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The reflector limbo: how low do you go?

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#1 Aperturefever

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Posted 07 November 2019 - 07:04 AM

I'm cobbling together a 24" build and have reached the stage where I have to cut out some of my front rocker board to accommodate the lower struts at an almost horizontal angle. I was considering it this afternoon with the struts hard down against the front rocker board, and thinking - hang on ... I never observe that low with my 16". Why would I bother with a 24? I've been around the block at this point and tried to nab the stuff down low north like notherners have done the same down south. OK, so maybe you do it on a whim and on the odd occasion ... but really ... the experienced observers know not to bother looking through pea soup. It's easier to jump on a plane and fly up (down?!) to an accommodating reflector owner in a better location.

So my question is this: at what latitude do you pull the pin for observing? And I'm not talking the bragging rights stuff where you saw something famous, but the working observing area where you hit the point of diminishing gains?

I have pretty clear skies where I am ... but geez ... not sure if I would bother at an attitude that warrant cuts in this rocker board at this point in my observing career.

Thoughts?



#2 Jon Isaacs

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Posted 07 November 2019 - 07:38 AM

OK, so maybe you do it on a whim and on the odd occasion ... but really ... the experienced observers know not to bother looking through pea soup. It's easier to jump on a plane and fly up (down?!) to an accommodating reflector owner in a better location.

 

So my question is this: at what latitude do you pull the pin for observing? And I'm not talking the bragging rights stuff where you saw something famous, but the working observing area where you hit the point of diminishing gains?

 

I have pretty clear skies where I am ... but geez ... not sure if I would bother at an attitude that warrant cuts in this rocker board at this point in my observing career.

Thoughts?

 

 

I consider myself an "experienced observer."  These days I spend about 500 hours a year out under the night sky with a telescope and I have been doing for quite a few years.. 

 

Jumping on a plane?  That's easier than making some cuts on my rocker box? It doesn't seem that way to me. 

 

I know that observers who have limited observing opportunities/time seem to not be interested in observing near the horizon but that's not me, I have plenty of time and so I will go as low a 5 degrees, even lower.  There's interesting stuff down there.  Sure, they are not what they could be if I were further south but I am not further south and the likelihood of being further south is about zero since I am about 3 miles from the US Mexico border.  

 

I do have a clear view of the southern horizon, that enters into the equation.  The hills in the background are in Mexico.

Starsplitter 22 inch Evening in Boulevard.jpg

 

Jon


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#3 TOMDEY

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Posted 07 November 2019 - 09:07 AM

Superb point! You don't owe it to anyone else to build your scope to their recommended specs. And it most certainly is true that the views get better and better at higher elevation angles. My southern horizon is ten degrees up, and I decided that's good enough for me. Most all of my observing is way above that. So, an observing friend comes over and enjoying looking thru the 36-inch scope, even getting slightly envious, and comments, "too bad you don't have more southern horizon." ... and I retort, "Well, that's your problem, not mine; let's try your 60mm refractor."    Tom



#4 25585

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Posted 07 November 2019 - 11:31 AM

I like seeing how far down I can pick things out. For bright stars, Fomalhaut is good. Saturn for planets.  



#5 MitchAlsup

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Posted 07 November 2019 - 12:33 PM

So my question is this: at what latitude do you pull the pin for observing?

 

And I'm not talking the bragging rights stuff where you saw something famous, but the working observing area where you hit the point of diminishing gains?

I pull the pin at about 1º--because this is the mountain I am looking ver when M42 comes up over the horizon.


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#6 stargazer193857

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Posted 07 November 2019 - 04:15 PM

I know a public observatory that won't go below 15 degrees because they don't want the mirror tipping. They are very cautious. The horizon has chromatic aberration and light pollution and smog. Not just atmosphere. Also ground thermals right in front of the scope.

But my best view ever of M81/M82 was 15 degrees up through a 10" dob. But that horizon was dark, away from town. Horizons away from town can show spectacular views. The bortle scale is for zenith.

#7 nirvanix

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Posted 07 November 2019 - 04:20 PM

Everything I ever looked at below 10 degrees looked like a pot of soup on the stove so I planned my own build around that. Unless you want to use your dob for birding? smile.gif



#8 kfiscus

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Posted 07 November 2019 - 04:26 PM

The only targets that warrant going down in the gunk, in my opinion, are comets that are really close to the sun.  Otherwise, I won't usually go down in the lower 20 degrees and look through all of our mid-western dust and humidity.  I wouldn't recommend modifying your dob to go very low.


Edited by kfiscus, 07 November 2019 - 04:27 PM.


#9 stargazer193857

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Posted 07 November 2019 - 04:44 PM

Build it so you can point it down a hill at buildings below. That might involve special mirror clips though. Push the technological envelop.
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#10 Old Rookie

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Posted 07 November 2019 - 05:08 PM

What price do you put on those objects near the horizon like the summertime objects in Sagittarius and Scorpius?  Typically, these constellations are just above the trees and normally in the soup.  But a couple of times a summer, the sky is transparent down to the ground and I can get some good looks at some of those dso's.  Maybe you should ask yourself if you'll be content avoiding bright, glorious objects so low on the horizon.


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#11 Jon Isaacs

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Posted 07 November 2019 - 05:24 PM

Everything I ever looked at below 10 degrees looked like a pot of soup on the stove so I planned my own build around that. Unless you want to use your dob for birding? smile.gif

 

The only targets that warrant going down in the gunk, in my opinion, are comets that are really close to the sun.  Otherwise, I won't usually go down in the lower 20 degrees and look through all of our mid-western dust and humidity.  I wouldn't recommend modifying your dob to go very low.

 

What price do you put on those objects near the horizon like the summertime objects in Sagittarius and Scorpius?  Typically, these constellations are just above the trees and normally in the soup.  But a couple of times a summer, the sky is transparent down to the ground and I can get some good looks at some of those dso's.  Maybe you should ask yourself if you'll be content avoiding bright, glorious objects so low on the horizon.

 

I'm with Old Rookie..

 

It just so happens that at my site, Omega Centauri culminates at 9.9°.  The views are generally spectacular. That's the nature of Omega Centauri. I am sure that were it at 45 degrees elevation, it would be even more spectacular.. But I make the best of the opportunities I'm offered..

 

Sometimes it's on the fuzzy side at 130x, sometimes it's clean and sharp at 215x.. 

 

Jon


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#12 Kunama

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Posted 07 November 2019 - 06:41 PM

Everything I ever looked at below 10 degrees looked like a pot of soup on the stove so I planned my own build around that. Unless you want to use your dob for birding? smile.gif

My previous build went 6º and what I learned from that was that we really have polluted the atmosphere a lot...... 

 

My current 'Excalibur' 18" F3.5 goes to +7.5º but hardly ever looks at anything that low.......

 

In normal use seldom view objects below 15º, I prefer to wait till that object is better sited. 



#13 John O'Hara

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Posted 07 November 2019 - 06:44 PM

If you have a sling style cell, you might want to make sure it will maintain collimation when aimed that low.


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#14 Kunama

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Posted 07 November 2019 - 06:53 PM

I'm with Old Rookie..

 

It just so happens that at my site, Omega Centauri culminates at 9.9°.  The views are generally spectacular. That's the nature of Omega Centauri. I am sure that were it at 45 degrees elevation, it would be even more spectacular.. But I make the best of the opportunities I'm offered..

 

Sometimes it's on the fuzzy side at 130x, sometimes it's clean and sharp at 215x.. 

 

Jon

You're lucky Jon, you get to challenge yourself viewing O Cent.... we have to contend with it being an easy naked eye sight at 77º....... undecided.gif


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#15 Asbytec

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Posted 07 November 2019 - 07:36 PM

If you have a sling style cell, you might want to make sure it will maintain collimation when aimed that low.

That's a good point. There may be a mechanical or collimation reason for not going too low. At some point as we approach horizontal, we may risk the primary tilting forward. Like others, generally my viewing is higher in the sky, but I'll go pretty low to see something I want to see because that's as high as it gets. Mirror clips loosely hold the primary in its cell, but YMMV. 


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#16 Jon Isaacs

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Posted 07 November 2019 - 08:44 PM

You're lucky Jon, you get to challenge yourself viewing O Cent.... we have to contend with it being an easy naked eye sight at 77º....... undecided.gif

 

It's easy naked eye at 10 degrees or even 5 degrees.

 

Obviously it's way better at 77 degrees.

 

Jon



#17 Oberon

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Posted 08 November 2019 - 01:56 AM

If you prevent your scope from going low to the horizon, I can guarantee the first thing you will want to do is look at something low to the horizon.

 

Horizons are like trees. Always in exactly the wrong place.


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#18 Aperturefever

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Posted 08 November 2019 - 07:37 AM

Great discussion all. My mirror cell has clips so I can go flat out as a lizard drinking if I wanted to. And yes Jon cutting some holes is easier than flying ... but the obsessive compulsive in me would prefer a straight line! I guess for me, if I observe something I want to tease out all the detail I can. So I would rather examine something high than look at something through the muck just to say I bagged it, and save it for later when I happen to be at another lattitude. Like at Tom's place!!! ( I can bring beer/port??)

If you prevent your scope from going low to the horizon, I can guarantee the first thing you will want to do is look at something low to the horizon.

 

Horizons are like trees. Always in exactly the wrong place.

Ah yes ... you read my mind. This is what worries me! You alway want what you can't have ...

 

If I get desperate enough to view things in the pea soup I have this for the task:

Of course Matt! The lovely brass refractor in the background eh?! poke.gif


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#19 stargazer193857

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Posted 08 November 2019 - 08:47 AM

Just prop up a leg. Bring a hockey puck. You don't need to modify the scope.
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#20 Jon Isaacs

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Posted 08 November 2019 - 10:22 AM

Just prop up a leg. Bring a hockey puck. You don't need to modify the scope.

waytogo.gif

 

That's a good workaround with a smaller scope. 

 

With a 24 inch, it would probably take a 5 inch lift to achieve10 degrees and it getting the piece in place is probably difficult to the weight.  One also has to preplan this since the foot should pointed more or less south (for someone in Australia.)  And it will mess up DSC alignment.

 

But for a one time observation, it is workable. 

 

I observe at 10 degrees and below on a regular basis so I modified my 16 inch so it would go below the horizon.  My 22 inch stops at about 8 degrees but since it has a horizontal focuser, it's very uncomfortable trying to view even at 8 degrees.

 

I don't consider a refractor a workaround since it requires giving up so much aperture.  

 

Jon



#21 Jon Isaacs

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Posted 08 November 2019 - 10:29 AM

Great discussion all. My mirror cell has clips so I can go flat out as a lizard drinking if I wanted to. And yes Jon cutting some holes is easier than flying ... but the obsessive compulsive in me would prefer a straight line!

 

 

When I cut the front board on my 16 inch, the edges are angled and the center is flat. I think it looks better than it did.  But your scope probably has a very low rocker box so that front cross piece is pretty short already.. 

 

Stargazers workaround might be all you need and if not, have you considered balloons?

 

6446676-Birthday Dob CN.jpg

 

 

:lol:

 

Jon



#22 25585

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Posted 08 November 2019 - 12:21 PM

Seems obvious to me cel design for mirrors is limiting use of Newtonians. While some aspects of modern Newtonian design are advanced, others are at standstill.

 

Is it the fragility of mirrors themselves, that seems to need slings & wood rather than clamps made from strong but light modern materials? 



#23 Kunama

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Posted 08 November 2019 - 04:51 PM

Seems obvious to me cel design for mirrors is limiting use of Newtonians. While some aspects of modern Newtonian design are advanced, others are at standstill.

 

Is it the fragility of mirrors themselves, that seems to need slings & wood rather than clamps made from strong but light modern materials? 

I don't consider mirrors fragile as such, just that their tolerance for any kind of clamp method is so very small without affecting the figure....... I don't think PLOP was designed to account for a mirror tilted beyond horizontal.....

 

As for mirror support, I am a fan of the 180º cable at COG with the weight hanging off linear bearings, mounted on polished shafts parallel to the optical axis of the mirror, that can track the collimation position freely....

 

 

For my next DIY project I am making a second cell for my Dob with wiffle tree roller supports just to see if there is a difference....


Edited by Kunama, 08 November 2019 - 04:52 PM.

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#24 Jon Isaacs

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Posted 08 November 2019 - 05:04 PM

Seems obvious to me cel design for mirrors is limiting use of Newtonians. While some aspects of modern Newtonian design are advanced, others are at standstill.

 

Is it the fragility of mirrors themselves, that seems to need slings & wood rather than clamps made from strong but light modern materials? 

 

Mirror cells design has advanced dramatically in the past 20 years. This has gone hand in hand with the development of thinner and thinner mirrors.

 

As Matt says, it's not that the mirrors are fragile,  it's that the must be properly supported to avoid small elastic deformations that affect the figure of mirror. A mirror made from a ductile material like aluminum would require the same care in mounting.

 

Mirrors can be supported regardless of aperture. There are mirrors over 8 meters in diameter.

 

The refractor ran into a limit at 40 inches more than 100 years ago.. glass sag.. that hasn't been exceeded.

 

Jon


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#25 macdonjh

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Posted 08 November 2019 - 05:34 PM

I don't own a Dobsonian anymore, but my answer would be the same if I did: when I have the choice I wait for objects to get high in the sky before observing them.  However, as some have said, some interesting objects simply don't rise high, so I observe them where they are.  To that end, I built my observatory such that I could point my scope at the tree tops in any direction so I could see anything visible from my site.

 

Someday, I'd like to travel, probably renting scope time at my destination rather than shipping a scope, and suffer through not being challenged by Omega Centrauri.

 

Oops, I forgot to answer the actual question: I've forgotten the trig I did, but I think our trees make an artificial horizon at something like 15o elevation, so that's as low as I go.


Edited by macdonjh, 08 November 2019 - 05:36 PM.

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