6" Refractor vs 10-12" Dobsonian

[Deadlake]

19 minutes ago, niallk said:

I keep a 15" dob ... assembled ... in our bedroom! 😂

That’s a great photo, always good to put things in perspective 😃👍

[faulksy]

2 hours ago, PeterW said:

I’m surprised you’ve not at least tried to hide it, You could drape a towel, hang some clothes on it….

Peter

or put the hoover in front of it, she will never look there

[Louis D]

3 hours ago, PeterW said:

I’m surprised you’ve not at least tried to hide it, You could drape a towel, hang some clothes on it….

Peter

What???  And cover up a fine piece of modern art and/or woodworking? 😉

[Rusted]

Do I detect a lack of creativity here?
You have to make them multi-purpose to be able get away with it.
A bit of ply and you have a stand up rest for the laptop.
Bung a doily over the top and you have a standard lamp.
TV stand? Place for a goldfish bowl? Trophy display area?
Ironing board for hankies? 😱 Hifi turntable stand? Coat rack in the hall?
If you were really clever you'd turn it into a make up mirror and storage shelf.
Have two! His and hers!
 

 

[Rusted]

8 hours ago, Deadlake said:

That’s a great photo, always good to put things in perspective 😃👍

I like the furniture's colour coordination with the "perfect ornament stand."

That pot looks completely lost on that chest of drawers.

[ollypenrice]

On 14/05/2021 at 19:45, neil phillips said:

Get your point. But it should never be a war of words or beliefs. Both have merits with advantages and disadvantages obviously. The idea refractors are better for photos, is a bit misleading, especially cost per performance criteria. Would like to see a refractor do this for anything less than many many thousands of pounds. Skywatcher 300p new price £900 

I got mine for £100 secondhand. Would like to see the cost of a refractor, any refractor match this for resolution. Either new or secondhand. 

 

 

Nothing resembling an amateur refractor can, as you say, match this resolution.  When/if lucky imaging comes to deep sky imaging, as it has come to lunar and planetary, then the refractor will lose out to the reflector. Although some lucky DS imaging is being done already it has some very considerable way to go. I'd be surprised if it didn't happen eventually, though.

Superb image from a bargain scope. Bravo.

Olly

[neil phillips]

26 minutes ago, ollypenrice said:

Nothing resembling an amateur refractor can, as you say, match this resolution.  When/if lucky imaging comes to deep sky imaging, as it has come to lunar and planetary, then the refractor will lose out to the reflector. Although some lucky DS imaging is being done already it has some very considerable way to go. I'd be surprised if it didn't happen eventually, though.

Superb image from a bargain scope. Bravo.

Olly

Having said all that. I am getting older and 300ps are not light. Recently got a secondhand SW 70mm F12.8 Achro. And a Evostar 120 F8.3 Achro. Could they produce a image as good as this ? Obviously not. Am i enjoying them ? most definately. As long as expectations are adjusted along with apeture loss.

Easy to set up. Quick cool down. No collimation needed. Grab and go fun. But if i want to get up close and personal. I will have to setup my 245mm Orion F6.3. For hi resolution lunar and planetary imaging. Different situations require different instruments and thats all we need to worry about. 

Dabbled lucky imaging deepsky. But would rather leave the dark art to those who truely know how to get the best out of it. Like yourself Olly. 

[dan_adi]

It depends. The "aperture rules" rule is not always true. The resolution of the scope will be limited by seeing. There is no point building a 10 meter scope in a back yard, cause it will perform no better that a 10 inch scope. Sure the image will be brighter but in terms of resolution ... I guess a more important factor is the quality of the optics and mechanics. A small scope with high quality will go head to head with bigger, less quality scopes. 

I used only a few designs, 10 inch newt, 12 inch SCT, 4 inch doublet frac and lately 8 inch Apo triplet. It might seem strange but the 8 inch frac performs better that the other scopes on DSOs. Is it the quality? Is it the design? I don't know, but all the other scopes were donated to friends. The major downside is cost, but for me the views and general versatility of the refractor justified the purchase.

Bottom line I think quality is more important than aperture

[neil phillips]

49 minutes ago, dan_adi said:

It depends. The "aperture rules" rule is not always true. The resolution of the scope will be limited by seeing. There is no point building a 10 meter scope in a back yard, cause it will perform no better that a 10 inch scope. Sure the image will be brighter but in terms of resolution ... I guess a more important factor is the quality of the optics and mechanics. A small scope with high quality will go head to head with bigger, less quality scopes. 

I used only a few designs, 10 inch newt, 12 inch SCT, 4 inch doublet frac and lately 8 inch Apo triplet. It might seem strange but the 8 inch frac performs better that the other scopes on DSOs. Is it the quality? Is it the design? I don't know, but all the other scopes were donated to friends. The major downside is cost, but for me the views and general versatility of the refractor justified the purchase.

Bottom line I think quality is more important than aperture

No better than a 10" ? Politely disagree. Show me a 10" scope that can match the resolution, and details in Luc Cathala lunar images. Just one example ?

 Ive had superb seeing in the UK from time to time over the years. At those times No 10" is going to match Lucs Newtonian. Not even a Zambuto. Ask Luc if he can resolve those details with a 10" at hes location ? Of course seeing matters. But even in the UK it does happen from time to time. Let alone many other back yard locations around the world.

UHD in Moretus 22 april 2021 with 625 mm and QHY5III178M focal 13650 mm - Imaging - Lunar - Stargazers Lounge

Edited May 20, 2021 by neil phillips

[Piero]

On 17/05/2021 at 18:52, Stu said:

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!

Yeah, those are not for wide-field.. those are for catching small faint galaxies and planetaries with your feet on the ground or at most on a step. Storage is another benefit. Something like that could be stored in a garage. The same aperture but f4-5 requires a dedicated house! 

 

On 17/05/2021 at 17:52, 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?

An F4 mirror is easier to make, but not necessarily better. Achieving an excellent smoothness is determined by the process the mirror maker has in place. You can have a terrific F3 and a so and so F5 for the same aperture. Of course, the former is going to cost more. Both f-ratio benefit from a coma corrector.

 

On 18/05/2021 at 07:20, niallk said:

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. 

 

I would not do that. Attaching the fans to metal brackets will cause vibrations. This is certainly not wanted while observing. In addition, fans for defeating the boundary layer should be off while observing. If on, the chance is that they will cause a lot of air turbulence.

I would find a way to temporarily (=e.g. consider velcro strips) mount 2 fans to the front panel of your mirror box (the one which is shorter). I would place them so that their point at the mirror surface, but not the mirror edge. Warm air moves up, not down, so those locations are sufficient. Then you turn them on for a few minutes, then off and observe. It should last for probably 45m / 1h . Then you give another blow for a few minutes while you have a break from the telescope. In addition to that, I would raise the light shroud up for about 10-12 cm from the bottom. Not sure how much distance your telescope has between the top edge of the mirror box and the top mirror surface. It might be that raising the light shroud up is sufficient for blowing the air boundary layer away. That is not really a laminar flow, it is the fact that the air boundary layer cannot form (which to me seems an even better solution).

 

[jetstream]

5 hours ago, dan_adi said:

the 8 inch frac

What brand of refractor is the 8"?

[dan_adi]

8 hours ago, neil phillips said:

No better than a 10" ? Politely disagree. Show me a 10" scope that can match the resolution, and details in Luc Cathala lunar images. Just one example ?

 Ive had superb seeing in the UK from time to time over the years. At those times No 10" is going to match Lucs Newtonian. Not even a Zambuto. Ask Luc if he can resolve those details with a 10" at hes location ? Of course seeing matters. But even in the UK it does happen from time to time. Let alone many other back yard locations around the world.

UHD in Moretus 22 april 2021 with 625 mm and QHY5III178M focal 13650 mm - Imaging - Lunar - Stargazers Lounge

If you are referring to planetary imaging, where seeing is not a problem because  of ultra fast exposures yes, otherwise no. 

[dan_adi]

3 hours ago, jetstream said:

What brand of refractor is the 8"?

CFF 8" f6.5 

[Piero]

10 hours ago, dan_adi said:

Bottom line I think quality is more important than aperture

 

Your comparison between a 10" and 10 m is unrealistic.. Cite one person in the UK having a 10m telescope.. It is hard to find one owning a 1m! 

 

I think both quality and aperture are important. However, if the two instruments are on par in terms of quality (as I said before, not just optics, but the whole thing), the larger is the better. This is said in a realistic context though. I don't know what the upper bound is, but I certainly disagree that 10" is the limit and beyond that, targets are just brighter. Seeing, transparency, and darkness are other critical factors to be taken into account. Some locations are better than others, but there are a few decent places in the UK where one can take serious advantage of aperture. 

As far as I can see amateur astronomers owning medium to large dobsons (15-30" aperture) are people who are not interested in more resolution.. They don't get those telescopes for observing Jupiter, the Moon, or double stars... They get them for catching SMALL FAINT planetaries and galaxies. These targets require DARK SKIES and EXPERIENCE. An 8" refractor, even with the most exotic and best polished optics, is still an 8" aperture. Nice all-around, despite the cost reaching the sky, but a few classes of targets are out of its range due to its limited aperture. 

My bottom line.. telescopes with different apertures have different use cases. Understanding the different use cases, your own interests, and your life context is a good starting point when choosing a telescope. 

[Stu]

I say this without wanting to stir up a load more trouble, but it actually happened....

Last night, I was observing with my local group, and one chap turned up without his scope. At one point I offered him a look at the Moon through my Tak FC100DC, which he accepted.

His words were ‘Wow, that’s better than my 10” dob”.

Now, I know from experience that my 8” f8 will show better detail at high power due to its resolution so I’m not going to argue that. I guess it’s a case that at the x148 I was using, the Tak gives a very nice, clean and contrasty view which is very appealing.

I shall now don my tin hat and seek refuge under the table 🤣🤣

#justsayin’

[cloudsweeper]

5 minutes ago, Stu said:

I say this without wanting to stir up a load more trouble, but it actually happened....

Last night, I was observing with my local group, and one chap turned up without his scope. At one point I offered him a look at the Moon through my Tak FC100DC, which he accepted.

His words were ‘Wow, that’s better than my 10” dob”.

Now, I know from experience that my 8” f8 will show better detail at high power due to its resolution so I’m not going to argue that. I guess it’s a case that at the x148 I was using, the Tak gives a very nice, clean and contrasts view which is very appealing.

I shall now don my tin hat and seek refuge under the table 🤣🤣

#justsayin’

I agree Stu.  Aperture is good, especially for faint stuff, but if you want real sharpness and contrast, fracs are the way.  I've never used a Tak, but my Bressers and Exp Sci Apo also give fine views!

Move over, I'm joining you under the table!

Doug.

[Deadlake]

34 minutes ago, Stu said:

I say this without wanting to stir up a load more trouble, but it actually happened....

Last night, I was observing with my local group, and one chap turned up without his scope. At one point I offered him a look at the Moon through my Tak FC100DC, which he accepted.

His words were ‘Wow, that’s better than my 10” dob”.

Now, I know from experience that my 8” f8 will show better detail at high power due to its resolution so I’m not going to argue that. I guess it’s a case that at the x148 I was using, the Tak gives a very nice, clean and contrasts view which is very appealing.

I shall now don my tin hat and seek refuge under the table 🤣🤣

#justsayin’

I was actually there. Seeing was quite good, better then expected. My ears did pick up as the above word where uttered. The cogs in my brain where turning about posting on this thread. 😀
One thing I would say and it’s the first time I went mobile was the flexibility of having a 4” refractor to setup, use and take down. 

Thanks @Stu for organising, shame @GavStarwas missing in action. 😀

[JeremyS]

47 minutes ago, Stu said:

I say this without wanting to stir up a load more trouble, but it actually happened....

Last night, I was observing with my local group, and one chap turned up without his scope. At one point I offered him a look at the Moon through my Tak FC100DC, which he accepted.

His words were ‘Wow, that’s better than my 10” dob”.

Now, I know from experience that my 8” f8 will show better detail at high power due to its resolution so I’m not going to argue that. I guess it’s a case that at the x148 I was using, the Tak gives a very nice, clean and contrasts view which is very appealing.

I shall now don my tin hat and seek refuge under the table 🤣🤣

#justsayin’

Wasn’t @mikeDnight was it by any chance Stu?
 

🤣

[jetstream]

2 hours ago, dan_adi said:

CFF 8" f6.5 

Very nice

I wonder how it compares to a 100mm Tak?

[John]

I'm always very, very polite when looking though someone elses scope

[neil phillips]

1 hour ago, Stu said:

I say this without wanting to stir up a load more trouble, but it actually happened....

Last night, I was observing with my local group, and one chap turned up without his scope. At one point I offered him a look at the Moon through my Tak FC100DC, which he accepted.

His words were ‘Wow, that’s better than my 10” dob”.

Now, I know from experience that my 8” f8 will show better detail at high power due to its resolution so I’m not going to argue that. I guess it’s a case that at the x148 I was using, the Tak gives a very nice, clean and contrasts view which is very appealing.

I shall now don my tin hat and seek refuge under the table 🤣🤣

#justsayin’

Each case can be different for a multitude of different reasons, How well a scope is collimated. How long its been cooling. Falling temperatures affecting larger scopes with bigger air cells. The optical figure. Perception of what constitutes better ? Someone being nice (He he ) on and on. You get the point. Of course there will be times a smaller scope. Can do better. It was the blanket statement i took issue with.  like it was set in stone.  Were all friends here. Come out from under that table. And take off that silly hat Stu 

[neil phillips]

2 hours ago, cloudsweeper said:

I agree Stu.  Aperture is good, especially for faint stuff, but if you want real sharpness and contrast, fracs are the way.  I've never used a Tak, but my Bressers and Exp Sci Apo also give fine views!

Move over, I'm joining you under the table!

Doug.

Oh God not again. How about this for contrast and sharpness. A mass produced 15 year old Celestron power seeker F8  4.5" with pin holes in the primary. Trust me on this, when well collimated its supremely tak sharp (no pun intended ) and contrasty. It cost how much ? ? ?  I can see you under that table quite clearly 

[Stu]

1 hour ago, JeremyS said:

Wasn’t @mikeDnight was it by any chance Stu?
 

🤣

I’m fairly sure not, I don’t believe he owns a 10” shaving mirro... I mean 10” dob does he? 😉😉

[Mr Spock]

It doesn't matter what anecdotal evidence you provide, a 250mm is considerably brighter than a 100mm, while black remains black.  A scope without a central obstruction can produce a better MTF curve which would favour picking out faint contrasts on such as Jupiter, but the 100 to 250 ratio is overwhelming; even with a central obstruction the 250mm has significantly greater contrast.

[Stu]

As I’ve said many times before, when conditions are good, I see more detail at high power in my 8” f8 than I do in my Tak. I don’t have @mikeDnights eyes unfortunately! When conditions are poor, it can be the other way around.

The key point is actually that the scope I had with me was the 4” refractor with a light weight mount (and actually a Heritage 150p), because my car was full of other stuff and the dob wouldn’t fit, plus I expected it to cloud over so not worth taking anything bigger. It’s the scope you have and use most that’s the best. Oh, and the chap wasn’t just being nice, it was a genuine unprompted comment.

But I still recommended the 12” dob in answer to the original question! 

I quite like it under this table anyway, so I’ll stick here for a while. The hat is keeping my head warm.... 🤪🤪