Why settle with reflector scopes when refractors have no collimation?

[Captain Scarlet]

Just now, Ships and Stars said:

That's impressive! Were those taken under urban or rural skies? Either way, I've got to try NV.

How much money do you have? 🤣🤣

[DaveS]

This is why don't do NV, much rather spend my money on a real camera and filters.

[tomato]

52 minutes ago, DaveS said:

This is why don't do NV, much rather spend my money on a real camera and filters.

Wow, a branch of visual astronomy that hurts the wallet as much as imaging.

[PeterW]

For NV you want speed of optics, using the Televue 67mm reduction to speed up the scope you choose for the focal  length and hence field of view. Big scopes simply give you a smaller field of view. Refractor, reflector, anything will do. For the best views you want to give each eye an NV view…. Double the fun, but double the cost. Depends what objects you want to look at…  different objects, different tools. 
collimation is nothing to worry about, though did watch a friend nearly unscrew his SCT secondary by twiddling the screws the wrong way….

Peter

[Deadlake]

40 minutes ago, tomato said:

Wow, a branch of visual astronomy that hurts the wallet as much as imaging.

It’s just like looking thru an EP, just black and white. And real-time, no need to spend hours collecting and processing the image. However for most DSO not much colour so don’t think much to miss…. As @PeterW says just need to choose focal length to frame object of interest and of course improves any telescope.

Easier to drag around an NV intensifier and a 9 “ scope then a 24” dob and a ladder… 😃😃😃

Edited April 30, 2021 by Deadlake

[DaveS]

I think I would rather put a proper camera on the end of my 'scopes and go deep.

[PeterW]

Depends how you want to spend your time… imagers take ages on each object to collect photons and then spend time to process  the result. Observers spend their time finding and looking at objects with their eyes out in the cold…. Personal choice, everyone has their own motivations.

Peter

@Deadlakeyou can have fun without even a scope… just a small camera lens sometime…. hand carry.

[neil phillips]

4 hours ago, Lockie said:

 Hi Micheal, can I just clarify? Are you saying the extra aperture of the C9.25 didn't resolve any finer detail? I might experiment with un obstructed glass verses larger obstructed mirrors on say lunar detail.   

Astronomical telescope 154mm series HD objective lens focal length F750 F1000 F1200 multi layer film single layer film|Monocular/Binoculars| - AliExpress

6" F15 Chris

154mm Diameter 750/1200/1500/1800/2250mm Focal Length Astronomical Telescope Objective Lens AR Multi coated Dual Separation|Monocular/Binoculars| - AliExpress

Edited April 30, 2021 by neil phillips

[Mr Spock]

3 hours ago, Lockie said:

 Hi Micheal, can I just clarify? Are you saying the extra aperture of the C9.25 didn't resolve any finer detail? I might experiment with un obstructed glass verses larger obstructed mirrors on say lunar detail.

Yes. With the C9.25 my usual mag was x235 and very rarely x296. In the last few weeks I've been routinely using the 120mm close to x300. Even with some loss of contrast and smearing due to CA I was seeing the same level of detail or more. The C9.25 has twice the resolving power but can't actually do it in the real world, even when the optics are really good. It can with imaging, which is why you see such good planetary images taken with it. It's all down to that big obstruction and how it affects MTF.

[PeterW]

… a lot is down to the atmospheric seeing and it’s spatial length scale which means that scopes much bigger than 6” (or so) are not necessarily any better than smaller ones. Of course if you are using lucky imaging then you can grab those very occasional moments of stable viewing and stack them together and get more from your bigger scopes.
 

Peter

[paulastro]

6 hours ago, Stephenstargazer said:

Oh, I don't know, the ladder has wheels and you just push it out of a very big shed 😀😀

Ah, that would be push and go, but not very far then 🙂.

[Chris]

10 hours ago, neil phillips said:

Astronomical telescope 154mm series HD objective lens focal length F750 F1000 F1200 multi layer film single layer film|Monocular/Binoculars| - AliExpress

6" F15 Chris

154mm Diameter 750/1200/1500/1800/2250mm Focal Length Astronomical Telescope Objective Lens AR Multi coated Dual Separation|Monocular/Binoculars| - AliExpress

Don't tempt me to bite off more than I can chew, neil. I went through a stage about 6-7 years ago where I really wanted to do this. Can you imagine how big a 6" f15 would be though 🤣 

 

[Chris]

9 hours ago, PeterW said:

 Of course if you are using lucky imaging then you can grab those very occasional moments of stable viewing and stack them together and get more from your bigger scopes.
 

I think any experiment between un obstructed aperture and larger obstructed aperture would need to be via AVI footage only in order to make things more akin to observing. 

[neil phillips]

13 minutes ago, Lockie said:

Don't tempt me to bite off more than I can chew, neil. I went through a stage about 6-7 years ago where I really wanted to do this. Can you imagine how big a 6" f15 would be though 🤣 

 

I know from my floor to my ceiling what a beast. Look at the price of the 6" F8  tubes and cells add considerably though 

[Carbon Brush]

Going back a little way on the thread, collimation.

I have owned various reflector scopes for almost 20 years. Some basic newtonian reflectors and some compound scopes.
Having read early on about collimation being essential, I suppose I got quite obsessed with regular checks and different methods or tools.
But after a while I started to realise that collimation often did not help.

The general observations have been....

If the scope is designed correctly, it tends to hold collimation.
By that I mean parts that don't flex and wobble in normal use and handling.

If the scope is assembled correctly it is easy to collimate.
By assembly, I don't just mean extending a Skywatcher flextube, or assembling a truss dob.
I have seen scopes arrive from the manufacturer with the primary mirror way off centre, or pointing way off tube axis.
I have seen focusser tubes that don't point directly  to the secondary mirror. Either off to one side, or not to the tube centre.

In use the reflector scopes have tended to hold collimation well.
If handled sensibly, what is the difference between standing on a shelf for a week and leaving on a pier?

Also if it is a night when seeing limits you x100, why get worked up about perfect collimation?
Just enjoy what you can see.
 

[Louis D]

15 hours ago, Mr Spock said:

Yes. With the C9.25 my usual mag was x235 and very rarely x296. In the last few weeks I've been routinely using the 120mm close to x300. Even with some loss of contrast and smearing due to CA I was seeing the same level of detail or more. The C9.25 has twice the resolving power but can't actually do it in the real world, even when the optics are really good. It can with imaging, which is why you see such good planetary images taken with it. It's all down to that big obstruction and how it affects MTF.

Try resolving M13 or M22 to the core with each.  I find there's no substitute for aperture for globs.

[Skyline]

On 29/04/2021 at 12:13, ollypenrice said:

I don't like messing about with optics so I use refractors. That means two things: I'm lazy and I enjoy astrophotography...

😁lly

You can also enjoy astrophotography with a reflector 

[Skyline]

Here is an informative article by Nils Olif Carlin about collimation.

http://web.telia.com/~u41105032/kolli/kolli.html

[ollypenrice]

14 minutes ago, Skyline said:

You can also enjoy astrophotography with a reflector 

With a working reflector!

But please don't take my comment seriously, it was tongue in cheek.

Olly

[Skyline]

Absolutely, I was not thinking any other way.

[Deadlake]

How sensitive are refractors to seeing and accilmiation when they get over 150 mm in aperture.

100-130 mm APO's are the ultimate grab n'go, but ignoring cost when the refractor gets over 150 mm does it lose it's main advantage as far as setup speed? 

At that point only collimation (which for some mirrored scopes you might every few years) is the difference?

[Louis D]

They tend to get quite long, so most are permanently installed in observatories on piers.  F-ratio must increase as the objective gets larger with a constant design type to maintain the same level of color correction.  This makes the length grow faster than the aperture.  This is why ED glass and triplets have become popular.  It allows for shorter f-ratios and thus shorter tubes while maintaining good color correction.