Piero

38 minutes ago, jetstream said:

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.

It's kind of shocking how much that school of thought is widespread..  I won't mention the location, but some years ago I had a chance to observe through a - what I believe was - 20" F4 Newtonian on an equatorial mount in an observatory. On the primary mirror the was a large (easily 4" diameter) bird squirt - possibly dropped by a seagull. Aside from that, the whole mirror was covered by plenty of dust and dirt. By the look of it, who knows when was the last time it was washed, if ever. A few guests pointed that out, and guess what, the person responsible for the observatory simply replied that, that dirt did not affect the views. We observed Saturn that time and the view was just a bit better than my prehistoric Celestron 114mm F8 Newtonian telescope (the one mounted on that kind of hyper shaky German "EQ 0" mount). 

11 hours ago, jetstream said:

@John

...

Smooth glass means less scatter, again my opinion.

...

Agreed. 

And light scattering is pretty much noise in the optical train. Effect? Star light spreads on a larger area, and faint stars can even become invisible as squashed with the background. 

In addition to that "structural" noise, there is the actual noise due to dust, each particle acting like a small scratch..... 

I also used a large magnet at the bottom of the tube, opposite to the focuser. No balance issues even when I the ES 30mm 82 deg was in the focuser.

10" is a nice aperture. Televue and Pentax XW?

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. 

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).

Have a look at the section "Tools for observation" on this website:

http://takitoshimi.starfree.jp/index.htm

Toshimi Taki's free star atlases are really good in my opinion.

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.

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.

6 hours ago, Stardaze said:

The point regarding the dob is that with cooling, collimation and the bulk to move, it is a longer drawn out process than a small grab and go frac. 

My mirror is 1.5" thick. I start the fan at the highest speed when the telescope is still inside. So, I "wheel" it out, then go back inside to get better clothes, then go back outside with eyepiece case, collimate the telescope, and start observing. At this point I reduce the speed of the fan by half - the views are already acceptable (that's about 1/2 h fan on). Within another half an hour I might switch it off or just leave it at low speed, depending on how the night temperature goes.

P.S.

grip material on wheelbarrow handles is now yellow (black looks cool, but invisible at night time). Wood blocks for making that step ramp were a very temporary solution. I now use the following:

Next step is to make a good adjustable chair to be used with both the refractor and the dobson.

My 16" F4, but also my 12" F6, regularly show far more details than my 4" Tak DF.

The refractor "wins" on wide field (3" Deg Vs 1 Deg fov) and solar observing (only possible with the refractor). 

Side note..

comparing top notch refractors like Tak, TEC, TV against low-end commercial dobsons is unfair. The optical quality is a major component, but there are other critical aspects in a newtonian telescope which must be considered to make it really fly.. at least the following: collimation (process+telescope mechanics for retaining it), mirror cell supports (back+side), mirror cooling, focuser quality, baffling, thermal currents, and (drum sound..) boundary layer above the primary. 

Whilst working on most of the list above reduces imperfections to a level in which these can be invisible to the eye, the last one can play like a "turbo" for magnification. After testing this, I really agree with the statement that a lot of the reported atmospheric seeing issues are actually 1-2" above the primary mirror surface.

As an example, the mirror box of my 16" is 11" deep and open at the back (Kriege design). Observing the moon, ~300x seemed the highest I could go before the image started softening. After pulling up the light shroud for 5-6" (this is enough to move the boundary layer away, as the mirror box is shallow) from the mirror box and defocusing the image intentionally, I could see substantial air turbulence going towards one direction (the warm air was blown away and moved up). Note that, this air flow only affects the air above the mirror surface, not the mirror side wall! An air flow directed at the mirror wall can impair the temperature across the mirror causing astigmatism and spherical aberration (tested on the other dob). After refocusing back, the image was getting better and better as the air flow stabilised.. the limit of 300x was pushed forward to about 650x (crisp). The images was still acceptable at around 800x (yeah.. viewing craters and details like from an Apollo spacecraft). What about stars? Thin diffraction cross on the bright stars, otherwise stars were pin points. Globulars? M13 or M3 at 300x were bright, with pin point stars all over and kind of star dust (not a diffuse cloud) near the core.

19 minutes ago, Stu said:

Indeed! I really dislike viewing the Moon in my 24mm Panoptic because of this issue. It is great for most other targets though.

Regardung doubles, I think most of us observe these on axis as much as possible, though obviously in a dob where you are nudging you need to be able to use as much of the field as possible.

I found this very useful link on the web for describing and explaining different optical Abberations, I’m sure you know it but useful for others perhaps (and me!)

https://www.handprint.com/ASTRO/ae4.html#distortion

Yeah, the 24 Pan is another great eyepieces (currently my only TeleVue!), not ideal for observing the Moon, but lovely for other things (like star fields).

You made a good point regarding on/off axis. Both AMD and RD are off-axis. Not everyone observes planets or double stars off-axis.. In a dob at high power, one could end up observing the target at the edge to avoid constant nudging. 

I am aware of that link, but I am not sure whether it is fully correct regarding AMD / RD. In fact, the text calls RD as positive distortion, whereas AMD as negative distortion, letting one feel that they are kind of opposite. As far as I know, both can be positive or negative. For example, regarding AMD, if positive the image is "magnified" on-axis, and "minimised" off-axis, whereas negative AMD is the other way around. Not sure how common negative AMD is. Regarding RD, it is easier to think as if the image is projected on a sphere. If positive, you are looking at this image from the inside of the sphere, whereas if negative, you are looking from the outside. Not sure how common negative RD is.

What we see near the edge with common eyepieces having these distortions: 

- AMD: the moon pretty much remains round, distance between stars shrinks

- RD: the moon becomes an egg, distance between stars pretty much remains unchanged

Anyway, this is a bit off topic. Personally, I would not bother too much about the presence of some AMD in this eyepiece. To me, there are more important factors (e.g. how much back focus needs, how much the view remains sharp at the high-end of the magnification range, comfort, colour tone, etc). 

Very pleased to hear that this new zoom will be available in the astro market. If it has the optical quality of a Pentax XW / Delos, ~ £400 does not sound much to me. The Zeiss zoom I have is out of production, the Leica zoom costs nearly two times. Zooms are great not just for slimming down, but for the added practicality of getting your favourite magnification for a certain target without having to swap eyepieces.

11 hours ago, DesertSky said:

7.7mm-15.4mm  is a very narrow range.

Although there are zoom eyepieces with a much larger range, I would not say that a range of 15.4-7.7 is narrow. All depends on the telescope focal length.

In my dobson, f.l. = 1869.9mm with PC2, this zoom has a range of magnifications from 121x to 242x . When used with a VIP Barlow at ~2x, it can deliver something like: 259x - 519x . Using an spacer of 40mm, it would go from ~317x to ~614x . Yeah, I would possibly like something more for the "exquisite" nights, but I can always stick a Vixen HR if needed.

Anyway, my point is that zooms really kick when a very good Barlow can be added, in my opinion.

One could just use a low power eyepiece, a zoom, and a barlow, as "eyepiece" set. Minimalistic yes, but not necessarily less powerful.  

6 hours ago, Virtus said:

I was going to get on the pre-order list but read a little more about it and it appears to be designed for daytime use with a lot of angular magnification distortion (AMD) present in the early prototypes. 

Not going to totally write it off but will definitely be waiting to read reviews once they're out before I will consider it. 

Some users dislike AMD. There is this long living (and artificial) statement that astronomical eyepieces should not have AMD. Being optimised for nearly lack of AMD, means that they have a lot of RD. TeleVue is a brand which goes in this direction. One of the main reasons for this is the separation of double stars near the edge. If the eyepiece has positive AMD, the separation between a pair of stars decreases as this shifts towards the edge. RD is not free of defects either, though. If you look at the moon with a 24mm Panoptic, you will see an oval as our satellite moves towards the edge.

As far as I know it is not possible to correct both AMD and RD at the same time. Choices really. Personally, I like a trade-off between RD and AMD. 

The Docter UWA 12.5mm (nowadays called Noblex) does show AMD as well as some RD. Those who don't like AMD, tend not to like this eyepiece (not a surprise). To me, this eyepiece is one of the finest eyepieces in the market. I would call it a gem.

I had a s/h Lunt 50mm.. it remained with me just one month.. To my taste, it does not allow enough mag, so I sold it. I  have never looked thought a Lunt 60.

An exit pupil of 2.0mm is a good all-around, a trade-off between brightness and magnification. In my opinion, it is good to consider it as a reference for a medium power when building up an eyepiece set, not as a rigid constraint.

2 hours ago, Pixies said:

Is that a Berlebach clothes dryer? Must get one of those!

That's correct. UNI19, with AOK AYO 2 head. It dries clothes incredibly well. Time to time it is also used for holding my 4" Tak.

7 minutes ago, Louis D said:

Looks less like a living room and more like a mud room/entry foyer.  Our living room has oodles of stuffed furniture, television, gas logs in the fireplace, etc.  It doubles as a home theater.  Different floor plan entirely.

Possibly yes. We are not snobbish, but just simple people attracted to simple things, like reading a nice book, rather than having a cinema at home and being fed by Hollywood products.

17 minutes ago, Louis D said:

My wife would kill me if I left my 15" Dob assembled in the living room.  Perhaps if I get a second home in retirement I can find a spot for it there to leave assembled.

Like this? 😁

Honestly, my girlfriend is actually happy about having it in the living room as it looks nice.

She only asked me not to store both this 16" F4 and the other 12" F6 next to each other! 

Almost 6ft wheelbarrow handles for me.

The telescope is left assembled in the living room and wheeled outside through ramps. Little effort and quick.

Kind of funny that I find easier to take a 16" out than a 4" tak + its tripod!

5 hours ago, CraigT82 said:

What a fantastic scope, we'll done! What mod did you do to the astrosystems secondary holder (I've just ordered one). Also what made you decide on a glatter sling over a whiffle tree arrangement? 

Thanks Craig! 🙂

I replaced the original wood plate where the 4 long collimation bolts connect to, with a custom one in aluminium. Probably, not necessary, but I wanted as little as possible secondary shift in collimation. This aluminium plate was drilled and tapped at the centre and each edge. The central thread allowed me to remove the nut fixing the stud at the bottom of the spider hub. As a consequence, the plate "binds" to the 4 vanes.

Regarding the Glatter sling VS whiffletree, i chose the former due to its simplicity. This is the first telescope I made, and also my first wood/metal project ever. To be honest, I never drilled a hole before this work, so there was a bit of learning curve for me. I think both solutions work without issues as long as implemented and installed correctly.

Feel free to PM me if you want to see more photos about the construction of my dob.

3 minutes ago, GavStar said:

I do like a 16 inch f4 dob. Here is mine, a Skyvision t400. Any more details you can share on yours, eg mirror maker, who built yours, etc?

SkyVision makes some very fine dobsons.

The 16" F4 primary mirror was made by John Lightholder (very nice guy) and coated by spectrum coatings in Florida. The 3.5" secondary mirror is from Antares (the shop, not the star). Focuser is a Feathertouch. Telescope designed and made by me, so that I didn't have to compromise with anyone. Secondary spider and holder are from astrosystems, although part of the holder was modified. The mirror cell is based on a slightly modified Kriege design and includes a (real) Glatter sling. Truss attachments were design based on Highe and John Pratte's work. Mirror box has tongue and groove joints like in Teeter, but more shallow. Trunnions are based on Fiske's 22" design, although mine have a little bit more maths. Rocker box has dowel joints, design taken here and there. Ground board was custom designed by me. Light shroud is by Heather Teeter. 

Not sure why 2 identical posts.. mobiles..! Mods, feel free to delete the latter one.

2 minutes ago, Piero said:

They are both middle size, but with different specs. In terms of primary mirrors, the other one is a 12" F6, whereas this one is 16" F4. Optics, design, and mechanics are generally different though. The 16" is fully custom, based on my requirements.

 

Here's an old photo of the 12":

2 minutes ago, JeremyS said:

Looks great Piero. Is this different from the large dob you got a while ago?

They are both middle size, but with different specs. In terms of primary mirrors, the other one is a 12" F6, whereas this one is 16" F4. Optics, design, and mechanics are generally different though.

Here's an old photo of the 12":

17 minutes ago, John said:

Very fine looking scope Piero

Thanks John 🙂

I started testing it only recently, but it has so far impressed me quite a lot.

My new 16" dobson cooling down 🙂