Piero

40 minutes ago, PeterStudz said:

Where did you buy yours from? I think that like me you are in the UK. I looked for embossed FRP but the places I found didn’t offer small-ish sections like and/or charged a lot for postage. Eg here…

https://www.whitecladding.co.uk/product/white-embossed-frp-fibreglass-wall-protection-panel/

The smallest size is £55.99. But add VAT and “economy” postage of £30 the total comes to £103.

I bought it from http://www.beplas.com/ and it was a sheet larger than 500x500mm. The extra material has been used in other projects. Considering the cost of building a dobson + mirror, this felt rather cheap. It is also something I don't want to compromise.

Agree with Gerry (jetstream) above.

"Bright DSOs" is used for "the brightest DSOs given aperture and sky darkness".

The vast majority of targets in this hobby require a certain amount of "study" and "re-visits" to be appreciated. It takes dedication and patience, a bit like fishing.. 🙂

"Bright targets" is also used for objects that can be spotted relatively easily, but they cannot really be observed. For instance, I spotted a few planetary nebulae with my TV60 under moderately polluted bright skies with or without filters. However, excluding a very limited number of bright and large PNs (e.g. M27, M57) this remains a tick exercise (and a few members have this kind of approach with this hobby) using small apertures. I really started appreciating them using at least moderate powers (at least 250-300x and much more) with my 16". That is when some structure starts showing up and therefore the target can be studied.

OP:

Between those two telescopes you have listed, I would choose the refractor.

However, in my opinion, a better all-around telescope would be the Bresser 8" (or 10" if money is not an issue) Dobson. This will also need the purchase of a collimator, but FLO sell a few inexpensive ones.

For my 16" dob, I used embossed FRP and teflon for both altitude and azimuth bearings.

Every 6 months or so I lubricate the FRP with a wet bar of soap to make the telescope as smooth as desired.

On 28/05/2023 at 13:49, Mutley said:

The unpaited plywood pier you see is fixed to the back of the moving plane of the mirror cell, just so I had somewhere to mount the brackets for the sling support. The sling itself I bought pre-fabricated from here : https://www.gsproducts.co.uk/

Thanks to all of you for all the helpful advice and information. Now off to do some painting and think about how to implement a cooling soultion.

Thanks for sharing the photos of your telescope and mirror cell.

My thought:

• Looks like you need a spanner to collimate the primary mirror. In more recent versions, David uses metal wing nuts which work on that kind of cell. Of course this is not required but it would allow you not to use an additional tool.
• The first thing I would do to improve the cooling is to drill, using a hole saw, where you have that marking point at the centre of the triangle plane of the mirror cell. To make a clean cut at the back, I would clamp some wood to the other side so that the wood does not split. Following that, I would install a fan (80mm is fine if it runs at good speed - you could also opt for 100mm, depending on how much space you have, considering the collimation bolts). Having said that, this requires some thought on where to install this fan as it does not seem that you have much space between the triangle panel and the bottom panel of the mirror box. One way could be to install it to the back of the triangle panel, but this depends on whether the gap between the two panels is sufficient. A possibly neater way is to make a groove to the bottom panel of the mirror box (outside side) and install it there. It looks like that panel is quite tick. Doing so, the gap size between this panel and the triangle panel does not really matter. I have a feeling that your fan positioned at the front of the mirror box improves the views, by cooling down the mirror. As it is positioned to front of the mirror, it will blow away the boundary layer, but my feeling is that this is a secondary effect really. How long does it last? I haven't done the maths, but I would imagine a mirror of that thickness takes about 1h to cool down to ambient temperature. I keep the fan of my 16" on all night as the temperature decreases. However, I reduce the speed once the mirror is at ambient temperature, so that the views are not affected by the fan being on, whilst the mirror temperature continues dropping following the decrease in ambient temperature.
• If possible, you should consider painting the additional wood panel supporting the sling. The sling angle brackets should also be sanded coarsely, primed and painted matt black to avoid reflections coming back towards the secondary mirror. This will improve contrast a bit. I would do this later once everything is fixed though.
• was that black ring structure at the edge of the mirror installed to fix a turned down edge issue?
• Is your telescope f4 or even faster?  In the third photo, the centre of the sling cable seems quite low. Now, I would imagine it to be visibly lower than half mirror thickness as your primary mirror is fast, but from the photo it seems lower than what I would have imagined. Just to double check, is the centre of the cable really at the COG of the mirror? Unfortunately, that installation of the cable follows David's design and the biggest downside is that the height of the cable is fixed. Therefore, to regulate it to the COG of the mirror, the only way left (maintaining the existing installation) is to trim/raise the pads. This would allow you to decrease the mirror height and therefore raise the position of the cable - assuming that the cable is positioned a bit lower than the COG. If the cable is positioned below the COG of the primary mirror, the effective mirror figure will not be parabolic as it should and this will cause spherical aberration. Depending on whether it is position higher or lower than the COG, you will have an overcorrection or undercorrection effect. You might also see astigmatism if the cable is positioned at slightly different heights around the mirror from the back of the mirror.

6 hours ago, Littleguy80 said:

Thanks Piero. I felt sure I’d read somewhere that the Taks do better with prism diagonals. That was one of the reasons I tried the prism. I wonder if getting the BBHS mirror version would remove the CA on Venus. I’m not sure why Venus seemed more prone to CA than even the moon. 

I wouldn't base all my CA tests on Venus. It is a challenging target: it is low (so the atmosphere plays a role in the views), incredibly bright, incredibly white, low contrast and smallish.

The best views of Venus I have had, are still with my TV60 + single polarising filter..

The Tak fc-100 is a doublet refractor. It reduces CA at super low levels, but it cannot fully correct it. I also have some doubts about fixing this using a prism as prisms slightly overcorrect, so whilst "potentially" fixing the blue, the introduced overcorrection would affect the other two colours.

I cannot quantify how much CA you have detected in your tak based on a post here, but I would observe a range of targets to asses this properly.

Said that, my tak FC 100 DF (now FT!) shows a touch of CA on very bright targets, but this is well controlled. I do not have a problem admitting this - perfection does not exist, but it is very often reported as people desire, even ending up believing, that their tool is perfect and can do better than the rest. Similar story about members claiming that their tak 100mm shows views which can be observed with at least 10-12" telescopes.

At some point I had both 2" bbhs prism and mirror diagonals.

I sold the prism as it showed CA, whist the mirror did not. The mirror version also cooled down a bit faster. In terms of details, again I prefer the mirror as the views seem more natural.

Lots of members prefer the prism though.. 

Some observations:

- nylon bolt vs felt pad. Felt pads can stick to the back of the mirror as well as get squashed. I used heavy duty felt pads on my 16" dob and they don't stick. Also the back of my 16" Lightholder mirror is smooth, so any friction is minimal. I used nylon bolts as back supports for my 12" Nichol mirror (which is not back smooth). In the original cell, I replaced the nylon bolts with pads and did not find any difference. Probably, where I live, it is not particularly humid. Dunno.

- the reason for the "cut" at the bottom / front of the mirror box is avoid that the box hits the central bolt connecting the rocker box to the ground board. Therefore that cut enables to reduce the height of the RB side panels, and therefore the telescope, by a couple of inches. This is well documented in telescope making books. This "cut" is not particularly high. Unfortunately, this other maker ignored the whole principle and used that opening for sliding out the mirror with its cell. The idea is crazy and a bad one too. It is crazy because it is much easier to access the mirror and its cell by rotation (2 locks, 2 pivots - see Astrosystems mirror cell). It is a bad idea because it exposes part of the mirror to colder air than the the internal side of the mirror. A minor breeze can cause astigmatism. Before redesigning my 12" (new MC, etc) I installed a panel of plywood closing that bottom / front cut on the MB. The astrosystems mirror cell does not suffer from this issue because the bottom front opening is still shallow (so air does not hit the mirror side) and it has two holes at the back. This enables a steady flow of air.

- side holes / fans are not added for cooling the mirror down, but to prevent the formation of warmer air above the mirror surface. To cool the mirror down faster, the most effective way is to install a fan to the back of the primary mirror. I would imagine that your mirror is F4 or similar, so the mirror box is not particularly deep. Therefore, lifting up the bottom of the light shroud might be sufficient to prevent the formation of air boundary layer (BTW my original light shroud was made by him and broke down within 6 months). I would recommend to get a copy of the book "the dobsonian telescope" by Kriege. There have been many advances to those ideas, but the design is simple and effective. It contains lots of way to redesign components. 

16 hours ago, Mutley said:

Yes, pointed the dob up at about 45 degrees and hooked the fan over the front of the mirror box so the airflow was over the mirror face. Left it there running for about 2 hours.  It seemed to do some good - much better star test. Practically speaking, easily split Izar, which was just a mess before I came across your original thread, and copied some of your improvements (part of the reason I started down this route was discovering the dob was being outclassed by my 4" refractor on this particular double).

I could but you probably wouldn't see much. The 2008 Lukehurst dob was quite a bit different from your model. There were no fans anywhere. The mirror cell is surrounded by solid ply, apart from the slot at the front of the mirror box for getting the mirror in and out.  Also, there was no wire sling. Lateral support was from 2 posts at 45 degrees either side of the centre line. Cork lined. This was for a 20inch mirror, 31mm edge thickness. It didn't work great.

I'll probably take it to bits again after I've had a look at the 1st quarter moon this coming weekend. I'll take some photos of the mirror cell then so you can view my handywork. No laughing.

So, if I understand this correctly, the bottom of your mirror box is fully closed with a plywood panel which forms the support of the mirror cell. How do you take the primary mirror out (e.g. for cleaning)?

Note that in the original MC/MB of my 12":

- the fan at the back of the mirror was ineffective as too distant from the back of the mirror and only blew on one part of it.

- the 3 additional ventilation holes were ineffective (I closed them for testing and there was no difference)

- the bottom/front opening, using his words "for taking out the MC" caused a lot of thermal issues and the side of the mirror was exposed.

- his sling is implemented incorrectly

- same for the 3 side mirror stops

- the triangles were nearly paralised and asymmetrical between each other.

Your mirror is particularly thin, meaning that it requires excellent support (back and size) to work well. What I don't understand is whether you have replaced his MC with your own, as I did, or, if not, what changes to the MC you have applied.

Did you direct the fan to the front of the mirror (e.g. from the top of the mirror box)? 

Would it be possible for you to take some photos of:

- mirror box

- mirror from the top of the mirror box

- mirror from the back of the mirror box (ideally showing the mirror cell)

- some photos of the mirror cell as fitted in the mirror box (no need to take it out)

Fans do help in my opinion, particularly the one fitted at the back of the mirror. The side fans are not for cooling the mirror down, but for avoiding the formation of 1-2" of slightly warmer air released by the mirror as this cools down. Wiping this out improves the views.  

21 hours ago, Mutley said:

Thanks for the suggestions, I'll have a read of the S&T article Merlin.  I have a David Lukehurst dob, circa 2008. Piero I read your thread about the extensive modifications you made to your Lukehurst. So much of it was familiar to me ( In a bad way). I've since made some similar modifications to the mirror cell, and converted to a sling support. My 2008 model used 2 posts for lateral support. The star tests were awful in the unmodified scope, and to be honest I had no idea why. I even contacted the mirror maker to confirm it was genuinely one of theirs (Oldham optical -it was genuine). Better now with the sling/ furniture pads/removal of those bolts for restraint of floating triangles (see Piero's thread). But still much room for improvement and I'm wondering if remaining problems are thermal in origin. Like the passive cooling idea for boundary layer. Certainly worth a try before fitting fans.

Piero if we ever meet I think I owe you a pint.

I see now..

Sorry to hear that you have been experiencing similar issues to mine. I really wonder how many of his telescopes share the same problems - my guess is many unfortunately.

Like you, I also contacted the maker of my mirror (John Nichol) to confirm that it was really made by him. It was before doing the large telescope redesign as I wanted to be sure that the mirrors were authentic. It is a shame that a customer has to go through so many issues. It isn't just about testing and some improvements. It is about real fixes needed due to poor and incorrect design choices. I feel a lot of people don't understand this.. these are the problems that can ruin a hobby because the whole time is invested dealing with tricky issues (a few of them unresolvable unless the component is redesigned), instead of enjoying observations.

I am not a telescope maker nor a wood/metal worker, but I spend quite a lot of time reading and understanding issues and solutions. The work I did on my two dobsons is nothing of innovative, but contains some of the best designs from talented engineers that I could implement with my basic skills and tools.

For unknown reasons (I raised the suggestion but no reply), SGL does not allow one to have more than one link in the signature. Here is the link to the album containing the redesign of my 12" Lukehurst dob (now called Phoenix as the bird reborn from its ashes..). I don't even called Lukehurst dob because a) it is significantly different, b) it works now, whilst the original telescope did not, and c) I don't want to remember the nightmare I experienced with it due to that maker. 

https://photos.app.goo.gl/qPdsWRQUiqxmm9ec9 

Whether in this thread or via PM, let me know if you would like me to share details about the changes. Happy to help.

From your original post, I didn't understand you had a Lukehurst telescope. I don't know in detail how far you have gone with improvements. Issues caused by boundary layer are very minor compared the rest. I would make sure that all the other issues are fixed before investing time and resources on improving the boundary layer.

In my 8" skywatcher dob the lock bolts were removed when the new springs were installed.

Originally, my 12" truss dob had heavy duty springs and they held collimation well. That mirror cell was replaced by a new one due to other issues and the latter does not have springs at all. Collimation is done via rotation of bolts which push up / pull down the mirror.

Old mirror cell with springs:

New mirror cell during telescope making:

Same cell design (but more supporting points) for my 16":

8 hours ago, Ratlet said:

What pickles my brain is that they use cheap spring then they add lock screws to help it keep collimation.  From my understanding, with decent springs you don't need to lock screws.  Surely the springs cost less than an extra 3 bolts?  Or am I wrong on that?

Correct. With good springs, the lock bolts can be removed.

I agree with the posts above that commercial telescopes should be fitted with decent springs from the factory. 

I've never had dew issues with the primary mirror, likely due to the fact that this is never really below ambient temperature because of size and thickness. I have experienced dew issues with the secondary mirror of my 12" dob as this is fully exposed, but this does not happen frequently. The cell of the secondary mirror of my 16" is an astrosystems and seems to shield the mirror better. With that telescope I also have an astrosystems dew heater installed behind the mirror, so the dew is never a problem with that telescope anyway. 

To me, it really depends on the required magnification. 

I am fine with 70-80 deg AFOV with my refractor (4" f/7.4) at high power (~200-250x), but when I use my 16" f4 + PC2 at > 300x, 70 deg AFOV starts being a pain to me and things get just worse when the target is invisible in the finder. That's one of the reasons why I decided to move from 8-6-4.5mm Delos to 9-7-4.77mm APM XWA. For completeness, the other reasons are: a) I prefer 9, 7mm f.l. to 8, 6mm with that telescope, b) the APM XWA feel a touch lighter but offer more view, c) the AFOV of Docter and N22T4 feels closer to APM XWA than Deloi.

Agreed! 

It is very comfortable, whether one uses a normal chair or observes standing up.

I assume you have a 20" truss dobson and that the reason you want to do this is for gaining steady views at higher powers.

I made 3 holes at the front and 3 at the back of the mirror box of my 12" f6 dobson, added fan grids, but not the fans. The height of the holes are right above the mirror edge so that the outside air coming from the ground flushes off the internal air above the mirror surface, without directly hitting the mirror edge, and eventually leaves from the back holes. The back of the mirror is fully exposed as this sits on a Kriege mirror cell. When testing, the views were very good in my opinion. Therefore, I decided not to install any fan. This mirror box is 15" deep. Also, the light shroud is generally lifted up at the bottom (approx 6"). 

I didn't make holes in my 16" f4 mirror box as this is quite shallow (11") and a bit wider. I found that pulling up the light shroud from the bottom for about 6" is sufficient for achieving the same effect.

In both cases the views become steadier and the magnification can be pushed an additional 100x, sometimes more.

6 minutes ago, happylimpet said:

Thanks for replies so far all - and yes I have been considering driving it out there - at least for delivery! We have friends in Lyons, just 3hrs further down.....

The cost of ferry+fuel isnt hugely bigger than courier, and way less hassle.....

And if stopped, you could just say that you took your telescope under darker skies!  

I imagine that you will be charged based on the cost of service he declares.

Yes, what we got is an expensive pseudo-freedom.

I'm more than happy with the telescopes I already have. Possibly they are even more than what I need!

+1 for a 10" dobson. I would choose the Bresser over the skywatcher or stellalyra as the design is much better. For instance the trunnions are more similar to those used in proper dobsons.

A 10" is a good size for planets and DSOs but it is still portable (e.g. it fits on the back seats of a normal car). Many people don't go beyond that aperture. 

Make sure you also get a good collimator as that is also needed.

For that kind of focal ratio (around f5), a minimal set of eyepieces with focal lengths of 25mm, 10mm and 5mm would work and cover the bases.

Piero

Here are my 3 cases (well there is also a case of Delos eps, but those are going on sale at some point this year time permitting).

The set for Nunki:

The set for Phoenix, Tak (DSO mainly) and TV-60 (24 Pan, N13T6, N9T6, N7T6 and Baader Q Barlow):

The set for the Tak (DSO and high power - 30mm apm uff, Zeiss zoom and Baader VIP barlow):

Make sure you get a collimator as well, regardless of the aperture you choose.

Between the two, I would choose the 10". I have an 8" in another country and a 12" here. There is a substantial difference between the two and I can imagine that something in between would offer a nice all-around telescope.

Having said that, I would choose the Bresser Messier dobson over the stellalyra. The mechanics look much better. Just look at the altitude bearings to see what I mean...

https://www.firstlightoptics.com/bresser-telescopes/bresser-messier-10-dobsonian-telescope.html

My journey: 30mm 82, 35mm Pan, 40mm maxvision (decloaked), 42mm vixen, apm 30mm uff. The pan was kept whilst comparing these eyepieces and sold after the arrival of the apm.

The stellalyra is a clone and sold to a bargain price. I haven't tried it, but hopefully it is a "true" clone, not like the Altair one with the non-black internal ring. That's a bit the issue with clones. You never know where the assembler has saved up to make it more competitive. At least aesthetically, the stellalyra looks identical to the apm one, which is a good sign.