6" Refractor vs 10-12" Dobsonian

[Piero]

1 minute ago, Deadlake said:

Amazing home build!!!! 😀 How high is the eye piece off the ground, do you need to use a step to use it?
It's the one thing thing that puts me off Dobs.

Thanks ☺️

I'm 1.76 m tall and the eyepiece is right where my eye is, when the telescope is vertical.

I have a ladder with 3 large steps, but this is used for sitting until the adjustable chair is ready.

[Piero]

3 minutes ago, Stardaze said:

Love that!! I need to knock some ramps up too. I’m thinking of getting a little truck so that I can do it all in one go instead of breaking down the base and OTA, but certainly need to negotiate the door step. Hopefully save my back which I occasionally suffer with.

Absolutely, health is first. I too have sore back time to time, that's way the whole use case was designed so that there was no need to dismantle Nunki (the telescope).

Of course, if needed, it can be dismantled anyway.

[Louis D]

On 14/05/2021 at 09:40, Rusted said:

There is clear logic to choosing an APO for its shorter focal length without suffering the "LSD trip" of false colours.
A shorter Newtonian is not quite so compromised by coma as is a short refractor by its rainbow hues.
Now what was the question again?

At f/2.7 or thereabouts, there is no comparison between a well figured 28" Newtonian with Paracorr II and any 28" refractor I know of.

 

[Stu]

6 hours ago, Louis D said:

At f/2.7 or thereabouts, there is no comparison between a well figured 28" Newtonian with Paracorr II and any 28" refractor I know of.

 

I’ve never quite understood those dobs. Not exactly widefield with a 1920mm focal length, stick a 31mm Nag in and you get an 11.5mm exit pupil. In reality you are limited to about a 16mm eyepiece so your starting point is x120. Fine for small galaxies, globs etc which is the main target I guess.

[PeterW]

What fer seeing faint stuff… designed fer the E-bomb (E21). https://www.bbastrodesigns.com/HET.html

Why I’ve got an f3 mirror waiting for a scope to appear around it.

Peter

Edited May 17, 2021 by PeterW
Additiom

[Deadlake]

2 hours ago, PeterW said:

What fer seeing faint stuff… designed fer the E-bomb (E21). https://www.bbastrodesigns.com/HET.html

Why I’ve got an f3 mirror waiting for a scope to appear around it.

Peter

Presume a Paracorr is mandatory using such a fast scope?

[PeterW]

Yes, the ES corrector doesn’t “add 15%) to your focal length. You want to keep your speed:

Peter

[Deadlake]

2 minutes ago, PeterW said:

Yes, the ES corrector doesn’t “add 15%) to your focal length. You want to keep your speed:

Peter

This will decrease the exit pupil, so apart from not standing on tippy-toes what advantage does this give?

An F4 would be maybe better, and have a smoother mirror?

[Stu]

59 minutes ago, Deadlake said:

This will decrease the exit pupil, so apart from not standing on tippy-toes what advantage does this give?

An F4 would be maybe better, and have a smoother mirror?

I guess it must just purely be manageability and eyepiece height, I can’t think of any other benefits. I doubt it gives you more field of view because as I said earlier the exit pupil just gets too large. Mind you, it is quite a consideration, not having to stand precariously on a ladder!

[Rusted]

There was a thread on CN about "the best telescope for planetary and lunar detail regardless of size or price."
A fully optimised 14-16" Newtonian, of fairly long focus, seemed to ruin many a big spender's day [or rather night.]
This was in repeated, side by side comparisons, in the USA, from a site with known good seeing.
Fans for thermal scrubbing, of the primary's boundary layer, a thermally neutral, cardboard tube, curved vane spiders and quality optics seemed to excel.
This was some time ago. So I may well have conjured up, false memories of the exact details.

 

[Stu]

2 hours ago, PeterW said:

Yes, the ES corrector doesn’t “add 15%) to your focal length. You want to keep your speed:

Peter

So long as you have enough in focus with it fitted, isn’t that why the paracorr has the built in barlow effect?

[Stu]

1 hour ago, Rusted said:

There was a thread on CN about "the best telescope for planetary and lunar detail regardless of size or price."
A fully optimised 14-16" Newtonian, of fairly long focus, seemed to ruin many a big spender's day [or rather night.]
This was in repeated, side by side comparisons, in the USA, from a site with known good seeing.
Fans for thermal scrubbing, of the primary's boundary layer, a thermally neutral, cardboard tube, curved vane spiders and quality optics seemed to excel.
This was some time ago. So I may well have conjured up, false memories of the exact details.

 

Quite some years back I viewed Jupiter through a 16” f4 dob at full aperture and also with a 170mm aperture mask. The masked view was more stable and nicer to view, but the full aperture ultimately showed more detail and colour despite being much more susceptible to the seeing conditions.

I do think boundary layer warm air is a significant factor in poor planetary views. My Portaball 8” had boundary later fans and I’ve often wondered about fitting some to my 8” f8 to see if that helps. I suspect it would.

[niallk]

16 minutes ago, Stu said:

I do think boundary layer warm air is a significant factor in poor planetary views. My Portaball 8” had boundary later fans and I’ve often wondered about fitting some to my 8” f8 to see if that helps. I suspect it would.

I want to try an air 'stirrer' fan in front of the mirror on my 15" too.  I can't bring myself to drill holes in the side walls of the mirror box though!  I'll just mount some fans on the inside angled to move the boundary layer, and assist with cooling from the front, in addition to the one I have on the rear.  I normally throw the scope out with the fan on for a good 2-3 hrs beforehand if I want to observe planets.

[Deadlake]

6 minutes ago, niallk said:

I normally throw the scope out with the fan on for a good 2-3 hrs beforehand if I want to observe planets.

Be quiet, since APO’s are ready to go immediately observers who have busy lives will buy them. That or some Deep Space products. 

Does make be wonder if I bought a C9.25 or Mewlon in 2022 when they are back in stock ( No jokes really) how much I would use it.

[JeremyS]

4 minutes ago, Deadlake said:

Be quiet, since APO’s are ready to go immediately observers who have busy lives will buy them. That or some Deep Space products. 

Does make be wonder if I bought a C9.25 or Mewlon in 2022 when they are back in stock ( No jokes really) how much I would use it.

2 or 3 times a year based on my experience 🤣

[Deadlake]

1 minute ago, JeremyS said:

2 or 3 times a year based on my experience 🤣

I’m lucky to get 2 hours in before being up for work in the morning or clouds stop play… 🤨 Choose a scope for your style of observing….

[John]

The best "accessory" for getting more out of astronomy whatever scope you use - retirement 

[Deadlake]

8 minutes ago, John said:

The best "accessory" for getting more out of astronomy whatever scope you use - retirement 

Working from home helps, on schools days I get up at 8 am. However unlike last year the clouds and the school days are not overlapping. 😩

[JeremyS]

15 minutes ago, Deadlake said:

Working from home helps, on schools days I get up at 8 am. However unlike last year the clouds and the school days are not overlapping. 😩

Long school holidays though 

🤣

[jetstream]

4 hours ago, John said:

The best "accessory" for getting more out of astronomy whatever scope you use - retirement 

👍

Its why I can observe so much.

[faulksy]

6 hours ago, niallk said:

I want to try an air 'stirrer' fan in front of the mirror on my 15" too.  I can't bring myself to drill holes in the side walls of the mirror box though!  I'll just mount some fans on the inside angled to move the boundary layer, and assist with cooling from the front, in addition to the one I have on the rear.  I normally throw the scope out with the fan on for a good 2-3 hrs beforehand if I want to observe planets.

im tempted with this nial. bought the fans when i first started my build. not sure were they should go though

[Louis D]

6 hours ago, Deadlake said:

Be quiet, since APO’s are ready to go immediately observers who have busy lives will buy them.

Speak for yourself.  My TS-Optics (Sharpstar) 90mm FPL-53 triplet APO takes at least 30 minutes to acclimate just 15 degrees F (7 degrees C ?) before stars quit having spikes.  My 8" Dob acclimates in about the same time, but to get rid of blurriness associated with tube currents.  If I put fans behind the primary, I'd probably be ready to go in under 10 minutes.  Low power views are fine in the Dob while waiting.  Not so much with the APO.  Those spikes are still plainly visible on bright open clusters like the Pleiades.

[niallk]

4 hours ago, faulksy said:

im tempted with this nial. bought the fans when i first started my build. not sure were they should go though

I'm thinking small fans, on bendable metal brackets in the 4 corners angled downwards on to the front of the mirror.  I won't attempt to achieve "laminar flow"... just keeping the air moving to aid cool down.  With a speed control, I could possibly leave on while observing. 

Otherwise, I'm thinking to just have a bigger fan on frame I can just place over the front of the mirror box during cool down, removing when observing. 

[ollypenrice]

10 hours ago, niallk said:

I want to try an air 'stirrer' fan in front of the mirror on my 15" too.  I can't bring myself to drill holes in the side walls of the mirror box though!  I'll just mount some fans on the inside angled to move the boundary layer, and assist with cooling from the front, in addition to the one I have on the rear.  I normally throw the scope out with the fan on for a good 2-3 hrs beforehand if I want to observe planets.

Nah, man up and get the drill out!    A Sky and Telescope article years ago convincingly demonstrated that you need to bring air in from one side, just above the mirror, and blow it out of the other. Shoot that boundary layer clean across the garden!

Olly

[niallk]

1 hour ago, ollypenrice said:

Nah, man up and get the drill out!    A Sky and Telescope article years ago convincingly demonstrated that you need to bring air in from one side, just above the mirror, and blow it out of the other. Shoot that boundary layer clean across the garden!

Olly

🤣🤣 I just can't do it!!  I've thought about it, but am too much of a wuss at the moment 😂

It's certainly a low cost feature which would be a welcome addition on Obsession Classics... along with a thinner mirror.

What I do is plan during the day if the skies are looking clear & the jetstream forecast looks good, and take a few mins to throw the scope outside and then get on with dinner / kids homework/ getting them to bed.  So, the cool down time is just something I work around, without it being a major problem.  The 2" mirror does need time to cool though, and the views show it of course.  Even for DSOs, where it may not be quite as critical, I tend to try to allow good cooling anyway - I drive to a darker site for DSOs, so want to take advantage of the best views I can get.

So I guess I have a 'grab-wait-and-go' setup.  The views are worth the wait

I even 'cool my car' - wind down the windows to let the inside cool towards ambient for using later on to transport the scope!  Probably unnecessary😄

Edited May 18, 2021 by niallk