How important is mirror quality for low magnification views

[GavStar]

I’m considering my first dob purchase. However my requirements are relatively specific. I’m likely to only be doing low magnification observation - around 20 to 40x. My main use would be Night Vision (monoculars), so getting as fast a scope is generally key. However, for the scope I’m considering (Sumerian) it can either be supplied with a GSO f4 or a ts optics quartz f5. 

I think the ts optics is the better mirror (its more expensive anyway). But at low mag views would I notice any real difference?

[iPeace]

Good question. I've yet to notice the difference between a 6" f/6 from GSO and a 6" f/8 from OO UK, certainly not at low mag. The report that comes with the OO mirror supposedly says it's pretty good, while the GSO mirror just works.

P.S. just some further thoughts -

• Given the options, I think I'd be considering whether I'd want a f/4 (shorter assembly, more difficult to collimate, more coma) or an f/5 (longer assembly, less difficult to collimate, less coma).
• In my own quest for the (for me) 'perfect' newt, it has been suggested to me that (smaller) f/4 mirrors are mostly meant for astrophotography, for whatever that's worth.
• I've read that it's 'easier' to get a slower mirror manufactured to a high standard than a faster one.
• If I understand correctly, Sumerian dobs are designed to break down for travel, and assuming an f/4 breaks down smaller than an f/5, does the f/5 break down small enough for your intended use?

[NGC 1502]

7 hours ago, GavStar said:

I’m likely to only be doing low magnification observation - around 20 to 40x. But at low mag views would I notice any real difference?

 

At those low magnifications, mirror quality is not very important, as long as you are certain that’s all you need.

But there’s another thing best to consider. At 20 - 40x with a fast mirror, your exit pupil will be very large indeed and for visual observation that will give a washed out bright sky background. However you mention  ‘night vision monocular’ and I don’t know how well that works with a large exit pupil.

Ed.

[GavStar]

28 minutes ago, NGC 1502 said:

 

At those low magnifications, mirror quality is not very important, as long as you are certain that’s all you need.

But there’s another thing best to consider. At 20 - 40x with a fast mirror, your exit pupil will be very large indeed and for visual observation that will give a washed out bright sky background. However you mention  ‘night vision monocular’ and I don’t know how well that works with a large exit pupil.

Ed.

Thanks Ed. With NV there is no issue with exit pupil. In fact the aim is it get the exit pupil as large as possible to get as much light into the NV monocular as possible (up to a 20mm exit pupil)

[NGC 1502]

2 hours ago, GavStar said:

Thanks Ed. With NV there is no issue with exit pupil. In fact the aim is it get the exit pupil as large as possible to get as much light into the NV monocular as possible (up to a 20mm exit pupil)

 

Wow, 20mm exit pupil, now where’s my friendly neighbourhood owl ? but thanks for NV info to make use of that.....

Ed.

[alanjgreen]

10 hours ago, GavStar said:

Thanks Ed. With NV there is no issue with exit pupil. In fact the aim is it get the exit pupil as large as possible to get as much light into the NV monocular as possible (up to a 20mm exit pupil)

Go for as fast as you can afford. As you say, you want a large exit pupil so you want it as fast as possible to increase the exit pupil size.

Add in the cost of a Paracorr2, which would also give you somewhere to mount your filters.

The faster scopes will be more compact (shorter) as said above so transportation will be less of an issue too (avoiding the need for "steps").

What size mirror are you thinking?

- I see there is a 14” Sumerian with John Nicol mirror on astrobuysell, does not say what the f ratio is

http://www.astrobuysell.com/uk/propview.php?view=137530

Plan on using your 18.2 Delite for bright globs and clusters, so calc your max magnification as seen with that EP.

But the 55mm will be the "most used" for sure, (although the Panoptic 35mm is a much better eyepiece in my big dob), you need all the speed to get galaxy arms to show.

Consider encoders and Nexus if you want to add push-to or are you thinking goto and tracking?

Alan

[PeterW]

With mirror testing the quality of a mirror can be (roughly) related to the magnification itncan take before the defects hurt the view... good mirrors can take 50x per inch... you’re well below 10x per inch... I got a quality not well known f3 as my interest is low power.

cheers

 

peter

[Arcturus D]

It is like with all hobbies, there is a sweet spot and then every marginal increase in mirror quality costs a lot more. Today's Dobsonians are very good value for money. An 8" or 10" Skywatcher dobs can keep you busy for years, until you have figured out exactly what your object viewing preferences are, which might mean moving on to a more specialised scope or staying with the dobs, as you are hooked on DSOs.

[Merlin]

Good mirror or mediocre mirror is the wrong way to look at it. For astronomy, we should get the best possible mirror we can afford. Besides, if someone buys a secondhand reflector not knowing that the mirror is mediocre, they are going to be disappointed when they up the magnification. Let's keep the mediocre off the 'scopes market.

 

[CraigT82]

On 13/05/2018 at 22:23, GavStar said:

supplied with a GSO f4 or a ts optics quartz f5

GSO mirrors are no slouches, as with Synta they may be mass produced in China or Taiwan but they are not bad at all. 

Problems you hear about with Chinese mirrors is more to do with QC that manufacturing quality... they tend to let more of the duds through than the specialist high quality Japan/UK/USA manufacturers.

Who makes the TS quartz mirrors out of interest?

[GavStar]

23 minutes ago, Merlin said:

Good mirror or mediocre mirror is the wrong way to look at it. For astronomy, we should get the best possible mirror we can afford. Besides, if someone buys a secondhand reflector not knowing that the mirror is mediocre, they are going to be disappointed when they up the magnification. Let's keep the mediocre off the 'scopes market.

 

The way I’m looking at it is to get the best possible set up for my specific requirements which are different to the typical dob user.

I want to get as fast optics as possible, I want lightweight and I’m not going to use high magnification (I have other scopes for this).

If it meets these requirements then it won’t end up on the secondhand market ?

[CraigT82]

There is a 14" F4.5 Sumerian with John Nichol mirrors for sale on UKABS now, it's in Reading and on at £2700. (and nowt to do with me - just a heads up!)

[jetstream]

I own 2, f3.8 mirrors, both with VG figure. One of them has bad scatter and blows light all over the place, reducing contrast considerably on DSO. Our Heritage 130 f5 is an example of an excellent mirror from a mass producer and if GSO can make their mirrors to its quality the views will be excellent.

You can get Wolfi at TS to test a mirror- he is good to work with and I believe he uses Wolfgang Rohr to test .

[25585]

22 minutes ago, jetstream said:

I own 2, f3.8 mirrors, both with VG figure. One of them has bad scatter and blows light all over the place, reducing contrast considerably on DSO. Our Heritage 130 f5 is an example of an excellent mirror from a mass producer and if GSO can make their mirrors to its quality the views will be excellent.

You can get Wolfi at TS to test a mirror- he is good to work with and I believe he uses Wolfgang Rohr to test .

Heritage is an excellent scope off the shelf, only let down by its pathetic toy focuser. 

[jetstream]

11 minutes ago, 25585 said:

only let down by its pathetic toy focuser. 

 

Well..... this focuser always works, including in my -30c winters, which is more than some offer. This focuser is much better than a cheap crayford focuser IMHO.

[25585]

12 minutes ago, jetstream said:

 

Well..... this focuser always works, including in my -30c winters, which is more than some offer. This focuser is much better than a cheap crayford focuser IMHO.

The other Heritage models do have r&p focusers, which at least can be replaced with upgrades. A 130 Flextube however has no base to fix a plate to, unlike larger Flextubes or solid tube OTAs. 

[John]

1 hour ago, 25585 said:

Heritage is an excellent scope off the shelf, only let down by its pathetic toy focuser. 

Have you actually used a Heritage 130 ?

I have and the focuser works well. I'd be careful about throwing around terms like "pathetic". It tends to annoy people.

Neil English knows a bit about scopes (much more than I do) and enjoyed his Heritage 130 experience:

https://neilenglish.net/a-newtonian-travel-scope/

 

 

[Mark at Beaufort]

I used my Heritage 130P last night and was able to use high mag (186x) on the Moon, Jupiter and Venus and it was easy to get sharp focus.

Anyway back to the thread's question - I have a Revelation 12" Dob F/5 and I rate the mirror very well - I usually start with a mag around 63X and the image is very sharp across the FOV.

[25585]

1 hour ago, John said:

Have you actually used a Heritage 130 ?

I have and the focuser works well. I'd be careful about throwing around terms like "pathetic". It tends to annoy people.

Neil English knows a bit about scopes (much more than I do) and enjoyed his Heritage 130 experience:

https://neilenglish.net/a-newtonian-travel-scope/

 

 

Yes, my brother owns one I gave him

I also gave him a couple of good eye pieces for wide & mid magnifications, both Plossls. Lucky him. 

The focuser coped with their weight OK, but it's use at mid to higher magnifications was too hit-and-miss & tried our patience. Had the design been upgrade able that would have been done. 

[CraigT82]

16 minutes ago, 25585 said:

Had the design been upgrade able that would have been done. 

Just need a few tools and A bit of ingenuity... fitted a 2" Crayford to my H130

[John]

23 minutes ago, 25585 said:

The focuser coped with their weight OK, but it's use at mid to higher magnifications was too hit-and-miss & tried our patience. Had the design been upgrade able that would have been done. 

Other users seem to manage OK. Perhaps some more practice ?

[Stu]

21 minutes ago, John said:

Other users seem to manage OK. Perhaps some more practice ?

Yes, it does work surprisingly well even as standard. I did the PTFE tape mod and it is even better now  

[mdstuart]

I use a 16 inch with a basic standard mirror with a bit of astigmatism.

Works well with up to a 12mm eyepiece but push it up to a 7mm and you start to see the stars as small blobs rather than dots and some of that astigmatism..so if you want to go to x300 and see stars as sharp dots then the quality of the mirror does matter. Of course on fuzzy objects its not as critical :)

I am grateful for what I have but one day I do fancy getting my mirror re-ground to a better shape as I think this might allow me to go deeper and see detail such as close double galaxies better.

Mark

[Piero]

I guess a 8" Newton F4 should also do the job. Still light and compact to be set up on a mount. Imagers often get fast tools, so finding one in the s/h market should not be too difficult. 

If I am not wrong, NV observation works well on emission nebulae. How much a coma corrector is needed for these targets, particularly at very low power? 

One issue with cheap dobson greater than 12" is that the base can be quite bulky. On the other hand, getting a decent structure and place a mediocre mirror seems a shame, particularly if you sometimes want to observe w/o NV..  ?

[GavStar]

9 hours ago, Piero said:

I guess a 8" Newton F4 should also do the job. Still light and compact to be set up on a mount. Imagers often get fast tools, so finding one in the s/h market should not be too difficult. 

If I am not wrong, NV observation works well on emission nebulae. How much a coma corrector is needed for these targets, particularly at very low power? 

One issue with cheap dobson greater than 12" is that the base can be quite bulky. On the other hand, getting a decent structure and place a mediocre mirror seems a shame, particularly if you sometimes want to observe w/o NV..  ?

For NV having a fast f ratio is key. So to get bigger image scale on smaller objects such as globs, galaxies and small emission nebulae (or blow up say the eagle nebula to see the pillars of creation), I need to get large aperture. I’d want to get good views to the edge so a coma corrector would be necessary I think.

However my question has been answered. The Sumerian is a light, well built scope, and the gso mirror would almost certainly be v good for the relatively low mag views I would be using it for. For high mag use on planets, double stars etc, my 160mm frac would be my scope of choice.

The 14 inch Sumerian is a nice option!