6" Refractor vs 10-12" Dobsonian

[Stardaze]

3 hours ago, jetstream said:

Yes.

Contrary to some members beliefs I believe that a clean mirror is a must. A dirty mirror not only reduces DSO "contrast" through the eyepiece it adds scatter on bright objects reducing the quality of the view IMHO.

Some schools of thought maintain a dirty mirror is fine and brag about how long ago it was cleaned... I do not subscribe to this thought.

Begs the question Gerry, how often do you clean yours?

[jetstream]

3 minutes ago, Stardaze said:

Begs the question Gerry, how often do you clean yours?

When needed... sometimes 4-5 times a year but now they are in a sea can they stay much cleaner, longer.  My preferred method is a soapy water spray to rinse, followed by another soapy water spray, spray the fingers and very gently use the fingers to clean the mirror. Soap is a must as it lubricates. The final rinse is with distilled or "Aquafina".

This is just me, others will have different suggestions.

[Stardaze]

17 minutes ago, jetstream said:

When needed... sometimes 4-5 times a year but now they are in a sea can they stay much cleaner, longer.  My preferred method is a soapy water spray to rinse, followed by another soapy water spray, spray the fingers and very gently use the fingers to clean the mirror. Soap is a must as it lubricates. The final rinse is with distilled or "Aquafina".

This is just me, others will have different suggestions.

Slightly different method than almost everything I've read to date. How do you mean, "in a sea"?

[jetstream]

3 minutes ago, Stardaze said:

Slightly different method than almost everything I've read to date. How do you mean, "in a sea"?

I store my scopes in a shipping container- a "sea can". Theres a few of us that use the method - you can feel the dirt or if the mirror is clean.

[F15Rules]

On 13/05/2021 at 00:44, Louis D said:

A long focal length, 6" refractor is just about enough to start resolving large, close globular clusters like M13 at 200x and above

I don't dispute the rest of the post, but my Tak FS 128 resolves a lot of faint stars in M13 at less than 200x, and is superb on good nights at over 250x due to the excellent contrast.

Both my previously owned 4" Vixen ED103s refractors also served up very pleasing images of M13.

The combination of tight, sharp stellar images and high contrast shouldn't be underestimated.

I'd also add that a proper, extended viewing of these objects will deliver better views of these targets, whatever scope is used..you can't expect a 2 or 3 minute "glance" to show the level of detail you can see after a proper 15minute plus viewing - on almost any object😉.

Dave

[Stu]

2 hours ago, jetstream said:

When needed... sometimes 4-5 times a year but now they are in a sea can they stay much cleaner, longer.  My preferred method is a soapy water spray to rinse, followed by another soapy water spray, spray the fingers and very gently use the fingers to clean the mirror. Soap is a must as it lubricates. The final rinse is with distilled or "Aquafina".

This is just me, others will have different suggestions.

I sometimes do that too Gerry. I make sure I soak the mirror for a while, and soften my fingers in the water too. As you say soap is essential to lubricate things but done carefully it is highly effective. I can’t remember where I read about it, but it certainly seems to work.

[Stu]

47 minutes ago, F15Rules said:

Both my previously owned 4" Vixen ED103s refractors also served up very pleasing images of M13.

Indeed. My ‘scope that must not be mentioned’ will resolve stars in and around M13 with accurate focus and careful observation. Darker skies or an observing hood also help. Not that it competes with a larger aperture, but it is not correct to say you can’t resolve stars in larger globs in a decent 4” frac.

[Spile]

I totally get the desire to keep my mirror clean but this tells me that the desire is not based on logic...

https://astroanecdotes.com/2015/03/26/the-mcdonald-gun-shooting-incident
In addition, it would mean I am signing up to cleaning my mirror twice a year or more which means as well as the time and hassle I am increasing the risk of damage.

 

[Stu]

2 minutes ago, Spile said:

I totally get the desire to keep my mirror clean but this tells me that the desire is not based on logic...

https://astroanecdotes.com/2015/03/26/the-mcdonald-gun-shooting-incident
In addition, it would mean I am signing up to cleaning my mirror twice a year or more which means as well as the time and hassle I am increasing the risk of damage.

 

A small area of damage on a large mirror, is not the equivalent of the entire mirror being reduced in reflectivity by a significant amount I would say?

[Piero]

I think one should clean it when it is clear that the mirror needs a wash. This depends on how much time the telescope has been used. 

Hard to say how often. The problem is that there seems to be a phobia associated to scratching it. It's just about learning a good way and be careful during the process. Said this, a large amount of dust / dirt is certainly worse than a tiny scratch. Both cause light scattering = noise, not visual aberrations or distortions.

[niallk]

I wash mine ~ once per year, but if it needed it - eg something unidentified splatted on it, I'd wash if required.  I too use finger tips.  I was very nervous to do it the first time, but quickly discovered it's not so big a deal.  Just have everything prepared, remove any wedding ring and take your time

I hope to get my scope under bortle 2 skies in August, so I'll wash it ahead of that trip to the Kerry Gold Tier Dark Sky Reserve to get the best out of it

Edited May 22, 2021 by niallk

[Captain Scarlet]

Seems a bit drastic to have to get divorced just to wash your mirror 🤣. On balance, probably worth it 😉

Seriously though I think by August I’ll be permanently esconsed in SW Ireland so I look forward to meeting you there!

Edited May 22, 2021 by Captain Magenta

[PeterW]

Even darker skies @Captain Magenta, thought it always rained in Ireland?

Peter

[Stu]

2 hours ago, PeterW said:

Even darker skies @Captain Magenta, thought it always rained in Ireland?

Peter

At least his mirror will always be clean.....

[jetstream]

7 hours ago, Spile said:

not based on logic...

Wait until you see your coatings start to get eaten by the dust/dew mix 😀 mind you if you don't clean it you might not know until its too late

[niallk]

11 hours ago, PeterW said:

Even darker skies @Captain Magenta, thought it always rained in Ireland?

Ha ha - feels like it much of the time!  But when you get a clear night down there in West Cork / Kerry ... man the sky is incredible 😉

Forget 6" frac vs 12" dob: mk1 eyeball gives memorable views!

Edited May 23, 2021 by niallk

[Spile]

On 22/05/2021 at 16:46, Stu said:

the entire mirror being reduced in reflectivity

The issue for me is if that reduction actually makes a difference at the eyepiece. The gunshot incident indicates that it doesn't so personally I don't bother.

Edited May 23, 2021 by Spile

[Captain Scarlet]

It will make a difference at the eyepiece but not because of reduced mirror reflectivity. It’s because of the reflectivity of the dust particles causing stray scattered light, plus the extra diffraction from all the edges of the dust particles: both will reduce contrast at the eyepiece.

[Piero]

Exactly. More light scattering means faint stars washed out in the back background sky, which becomes brighter, and a general reduction in contrast.

Here's a direct example: take your sight (or sun) glasses off and intentionally add dust on the lens surfaces. Then put them on and look around. Then blow the dust off with your breath and put your glasses on again.

I experienced that so often, when I was making Nunki, my 16" dob, and was covered with wood dust... 🙄

Edited May 23, 2021 by Piero

[scotty38]

If seven 9mm bullet holes and being attacked with a hammer don’t effect any viewing change then should there be any desire for absolute perfection in the first instance?

[Stu]

2 minutes ago, scotty38 said:

If seven 9mm bullet holes and being attacked with a hammer don’t effect any viewing change then should there be any desire for absolute perfection in the first instance?

I think there is a misunderstanding of what’s going on here. The bullet holes are a very small part of the mirror surface, the rest of which is presumably pristine in quality and cleanliness, so is performing as it should.

That is very different from having an entire mirror which is covered in dust, scattering light and reducing contrast. I assume there is also a reduction in reflectivity as a dirty mirror looks dull compared with a clean one. We put so much emphasis on mirror specs, it doesn’t make sense to me to leave the dirty and expect them to perform to specification.

One caviat here. Under the kinds of rubbish conditions I experience near Heathrow, with poor seeing, light pollution and often poor transparency (worse when the planes start again), it is quite likely I wouldn’t be affected by the performance drop, or might not notice it. Under good or excellent skies, you are much more likely to notice the drop in contrast, and if, say, you have driven quite some distance to get to dark skies, it makes sense to have your scope performing at its best. That’s doesn’t mean washing your mirror every week, but it does mean maintaining it when it needs it.

[scotty38]

Just now, Stu said:

I think there is a misunderstanding of what’s going on here. The bullet holes are a very small part of the mirror surface, the rest of which is presumably pristine in quality and cleanliness, so is performing as it should.

That is very different from having an entire mirror which is covered in dust, scattering light and reducing contrast. I assume there is also a reduction in reflectivity as a dirty mirror looks dull compared with a clean one. We put so much emphasis on mirror specs, it doesn’t make sense to me to leave the dirty and expect them to perform to specification.

Agreed and it’s why I mentioned absolute perfection in the first instance, that is during manufacturing. I can well imagine someone getting their nice new 10” dob home and noticing half a dozen air bubbles in the mirror would be sending it straight back yet (give or take but you know what I mean) it’d be akin to the bullet holes in the bigger one.....

[jetstream]

The 20% (or so) missing from the primary mirror due to obstruction is not missed...

[jetstream]

APM contract with LZOS says bubbles are OK. At least they are telling us, most (all?) other manufacturers dont.

"5. Bubbles and inclusions requirements
Permissible dimensions of bubbles and inclusions:
For systems of diameter up to 152mm
In lens material bubbles and inclusions of no more than 0.3mm in size are allowable , and a number of bubbles of
diameter 0.03mm to 0.3mm in each lens shall not exceed 3 pieces.
For systems of more than 152mm up to 254mm diameter
Number of bubbles and inclusions of size up to 0.3mm - not more than 20 pieces. - up to 0.5mm - not more than
3 pieces.
Bubbles and inclusions in material exceeding the requirements above shall be coordinated on the stage of optical
blanks selection for the manufacturing of each lens"

https://www.apm-telescopes.de/en/telescopes/apochromatic-refraktors/apm-lzos-apo-refractors/apm-lzos-telescope-apo-refractor-130-780-3.7-inch-focuser.html

Edited May 24, 2021 by jetstream

[Piero]

7 hours ago, jetstream said:

The 20% (or so) missing from the primary mirror due to obstruction is not missed...

That's linear obstruction, which does affect contrast, but not via light scattering caused by dirt on the optics. The reduction in light grasp reduction is minimal, as this depends on the quadratic (non linear) obstruction.

Said this, linear obstruction is something to consider, but, to me, not something to get obsessed about. Reducing linear obstruction from 20-21% to 16-17% has a very limited gain on the views - assuming it is implemented and maintained correctly -, whereas it adds a lot of technical constraints. These constraints not only can make your life more difficult, but can also damage the views. With this, I don't mean we should favour large central obstructions. Not at all. What I mean is that choosing the smallest possible or obsessing with this parameter, has more disadvantages than advantages.

Examples of constraints due to (obsessed) minimisation of the secondary mirror (NOTE: these constraints are not just at design time.. they live during the whole lifetime of the telescope..): 

- focuser must be perfectly squared.

- focuser axial alignment must be perfect, otherwise not all the primary mirror area is reflected, effectively acting like an aperture stop (=> smaller primary mirror..!). This means that there is no shift in the focuser axial alignment over any altitude or in the drawtube (hard to believe this is the case... e.g. Moonlight focusers do have an inevitable shift in the drawtube as the stainless steel bearings dig into the aluminium drawtube...).

- the whole OTA must be top notch and retain collimation perfectly at different altitudes, otherwise, a minor shift in orientation in the primary mirror will cause this not to be in the exact centre of the secondary mirror, which again can cause a loss in aperture. 

- the edge of the secondary mirror is also used for reflecting the light from the primary. The edge is generally the worst part of the secondary mirror. It is generally "trimmed" in interferometer tests for this reason. It is also the part of the mirror touching the holder if this is a "container" holder like astrosystems.. This means that it can also be more sensitive to thermal differentials.

- the focal length can be described as the sum of (a) Primary-to-Secondary and (b) Secondary-to-FocalPlane. An undersized secondary constraints the measures for (a) and (b), significantly

- an undersized secondary constrains errors in UTA diameter, the length the focuser drawtube can be ranked out / in (which affects eyepieces/PC2..)

- an undersized secondary reduces the FIF (fully illuminated field) diameter for large / medium field stop eyepieces (basically those used in low / medium power deep sky observing..). This is another parameter people often obsess about, but it is actually far less critical than all the constraints above.

 

The UTA is all about compromises. Because of this, having some degree of freedom is much better than constraining yourself and being forced to accept side effects.

Not all optimisations are golden. To me, undersizing the secondary mirror is an example of these.

 

Just my two cents.

Edited May 24, 2021 by Piero