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Short Tube 80 but better optics?

Beginner Equipment Refractor Mount
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#51 Tony Flanders

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Posted 01 December 2021 - 07:52 AM

What does 80mm limit me to in a suburban bortle 5/6 skies?


I will assume that the skies are genuinely Bortle 5/6, which is to say that all 7 main stars of the Little Dipper are readily visible even when the Little Dipper is pointing down, that the summer Milky Way is obvious overhead but perhaps not down near the horizon, and that M31 appears small and fairly faint to the unaided eye.
 
That roughly matches the conditions where I did the suburban observations for my Urban/Suburban Messier Guide. Under such conditions most of the Messier objects are obvious to the experienced eye through an 80-mm scope, and the brighter ones, including most of the open clusters and a few of the nebulae, show significant detail.
 
You would get a significant though not Earth-shattering improvement going from 80 to 100 mm. But to get significantly better views you need a LOT more aperture -- which is to say, not a refractor. Through an 8-inch scope all the Messier objects are obvious under such skies, and almost all of them begin to take on a real life of their own. In particular, a fair number of the globular clusters begin to resolve into individual stars.

This is why it often makes sense for the photographic instrument to be different from the visual instrument.


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#52 AstroDog77

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Posted 01 December 2021 - 09:57 AM

As SeattleScott said, “in use” was the key words. And aside from that, the AT80ED is going to be 8+ pounds with 2 inch diagonal and eyepiece. And that’s without a finderscope, if you need one.
It’s listed as 6.32 pounds without these accessories on Astronomics website.

The ST80 is listed as 4 pounds 10 ounces with rings, dovetail, diagonal, eyepiece, and finderscope.

 

I don’t use a finderscope or dovetail on my ST80. And only one ring. But I might use an eyepiece that weighs a few ounces more.

 

The 80mm f7 apo feels twice as heavy. And is at least 6 inches longer in use, with dew shield extended.

So couple the extra weight with the extra moment arm, and an 80mm f/7 really is at the max for a much larger photo tripod, like this (80 f7.5 shown)

attachicon.gif49526376-CC20-408E-B51C-E12B9D4DC781.jpeg

 

To get the stability of the ST80 on the 3021 with ball head, from my previous post, you’d probably need to move up to a Porta II class mount.

 

I guess you’d have to see them side by side and feel the large difference in weight to know what I mean.

Not sure how is a 5lb OTA going to hit 8lbs with a diagonal and an EP but I do use a red dot for a finder which doesn't weigh very much (1/4lb with the bracket I think) so maybe there?



#53 SeattleScott

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Posted 01 December 2021 - 11:52 AM

Not sure how is a 5lb OTA going to hit 8lbs with a diagonal and an EP but I do use a red dot for a finder which doesn't weigh very much (1/4lb with the bracket I think) so maybe there?

A typical Chinese 2” mirror diagonal weighs 1.25lbs. Personally I prefer lighter 2” eyepieces, 19 ounces to 25.6 ounces in my case. A typical diagonal plus my heavier 2” eyepiece is about three pounds.

Typically a 2” diagonal and eyepiece will weigh a combined 2.5-3.75lbs. Sure one can come in a bit higher or lower. Seibert and Russell are known for light weight 2” eyepieces. On the other hand some 2” eyepieces weigh three pounds alone. But three pounds combined weight is a pretty good rule of thumb.

Scott

Edited by SeattleScott, 01 December 2021 - 12:00 PM.

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#54 Echolight

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Posted 01 December 2021 - 12:51 PM

Maybe I can't read (I think I can but ask my wife after using her shopping list) but how is the same 15" length of the OTA and a half a pound much heavier and much longer (are you referring to focal length)?

Maybe?

 

Not sure how is a 5lb OTA going to hit 8lbs with a diagonal and an EP but I do use a red dot for a finder which doesn't weigh very much (1/4lb with the bracket I think) so maybe there?

Here’s what Astronomics website says about the AT80ED:

Weight:  6 lbs 3.2 oz with the rings and dovetail,  5 lbs 4 oz without rings and dovetail

https://www.astronom...ractor-ota.html

 

I guess you could zip tie it to the mount use a 25mm Plossl without a diagonal. But you’re still starting at 5 pounds 4 ounces.


Edited by Echolight, 01 December 2021 - 12:54 PM.


#55 AstroDog77

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Posted 03 December 2021 - 07:10 AM

The OP has stated that he intends to use a mount with a max weight rating of 5kg, so I don't understand what the hang up over the weight is here? A mount that can handle 5kg can handle these scopes. If I missed something in the posts that said there was a requirement of a 2" diagonal and EPs, I guess that's a bad on me.



#56 SeattleScott

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Posted 03 December 2021 - 11:10 AM

The Star Tracker he plans to use has a max load of 5kg. I think there was some concern about the camera tripod with ball head handling 8lbs.

Scott

#57 Arthur L

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Posted 04 December 2021 - 11:35 AM

I usually get a shipping notice from High Point within a day, I live in PA and I get my stuff in under a week.

Only if the item is in stock.

Also, does anybody know the estimated shipping time on the things on highpoint scientific and other website that say "more on the way" or "expanded lead time". As I have my eyes set on that Takahashi StarBase 80mm achromat,  but it says "expanded lead time", so am I going to have to wait months to get it? 

No one knows, if they did they would say so. If you want an item and it is in stock, best not wait.

Backordered items could be a while.  "on the way" could be just in but not in inventory, or still in China waiting

for a boat or waiting in the ocean for a port.  Or waiting for a truck.  You get the idea.  I just ordered an item this

morning and ten minutes later the availability was updated to not in stock.  Probably I got the last one.


Edited by Arthur L, 04 December 2021 - 11:36 AM.


#58 AstroDog77

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Posted 05 December 2021 - 03:30 PM

The Star Tracker he plans to use has a max load of 5kg. I think there was some concern about the camera tripod with ball head handling 8lbs.

Scott

You do understand that 5kg is basically 11lbs right?!

 

Yes?

No?

You don't understand the difference!?!?!!

If you don't you should not be providing specs to people because were response was completely ignorant.



#59 SeattleScott

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Posted 05 December 2021 - 03:50 PM

Yes I am well aware that 5KG is 11lbs. Personally I wasn’t the one who suggested a beefier camera tripod would be beneficial. I simply pointed out that the 5kg weight rating was the advertised weight rating for the tracker, not the tripod. The person who expressed concern suggested using a more robust camera tripod with the tracker. Now personally I don’t put telescopes on camera tripod so I don’t know the difference in the models. I was just highlighting the concerns noted and making the point that the weight rating of the tracker is irrelevant if it is put on a flimsy tripod.

The mount has to be viewed as a complete unit. An Astrophysics Mach 1 won’t be adequate support for a C11 if it is placed on a Twilight I tripod.

Scott
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#60 Echolight

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Posted 05 December 2021 - 05:35 PM

You do understand that 5kg is basically 11lbs right?!

 

Yes?

No?

You don't understand the difference!?!?!!

If you don't you should not be providing specs to people because were response was completely ignorant.

It may hold 11 pounds off the ground. And if you put a ball of lead on there whose weight was centrally located, it might not exhibit too much flex. 
But it won’t operate optimally with a telescope that weighs the same as the maximum rating but has it’s weight spread out and possess a moment arm that increases the actual load that it puts on the mount. And the mount would be prone to failure in a short time. Not to mention that your scope would vibrate excessively every time you touch it. Settle times would be unacceptable. You may never achieve sharp focus because the scope would likely always be moving.

 

And this is what you don’t understand.

 

And, that AT80ED that you keep saying is 5 pounds,... with rings and dovetail and loaded with an eyepiece and diagonal that many would choose to use with it for wide field viewing would really be over 9 pounds.

And 9 pounds spread out over 2 feet of length. 

 

And then there is the “sail factor”. Or how much wind will catch on a certain scope. Increasing the load further.
 

For my ST80, which weighs about 4 pounds with a single ring, a 1.25 inch diagonal, and a 1.25 inch eyepiece, I use a tripod rated for 17 pounds and a ball head rated for 77 pounds. And I wouldn’t put a heavier or longer scope on the mount. It does handle the scope very well though.

 

For my C8, I use a tripod and mount rated for 40 pounds each. The bare scope weighs 12 pounds. The CGE rail, reducer, visual back, 2 inch diagonal, and 2 inch eyepiece... let’s say another 4 pounds. So 16 pounds in a fairly short package on a mount rated for 40 pounds. It does pretty well. But still I’d be hesitant to put anything much longer or heavier on there and expect it to perform optimally.

 

Now my C6R, which weighs about 20 pounds, but with diagonal and eyepiece...say 23 pounds. But it’s nearly 5 feet long with dew shield and diagonal. Plus the eyepiece is fairly heavy and sticks up three to four inches above the diagonal creating a greater moment arm.

I use this on a mount rated for 40 pounds by the manufacturer. But I would only call it acceptable. Not optimum. There are a lot of variables that have to be considered when weighing the difference between a manufacturer’s stated capacity versus real world capacity.

9B8E5B34-8189-4859-906F-D6CF60CD022B.jpeg


Edited by Echolight, 05 December 2021 - 06:03 PM.


#61 SeattleScott

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Posted 06 December 2021 - 12:30 AM

A good rule of thumb is never put a telescope on a mount that weighs less than the scope. That would be like using a Corvette to haul an RV. Technically the Corvette may have enough rated horsepower, but…

That being said I don’t know the weight of the camera tripod and star tracker. Just passing on a rule of thumb. Honestly, even putting a 24lb scope on a 26lb mount is really pushing it.
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#62 gnowellsct

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Posted 06 December 2021 - 03:53 PM

A good rule of thumb is never put a telescope on a mount that weighs less than the scope. That would be like using a Corvette to haul an RV. Technically the Corvette may have enough rated horsepower, but…

That being said I don’t know the weight of the camera tripod and star tracker. Just passing on a rule of thumb. Honestly, even putting a 24lb scope on a 26lb mount is really pushing it.

Yes I've never thought this through in terms of ratio but my reaction offhand is more like 1:2 telescope:mount, where the scope is about 33% of the total weight.  

 

Edit: it makes a difference to what the mount is attached.  A mount on a concrete pier has infinite mass relative to typical telescopes.  With the Berlebach on a grassy surface, I drive the spikes into the ground, that's a very tight grip probably equivalent to 50 lbs of additional mass, or more.  Greg N


Edited by gnowellsct, 06 December 2021 - 03:55 PM.


#63 SeattleScott

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Posted 06 December 2021 - 05:37 PM

Yes I think more people are generally happier with a 2:1 ratio. I view 1:1 as kind of the extreme boundary not to cross. I don’t know the specific weights, but putting a 8-9lb loaded refractor on a medium duty camera tripod with star tracker feels like it is pushing it. It might be ok, idk.

This subject reminds me of another current thread where a person bought a 5” triplet Apo to go on a Twilight I mount based on a vendor advertising that the mount has a rated capacity of 19lbs. Now he is scrambling trying to put together a more suitable mount. Weight rating on mass market stuff is primarily for marketing purposes. When you get to high end stuff where cost is not the object, you get more realistic weight ratings.

Scott

Edited by SeattleScott, 06 December 2021 - 05:43 PM.

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#64 csrlice12

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Posted 07 December 2021 - 12:07 PM

I can say the Vixen ED81S works well with the Twilight 1 mount.  A bit back heavy with 2" gear, but tightening down the clutches works.  Vibration times are only a second or less.


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