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Some more stupid questions


spacegalaxy

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Hi Spacegalaxy,

Not a stupid questions at all. I think your answer really depends on a few things, so it's hard to say if it's better or not exactly.

First thing I would ask is how experienced are you with scope building, grinding mirrors etc? If this is the first time you've done something like this, perhaps it would make sense to stick with the 6in which will be a simpler design and probably a bit more forgiving. Also thing worth considering is whether you've had experiences using a binoscope and whether it's the right thing for you (will be heavier etc., potentially more complex to use etc.)?

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1 minute ago, badhex said:

Hi Spacegalaxy,

Not a stupid questions at all. I think your answer really depends on a few things, so it's hard to say if it's better or not exactly.

First thing I would ask is how experienced are you with scope building, grinding mirrors etc? If this is the first time you've done something like this, perhaps it would make sense to stick with the 6in which will be a simpler design and probably a bit more forgiving. Also thing worth considering is whether you've had experiences using a binoscope and whether it's the right thing for you (will be heavier etc., potentially more complex to use etc.)?

Thanks for the detailed answer! I am not experienced in mirror grinding, infact I have never grinded a mirror, I didn't got the meaning of "potentially more complex to use", And I never had a experience with binoscopes, but it seems that binoscopes give more 3d effect to objects. Also why is it harder make a binoscope?

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1 hour ago, spacegalaxy said:

I have a question is that I can get two 4 inch  mirror grinding for the price of one 6 inch mirror grinding kit, I am thinking of building a 4 inch binoscope, so is the binoscope be better than a 6 inch?

Have you ground any telescope mirrors before? If you're going to make two 4" for a binocular scope they will need to be of equal focal length. It is very easy to get it wrong when making identical optics. Once I'd achieved the right initial curve I would use the same tool for both, swapping mirrors regularly as i went along, so at least I'd keep identical curves during fine grinding. The 6" scope would be less challenging as you'll have less to worry about.

Edited by mikeDnight
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15 minutes ago, spacegalaxy said:

Thanks for the detailed answer! I am not experienced in mirror grinding, infact I have never grinded a mirror, I didn't got the meaning of "potentially more complex to use", And I never had a experience with binoscopes, but it seems that binoscopes give more 3d effect to objects. Also why is it harder make a binoscope?

No worries, to be clear I'm far from an expert and there will be others who can perhaps give better answers but I do have some experience grinding mirrors. 
Regarding difficulty of the build, it really depends on your design but firstly there will be two of every component to be made which need to be identical, as well as possibly special mirror setups, diagonals etc to ensure you converge your light paths to each eye etc., A standard dobsonian design will be much simpler to build, and only one mirror to worry about getting right.  Regarding complexity of use, again it's the same thing - a standard dobsonian reflector is pretty simple to get outside, set up and observing quickly whereas a binoscope will be heavier and probably take longer to get setup, cooled down etc.

Edited by badhex
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Just now, mikeDnight said:

Have you ground any telescope mirrors before? If you're going to make two 4" for a binocular scope they will need to be of equal focal length. It is very easy to get it wrong when making identical optics. Once I'd achieved the right initial curve I would use the same tool for both so at least I'd keep identical curves during fine grinding. The 6" scope would be less challenging as you'll have less to worry about.

No, this will be my first mirror grinding project, the problem with 6inch is I am a bit hesistant to make a parabolic mirror, so I might be going for a sperical mirror, but according to this thread:

https://www.cloudynights.com/topic/447807-parabolic-or-spherical-mirror/ (Read Dave Knisely's reply)

The mirror should have to be a f/12.1, Which is 76 inches! That's why I have been looking at either two 4inch f/12 or a 5inch f/12.5

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What mount will you be using.  Two 4" f12 OTA's is going to weigh a fair amount, so even if you managed to grind two sets of optics to the exact focal length so they collimated correctly they would be useless unless the mount can support them and keep them solid.

Personally, if you have never ground any optics before then you would be better off attempting the 6" scope, but even then I would prepare for a lot of frustration, it's not easy.

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There are many 4" f/9 telescopes working well with spherical mirrors. They might not be "perfect" in some eyes but they will give acceptable views.

A pair of mirrors for a binoscope do not need to be EXACTLY the same focal length, within 1% is acceptable. Use mikeDnight's method ( above) to get the mirrors close to the same fl. In my experience, quite a high level of skill is needed to build the mechanical structure to collimate everything ( both optical tubes separately and together ) and keep it collimated as you move it around the sky.

Once done the views will be amazing.

Nigel

 

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39 minutes ago, malc-c said:

What mount will you be using.  Two 4" f12 OTA's is going to weigh a fair amount, so even if you managed to grind two sets of optics to the exact focal length so they collimated correctly they would be useless unless the mount can support them and keep them solid.

Personally, if you have never ground any optics before then you would be better off attempting the 6" scope, but even then I would prepare for a lot of frustration, it's not easy.

What do you think about a 5 inch? My hands are quite small compared to an average person!

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16 minutes ago, Astrobits said:

There are many 4" f/9 telescopes working well with spherical mirrors. They might not be "perfect" in some eyes but they will give acceptable views.

A pair of mirrors for a binoscope do not need to be EXACTLY the same focal length, within 1% is acceptable. Use mikeDnight's method ( above) to get the mirrors close to the same fl. In my experience, quite a high level of skill is needed to build the mechanical structure to collimate everything ( both optical tubes separately and together ) and keep it collimated as you move it around the sky.

Once done the views will be amazing.

Nigel

 

What do you exactly mean by "perfect" views? Does that mean that it has sharp fov for a larger magnification? 

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42 minutes ago, malc-c said:

What mount will you be using.  Two 4" f12 OTA's is going to weigh a fair amount, so even if you managed to grind two sets of optics to the exact focal length so they collimated correctly they would be useless unless the mount can support them and keep them solid.

Personally, if you have never ground any optics before then you would be better off attempting the 6" scope, but even then I would prepare for a lot of frustration, it's not easy.

Also, I might go with a f/10 4inch and mount it on a dobsonian

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1 hour ago, malc-c said:

What mount will you be using. 

 

1 hour ago, spacegalaxy said:

What do you think about a 5 inch? My hands are quite small compared to an average person!

 

1 hour ago, spacegalaxy said:

Also, I might go with a f/10 4inch and mount it on a dobsonian

Dobsonian are the simplest to construct, but f10 ??? - 

  • The resulting image form a 4" f10 will be dark when used with high magnification
  • Dobs are normally associated with large mirrors (termed light buckets)

The size of the mirror becomes important when considering long focal lengths - Long focal lengths are typically aimed at planetary observations, and so you need a lot of light to give good contrast when using short focal ratio eyepieces otherwise the resulting image will be dull and lack contrast at high magnification.  Long focal length scopes are not best suited for imaging faint DSO's

You really need to think of the overall package and what it is that you want to view.  That will dictate to a degree what size mirror and what focal length to go for

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1 hour ago, malc-c said:

The resulting image form a 4" f10 will be dark when used with high magnification

What do you exactly mean by dark? Did you mean the background or the image?

 

1 hour ago, malc-c said:

Dobs are normally associated with large mirrors (termed light buckets)

I have seen dobsonian bases on the smallest of apertures as well, so what's the problem with that?

 

1 hour ago, malc-c said:

Long focal length scopes are not best suited for imaging faint DSO's

Even though I am going to use it for planetary, planets don't rise high for about 6 months so,what about DSO's?

 

1 hour ago, malc-c said:

The size of the mirror becomes important when considering long focal lengths - Long focal lengths are typically aimed at planetary observations, and so you need a lot of light to give good contrast when using short focal ratio eyepieces otherwise the resulting image will be dull and lack contrast at high magnification.

So, is the six inch better than the 4, 5 inch?

Also, the tube is the main question, I want a grab n' go dobsonian, so the 6 inch is not feasible, refer to this post:

4 hours ago, spacegalaxy said:

No, this will be my first mirror grinding project, the problem with 6inch is I am a bit hesistant to make a parabolic mirror, so I might be going for a sperical mirror, but according to this thread:

https://www.cloudynights.com/topic/447807-parabolic-or-spherical-mirror/ (Read Dave Knisely's reply)

The mirror should have to be a f/12.1, Which is 76 inches! That's why I have been looking at either two 4inch f/12 or a 5inch f/12.5

 

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1 hour ago, spacegalaxy said:

What do you exactly mean by dark? Did you mean the background or the image?

The image

Quote

I have seen dobsonian bases on the smallest of apertures as well, so what's the problem with that?

Nothing, just that £ for £, $ for $ you get more aperture with a dob than any other type of scope.  It's your scope, make it how you like

Quote

Even though I am going to use it for planetary, planets don't rise high for about 6 months so,what about DSO's?

For DSO's ideally you need a fast (f5 or less) scope.  

Quote

So, is the six inch better than the 4, 5 inch?

Yes - it gathers more photons :)

Quote

Also, the tube is the main question, I want a grab n' go dobsonian, so the 6 inch is not feasible, refer to this post:

So why ask if a 6" is better than a 4" or 5".   I can see the difference if you were questioning a 4" vs an 8" mirror, but does an inch really make things not feasible ?

May I suggest that you spend some time reading up on the relationship between aperture, focal length and focal ratio, together with the practical magnification you will get and thus the image size you'll probably get.  This may probably help you refine your options 

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29 minutes ago, malc-c said:

but does an inch really make things not feasible ?

Not that it's about the aperture, it's about the f.l., according to a post made by Dave Knisely's reply (Number 14th) which can be found here: https://www.cloudynights.com/topic/447807-parabolic-or-spherical-mirror/

Says that the minimal size for a diffraction limited spherical 6 inch Newtonian is f/12.1 or around 80inches of tube! But a 4 inch (and potentially a 5 inch) has a minimum of f/10.6 which is around 40inches of tube, I find a 30 inch tube pretty portable and grab n' go but 80inch? Might be a problem for grab n' go

29 minutes ago, malc-c said:

May I suggest that you spend some time reading up on the relationship between aperture, focal length and focal ratio, together with the practical magnification you will get and thus the image size you'll probably get.  This may probably help you refine your options

Thanks for the suggestion, and that's what I exactly did when I thought of buying/building a scope (with the help of SGL, astronomy.tools, and other forums ). It seems that long focal ratio is good for planets, moon, and other objects that need high magnification (Doubles) and it has a small secondary, Low focal ratio is good for widefield (DSO's, as you said) and has a large secondary. I was looking at a 6 inch f/6 which would have been portable, but many people say it is very hard to parbolise a mirror, or regard it as the hardest part, and some people say it has some coma, so I think I am heading towards spherical mirror, which performs bad when used at high focal ration

Edited by spacegalaxy
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I had a 4" spherical  f/9 mirror telescope and it showed me detail on Jupiter and Saturn, fine views of the craters on the moon and  dso's like the Crab nebula. The comment that a 4" spherical mirror needs to be f/10+ is for the writers own satisfaction. A 4" f/10 mirror is only 1/5th wave from parabolic and f/9 is only a bit worse. At f/12 that mirror is only marginally better than 1/8th wave. (Just for a horror moment, when the Chinese started producing telescopes in earnest some 25+ years ago they supplied  5" f/5 spherical mirrors. That is close to 2.0 waves off parabolic. Not only that, many had severe zones  and some still had grinding marks.😱 These were sold around the world. I had a batch of 25 of them, 23 needed corrective polishing/grinding, one I passed as acceptable and one was binned with a large crack. I think that their manufacturing is better now!)

I would suggest that you try to contact a local astronomy group so you can meet up and view through their scopes to decide what you want. If you really want to make your own mirror then go for a 4" spherical f/9 dobsonian. If you do a good job, you will be able to sell it ( if necessary ) to fund your next project and with the experience gained you'll do an even better job of that. In the meantime use binoculars for your astronomy fix.

Nigel

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2 hours ago, Astrobits said:

(Just for a horror moment, when the Chinese started producing telescopes in earnest some 25+ years ago they supplied  5" f/5 spherical mirrors. That is close to 2.0 waves off parabolic. Not only that, many had severe zones  and some still had grinding marks.😱 These were sold around the world. I had a batch of 25 of them, 23 needed corrective polishing/grinding, one I passed as acceptable and one was binned with a large crack. I think that their manufacturing is better now!)

2 waves? Can I have a interferometry test report, just for the laughs!

 

2 hours ago, Astrobits said:

If you really want to make your own mirror then go for a 4" spherical f/9 dobsonian. If you do a good job, you will be able to sell it ( if necessary ) to fund your next project and with the experience gained you'll do an even better job of that.

Thanks for the idea, it actually seems a very good plan, I might actually do it!

 

2 hours ago, Astrobits said:

I would suggest that you try to contact a local astronomy group so you can meet up and view through their scopes to decide what you want.

I actually wanted to do that, the local astronomy group is actually a bit small, and what I could know from online reviews it has a 6 inch custom-made dobsonian, the problem is that it is closed because of COVID. I will visit them before building/buying/renting a scope.

 

2 hours ago, Astrobits said:

I had a 4" spherical  f/9 mirror telescope and it showed me detail on Jupiter and Saturn, fine views of the craters on the moon and  dso's like the Crab nebula. The comment that a 4" spherical mirror needs to be f/10+ is for the writers own satisfaction.

Oh, I was having a thought on f/9, link here: https://tejraj.com/telescope-making-kit-indian.html (first one), so if I have satisfaction using a 70mm f/10 of probably 1/4 wave, will this be good? Also was the telescope you were talking about, what bortle sky were you using it? Was it a good mirror? What secondary did you use?

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On 16/02/2022 at 04:12, spacegalaxy said:

Thanks for the detailed answer! I am not experienced in mirror grinding, infact I have never grinded a mirror, I didn't got the meaning of "potentially more complex to use", And I never had a experience with binoscopes, but it seems that binoscopes give more 3d effect to objects. Also why is it harder make a binoscope?

Binoculars give a 3-D effect, so I assume a binoscope would do the same.  One thing that isn't mentioned much is that not everybody can always merge images resulting in doublevision.   Most often it's caused by miscollimation, but not always.  I'd also think a binoscope will probably weigh 2X the same aperture "monoscope".  And double the cost of eyepieces.

OTOH, somebody wrote a comprehensive book on deepsky observing and he used an 8-in (?) binoscope very successfully.

Edited by jjohnson3803
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2 hours ago, jjohnson3803 said:

One thing that isn't mentioned much is that not everybody can always merge images resulting in doublevision.   Most often it's caused by miscollimation, but not always.

I have pretty bad eyes, one a +2.75 and other +4.5, so will this work?

2 hours ago, jjohnson3803 said:

OTOH, somebody wrote a comprehensive book on deepsky observing and he used an 8-in (?) binoscope very successfully.

I will try to search it!

 

2 hours ago, jjohnson3803 said:

And double the cost of eyepieces.

Well that settles it, binoscope needs to 2 mirrors to be grinded, and both have to alumnised, they have to be mounted on a strong mount, And double the cost of eyepieces (which are very expensive here)

Thanks to everyone who answered my question!

(Mods: Please lock this thread, my question has been answered!)

Clear skies!

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