (true field of view), and improve your viewing experience. 

 

Your scope shouldn't be too hard to move along.  Maybe there is a clutch or something overtightened.  Share some pics if you can.  For coping with small field of views, it is common to allow the object to drift across the field as you watch it, then reposition and repeat.  So you've already figured that out. 

 

Brian: Yes, my current eyepiece came with the scope, a 1.25" Plossl. I also have a 28mm Kellner that was handed down with the x2 barlow. x3 for a wide AFOV eyepiece. The current scope I have can handle the 1.25" and 2" eyepieces. I have been playing with an online calculator and the 15mm with 82 degrees AFOV is the candidate for the general use eyepiece and the 7mm the high magnification eyepiece. x4 for the wider AFOV eyepiece.  the break mechanism for the horizontal axis is a simple screw that tightens the lazy susan mount ... chances are that the bearings are dirty as other members noted. I will post pictures if the stiction does not improve after cleaning and grading the bearings.

 

 

It's practice for the most part.  The scope's physical limitations will be reached eventually.

...

 

A "wedge" can equatorialize your dob.  It's incline is the same as ninety degree minus your latitude.  You stick it under the dob so that what would have been zenith is now the pole.  Some dobs can't use this either because there are hard stops in alt built into it, or the mirror would go crashing out of its support at some angles.  If it can work, then motion is mostly on one axis, depending on how well you polar align.

 

An equatorial platform isn't hard to build.  It tracks for you, essentially tipping the dob by just the right amount so that its alt and az change without having to move the scope.  About an hour of tracking is normal, then you reset the platform again.

ButterFly: thanks for your comment. I think i did not explain well the purpose of the wedge ... I use the wedge to stop the telescope at the precise point when adjusting the horizontal axis not under the scope to create a equatorial platform. Although I do like the idea of one equatorial platform ... maybe for a future project.

 

 

It's easier to get better at judging the nudging if you practice with lower-mag EP's first. Nudging with a 30mm EP will be a lot easier to get the hang of than a 9mm EP.

Neanderthal:  I will definitely have practicing with the lower magnification eyepieces I currently have before moving to a higher magnification eyepiece.

 

You may want to look at your bearings and ensure they are clean.  I can easily track at 250X and it takes about 30 seconds for the planet to go through the 82 degree filed of view.  With a little effort I can track at 400X plus but the planet is only in the filed of view for a short time before I have to move the scope again.  If my bearings are sticky, when I try to move the scope, it jumps too far and I loose the planet.  If the bearings are too smooth it is hard to get the telescope to stop in the right place.  You need just the right amount of friction and stiction (force to move it from its current position).  I have used soap and silicone oil on my bearings to keep them running smoother. 

 

FYI... I have tracked the ISS and seen the shape of the ISS in a DOB... The ISS moves pretty fast and tracking it is difficult but doable (I do lose it a few times in a pass).  My 18 year old daughter tracked the ISS 2 weeks ago in my 16".   If we can track the ISS... you can track planets.   It just takes practice and making sure your bearings are running smoothly (you may also want to consider tube balancing weights so you need less friction on the altitude bearing).

 

If you really want to solve the tracking problem, you can always get and equatorial platform which will track for you.   

 

Rob

Rob: You explain exactly the situation I am encountering with the telescope so chances are that it is the bearings that need some cleaning and lubed. x5 for a wider AFOV eyepiece.

 

Thanks everyone for sharing your thoughts on this. I think my next steps will be: 1. keep using the 25mm Plossl and 28mm Kellner eyepieces as much as I can. 2 Save for a 15mm with 82degree AFOV. 3. I will get the 7mm later on.

 

... Although I just found a 7.5mm celestron Plossl Halloween used that may change my plans if the specs are good on this eyepiece ... I am going to have to create a new post on this since I could not find any specs online.


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#10 vtornado

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Posted 11 August 2022 - 10:11 AM

Hi,  I saw mentions of this above, but I thought I would state it again.

 

In order do maximize the drift time in your scope you are looking to maximize the true field of view.   If everything else is constant TFOV decreases as you decrease the the focal length of the eyepiece. (hence increase the magnification).

 

To calculate the true field of view use this formula.

TFOV = field_stop_diameter / focal_length * 57

If you have an eyepiece with a field stop of 10mm and your scope has a focal lenght of 1200mm then your true field of view is 10/1200 * 57 = .475 degrees.

Now many eyepiece vendors do not publish the field stop diameter.  You can measure it yourself.  Flip the eyepiece upside down and see there is a baffle the restricts the size of the filed lens (lens that points toward the "field")   Also Don Pensak has an eyepiece buyers guide (in the eyepiece forum) that list hundreds of eyepieces and many have field stops.

 

There is another way to calculate the field stop which is not as accurate but close.

TFOV = apparent_field_of_view / magnification.

Putting in numbers for a 50 degree 10mm plossl in your scope it works out to

TFOV = 50 /.(1200 / 10) = .420 degrees.  You can notice that if an 80 degree eyepiece

is plugged into this equation the TFOV increases to .666 degrees.  So if  the 50 degree

eyepiece has a drift time of 30 seconds, the 80 degree eyepiece will have a drift time of 48 seconds.

 

One other thing to note is wide field eyepieces are difficult to make and have low distortion in the outter portiion of the field.  Also newtonians have coma as you move from the center of view.  So a widefield eyepiece will decrease bump time, but may interject other abberations in the outer field of view.

 

Regarding motions, I'm not sure what system your dob has for altitude, (springs or bearings) but if you take the tension off make sure the dob is pretty well balanced

with your typical accessories.  If it is not get a welding magnet and attach it to the tube

at the right point to get good balance.

 

Regarding azimuth.  If there is a speed bump thing going on there is definetly something

going on.  First make sure the base is level when you observe, otherwise you are pushing it up a hill or down a hill.  A star hopper??? Is this a used scope?  Are the  base boards flat? or is there some warping?  With a lazy susan there should be almost too little friction.

I have to put a small square of carpeting between my boards  to increase friction.



#11 CFM

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Posted 19 August 2022 - 03:09 PM

Hi,  I saw mentions of this above, but I thought I would state it again.

 

In order do maximize the drift time in your scope you are looking to maximize the true field of view.   If everything else is constant TFOV decreases as you decrease the the focal length of the eyepiece. (hence increase the magnification).

 

To calculate the true field of view use this formula.

TFOV = field_stop_diameter / focal_length * 57

If you have an eyepiece with a field stop of 10mm and your scope has a focal lenght of 1200mm then your true field of view is 10/1200 * 57 = .475 degrees.

Now many eyepiece vendors do not publish the field stop diameter.  You can measure it yourself.  Flip the eyepiece upside down and see there is a baffle the restricts the size of the filed lens (lens that points toward the "field")   Also Don Pensak has an eyepiece buyers guide (in the eyepiece forum) that list hundreds of eyepieces and many have field stops.

 

There is another way to calculate the field stop which is not as accurate but close.

TFOV = apparent_field_of_view / magnification.

Putting in numbers for a 50 degree 10mm plossl in your scope it works out to

TFOV = 50 /.(1200 / 10) = .420 degrees.  You can notice that if an 80 degree eyepiece

is plugged into this equation the TFOV increases to .666 degrees.  So if  the 50 degree

eyepiece has a drift time of 30 seconds, the 80 degree eyepiece will have a drift time of 48 seconds.

 

One other thing to note is wide field eyepieces are difficult to make and have low distortion in the outter portiion of the field.  Also newtonians have coma as you move from the center of view.  So a widefield eyepiece will decrease bump time, but may interject other abberations in the outer field of view.

 

Regarding motions, I'm not sure what system your dob has for altitude, (springs or bearings) but if you take the tension off make sure the dob is pretty well balanced

with your typical accessories.  If it is not get a welding magnet and attach it to the tube

at the right point to get good balance.

 

Regarding azimuth.  If there is a speed bump thing going on there is definetly something

going on.  First make sure the base is level when you observe, otherwise you are pushing it up a hill or down a hill.  A star hopper??? Is this a used scope?  Are the  base boards flat? or is there some warping?  With a lazy susan there should be almost too little friction.

I have to put a small square of carpeting between my boards  to increase friction.

Thanks for pointing me to Dan's spreadsheet ... it is a jewel for anyone seeking to purchase an eyepiece.

 

The distortion aspect is also something I will consider when selecting my next eyepiece.

 

The scope is currently well balance so the altitude adjustment is not much of a concern with the eyepieces I have, but the tip of using magnets to balance the scope is appreciated in the event I buy a heavier eyepiece in the future.

 

I am now making sure the base of the scope is level to minimize the up/down hill pushing.

 

Yes, this is a used telescope the base is in good condition as far i can tell. The base moves fairly smoothly until the minor adjustments need to be done. I have to say that I have gotten better at making fine adjustments while seeing objects since the original post.

 

Thanks!


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#12 Jon Isaacs

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Posted 20 August 2022 - 07:50 AM

Paul: I use a wedge between lazy susan platform to stop it at the right location. Otherwise moving the scope in the horizontal axis feels like going over a speed bump. I will work on lubricating the bearing and see if this helps. Wider eyepiece for higher magnification eyepiece noted.

 

 

There is definitely a problem with your Lazy Susan bearings.  The motion should be smooth with enough friction to avoid overshoot and jerkiness.  What can happen with the Lazy Susan bearings is that they become "notched".  They have little grooves that are all aligned so there is a preferred stopping point.  

 

Your bearings may just need cleaning but it is possible that they are damaged.  If they need replacement, you might consider replacing them with the more traditional Laminate-Teflon bearings. A piece of formica-like material is bonded to the bottom of the rocker box and three Teflon pads are mounted over the three feet on the ground board.  This can provide smooth motion with the right amount of friction.. 

 

With the bearings working properly and enough practice, tracking at very high magnifications is doable. Last night the "seeing" was very stable. (The seeing is stability of the air column you are looking through).  I was looking at a very close binary star and I was tracking by hand at magnifications up to 1200x. The scope was my 10 inch Dob, made in Taiwan 20 years ago.  It is very rare that conditions allow such magnifications but 200x-300x should be relatively easy if the bearings are working properly.

 

Jon


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#13 TayM57

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Posted 20 August 2022 - 02:48 PM

Agreed with Jon.

 

Also wanted to clarify a few points- by "horizontal axis", you mean azimuth axis. By "vertical axis", you mean altitude axis.

 

The motion on the azimuth axis should be smooth, as should the altitude. What I would do is add teflon pads between the goundboard and bottom of the rocker for a smoother motion. The bottom of your rocker probably already has formica laminate. If it doesn't put some on.

 

Likewise for your altitude bearings- add teflon pads to the rocker notches for the bearings.




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