Thoughts on 'best' 8" Newtonian's for imaging

[Robculm]

Hi All,

As you know from my profile, I'm currently using a standard Skywatcher 200PDS.

I love the 'light gathering' capability of reflectors & despite the immense frustration of collimation etc, my feeling is to stick with them, rather than switch to a refractor.

However, it does frustrate me that unlike refractors, where there are stacks of choice & a relatively straight forward link between price / performance, it's definitely not so clear cut with reflectors.

While I've seen some great images with low cost tubes like the 200PDS, there's no guarantee of optical quality compared for example with buying a Takahashi refractor!

Just wonder what people's thoughts / recommendations are for 'higher spec' reflectors.

As I see it, there's the PDS and various low end equivalents. The Skywatcher quattro & equivalents, but it's essentially the same in terms of optics. The Vixen R200SS which almost certainly has better optics, but has enormous diffraction spikes due to the heavy duty spider vanes & similar for the Takahashi Epsilon. Don't get me wrong, I'm a fan of diffraction spikes, but both of these are a little too much IMHO.

I've considered the Orion Optics AG8, almost certainly great optics, but very expensive & not sure of the quality aside the optics.

Aside that, I guess there's the option of a 'standard' tube, such as the PDS or quattro and upgrading the optics (again for example Orion Optics or I see some TS options).

Anyway, thoughts / suggestions welcome please.

Also to note, I think 8" is adequate in terms of aperture, certainly for my HEQ5Pro, longer term I'd like to upgrade that, iOptron perhaps, but aside the mount tube weight just from an ease of handling perspective is just about right, for me at least!

Cheers,

Rob

 

[ONIKKINEN]

I have the OOUK VX8 and do find the optics very good for when it matters. Speaking of that, it probably doesn't matter that much for long exposure since tracking and seeing will mask the little differences between a decent mirror and a great mirror (that is you could have a lemon in a Skywatcher and there are no guarantees its decent although rare to get complete shaving mirrors). For planetary/lunar/visual sure but i dont think there would be a big difference for DSO. Other than the optics the telescope is not really a stable imaging platform in its stock form. I have replaced the secondary spider with a sturdier one and the focuser with a Baader diamond steeltrack. Now i find myself looking for new rings and a losmandy plate for scopeside and a vixen rail for topside accessories. The vixen plate and rings this scope is meant to mate with are quite short so this is less than ideal with the bendy and very light aluminum tube so having a longer losmandy to the mount and vixen on top combo would strengthen the tube nicely. Overall the price will be quite high at that point but then i dont think i could complain about anything in the scope... The CT8 would definitely be cheaper, but that one still has the same focuser and the same spider so i would personally want to upgrade those.

The VX series is not great if you want to have a ready to go AP scope but its hard to beat the price, i got a 1/10 PV 0.991 strehl scope for 800e. Just the mirror could cost more than that in many cases so ill live with the modifications...

I was thinking of buying something like this as well: https://www.teleskop-express.de/shop/product_info.php/info/p5763_TS-Optics-8--f-4-5-ONTC-Carbon-Tube-Newtonian-telescope---fully-customizable.html

That one has a certified good mirror and sturdy mechanics ready to go for AP. Add a focuser to that (does not have one) and you are in the 2300-2500e pricerange so quite expensive but much cheaper than the AG8 for sure. I think i would go for that one if my VX8 somehow gets destroyed or stolen or something else.

[Second Time Around]

I believe that OOUK will fit a Baader Steeltrack to their scopes if you send it to them.

[Robculm]

Thanks for your comments Onikkinen & Steve.

Yes, that's a good point Onikkinen, for DSO's the mirror quality probably isn't of such importance compared with tube stability and mount / guiding.

The TS Optics options do look interesting & worth to keep in mind for the future.

I guess for now I will keep 'fettling' the 200PDS and maybe move a better mount further up my wish list!

Cheers,

Rob

 

[drjolo]

I have been imaging with 6 f/5 and 8 f/4 inches newtonians - one steel and one carbon tube. Both were quite heavily modded though.

In my opinion the mirror quality even in low budget newtonians are good enough to capture the quality images. The real pain is mechanics in entry level telescopes. The stock focusers are pretty useless for anything heavier than light DSLR camera. The drawtube tilts inside the focuser tube, but also the telescope steel tube (or even thin carbon tube) bends under the load of 1-2kg already. It is clearly visible when you put laser into the focuser, press the drawtube and watch the laser spot on the main mirror. 2 mm movement is enough to degrade f/5 telescope collimation. For f/4 the sweet spot is much less of course. 

Main mirror cell is also usually poor in entry level newtonians. It sometimes moves aside just under the mirror load at different telescope positions. Mirror holders are either too loose or too tight. Another examples are counter screws in GSO cells - all 6 screws are positioned symmetrically around the cell, and counter screw bends the mirror cell (and mirror as well) when tighten too much. 

Currently I just got back to imaging with newtonian. I collected second hand steel tube GSO 200/1000 in very good condition and now I am in the process of upgrading it - focuser replacement, external stiffening rings on focuser both sides, main mirror 195mm mask to get rid of three mirror holder shadows, changing the position of the counter screws in the main mirror cell, and probably flocking the tube interior. Then selection of coma corrector (probably TS 0.95x Maxvision) and that tube should work well with APS-C format sensor. I will write about the results for sure. 

I may recommend the book "Star Testing Astronomical Telescopes" by Harold Richard Suiter - quite complete source of knowledge and testing routines for amateurs. 

[ONIKKINEN]

25 minutes ago, drjolo said:

Then selection of coma corrector (probably TS 0.95x Maxvision) and that tube should work well with APS-C format sensor. I will write about the results for sure. 

I have the TS 0.95 maxfield (i assume its the same) and wouldnt buy it again now that i have used it a bunch. I also have an APS-C sized sensor and there is noticeable coma left in the corners while stars are generally larger than they should be even at the image center. 

I would look for the TS GPU corrector. I dont have that myself but i dont see the same issues with images taken with one and most users recommend it so i will probably change to that some day.

[drjolo]

It's bad news I knew that MPCC for example shows the same behavior - not quite corrected corners and soft center, especially for telescopes faster than f/5. But I hoped to spare some money with that 0.95x coma corrector. 

[Robculm]

Hi Lucas,

Just to say your website is great! Some fantastic images. My 'future targets list' has just got quite a bit longer! 👍

Cheers, Rob

[alacant]

Hi

Another gso203 f5 user here.

We had to invest a little time to being it up to imaging standard but the effort is minimal. I'm sure a similar investment on your pds would yield similar results. E.g. no Newtonian we've ever seen uses adequate mirror springs nor supports the tube with an adequate dovetail. 

I think we're more limited by seeing than the quality of the optics anyway. Here, it's only once or twice a month that the seeing will hold guiding at 1000mm, but when it does, you notice it. 

I'd lean toward bringing your pds up to speed, wait for the steady nights and spend the budget instead on an eq6-r.

Cheers

[Robculm]

Thanks Alacant.

If I was to upgrade the current vixen style bar to a Losmandy, I guess I need to change the tube rings too (wider spacing where it mates with the bar), any suggested bar & ring sets at a reasonable price!? I would then look to move the curent vixen bar to the top of the rings & mount my guide camera there (as I've seen in some of your previous set up photos).

Regarding the mount, I hadn't really considered the EQ6-R as although it's belt drive (and I've already belt modded my HEQ5 pro) & higher load capacity, I assumed it would still have the poor worm gear issues of the HEQ5 pro? Is it really a significant upgrade, in comparisson to something like the iOptron GEM45 (which for sure is somewhat more expensive).

Cheers,

Rob

[alacant]

54 minutes ago, Robculm said:

previous set up photos

Hi

No, you can keep the existing rings. They just need a much wider spacing. A (at least) 50cm long Losmandy width dovetail with a rigid box section to tie the top of the rings keeps the tube rigid. You can make the plate from 500 x 15 x 100 mm aluminium plate. We use 60 x 20 rectangular box for the top rail. You local window-frame supplier will get them for you; a round of drinks works wonders;) Don't forget to also replace and add extra proper mirror springs and seal the primary to its cell.

We only suggest the eq6-r because the one's we've witnessed just work with the minimum of fuss. iOptron mounts don't. I hasten to add, that is just our hands on experience in Spain. I'm certain that there are good iOptron mounts which just work too. 

HTH

Edited February 8, 2022 by alacant

[Clarkey]

12 hours ago, Robculm said:

Regarding the mount, I hadn't really considered the EQ6-R as although it's belt drive (and I've already belt modded my HEQ5 pro) & higher load capacity, I assumed it would still have the poor worm gear issues of the HEQ5 pro? Is it really a significant upgrade, in comparisson to something like the iOptron GEM45 (which for sure is somewhat more expensive).

I have an AZ-EQ6 and as well as the weight capacity, the quality is all round better. The guiding is typically twice as good as the heq5 and it is more forgiving of slight imbalances. I can't comment on iOptron as I have no experience.

With regards the scope I would have to agree with Alacant. Get your 200PDS fully optimised and you won't see the difference to the other scopes. Get a good focuser (personally I rate the Baader Steeltrack very highly) and carry out the other suggestions as above and you won't be disappointed.

[Robculm]

Thanks Alacant, Thanks Clarkey,

Will definitely keep the EQ6 in mind. Hopefully I might get a decent resale price on my HEQ5 pro (with belt mod)!

In the meantime, will work on my 200PDS. I already have the mirror glued & upgraded screws. Will look at getting a 500mm Losmandy plate & aluminium bar for the top (on which also mount the guide camera.

What's the advantage of upgrading the focusser? I don't tend to see any change in focus during a session, so don't think I'm getting any movement from the stock focusser.

I do see that my collimation drifts a little depending on the scope orientation, but I assumed that's down to flex in the tube or the weight of the secondary on the spider vanes.

Cheers, Rob

[drjolo]

I decided to upgrade focuser in my GSO200/1000, because there was significant tilt of focuser tube under the load. When I put laser into the focuser and simulated mono camera with filter wheel weight attached to the focuser drawtube, it turned out that laser dot at the main mirror moves about 3mm around. I assume it is too much to keep collimation even for f/5 mirror. I ordered Baader Diamond Steeltrack, but I will also need to stiffen the steel tube around the focuser, because it is a bit floppy. 

[alacant]

30 minutes ago, Robculm said:

I don't tend to see any change in focus during a session

In our experience, they're few and far between but you may just have found a good sw ds focuser. If so, and you can eliminate it from the collimation drift, keep it.

There are flexible rubber 'o' rings (sic) between the focuser barrel and the focusing spindle housing which can deform under load/changing angles. Best removed. Rely on the push-pull of the adjusters. 

 

31 minutes ago, Robculm said:

weight of the secondary on the spider vanes

In this case SW have it just right. A nice minimalist secondary mirror support. A far cry from the massive metal versions found on gso and joc.

HTH

[alacant]

16 minutes ago, drjolo said:

also need to stiffen the steel tube around the focuser

If you have the tube rings distanced and top-tied on a Losmandy dovetail, a reinforcement plate alone should fix it.

But yeah, spot on; the gso steel tubes really are hopeless.

Cheers

Edited February 9, 2022 by alacant

[drjolo]

What is that grey plate made of? Steel? I am thinking about two additional metal rings on both focuser sides. Or another option is to made a 15-20cm wide collar with carbon mate and resin around the tube. 

[Varavall]

 I have a the very same rig. As you can see in the photo it can be loaded with all the kit you may wish for. I have done the collimation,  Rowan Belt mod and adjusted the backlash out of the drives, and I made a pillar mount out of scrap aluminium to minimise risk of collisions with tripod legs. For guiding I started by using the 9x50 finder scope with QHY5L II C, but I have now bolted a TS optics 80mm finder/guider scope to the top which gives better pixel ratio for guiding, plus an additional lump of steel as an extra counterweight. I can do 10 minute guided subs without problems. Of course the tube is a sail when the wind blows and it can rock a bit. All in all I am very happy with it's performance., I configure NINA and go to bed and wake to all the marvelous images to process (well if no computer or weather glitch 😉). I have only been into mono imaging for 6 months so there is still a lot to learn about the rig, but I attach a photo I took recently in LRGB SHO of the Crab nebula.  It's a scope that provides enjoyment without breaking the bank, well at least until you start mono AP!

[alacant]

3 hours ago, drjolo said:

grey plate made of

@laser_jock99's telescope. I would guess aluminium.

**Please note that this thread is about the OP's SW 200pds. This particular bit is about a GSO model; I doubt whether the reinforcement plate would be necessary on the OP's sw steel tube.

Edited February 9, 2022 by alacant

[Varavall]

3 minutes ago, alacant said:

@laser_jock99's telescope. I would guess aluminium.

**Please note that this thread is about the OP's SW 200pds. This particular bit is about a GSO model; I doubt whether the reinforcement plate would be necessary on the OP's sw steel tube.

Yes, its aluminium alloy. Light and strong but not there for reinforcement, its to take the guide scope. Yes I know thread is about a 200pds as that is what I have and that is what is in the photo.

[alacant]

22 minutes ago, Varavall said:

not there for reinforcement

The question was about this plate. It is to reinforce the tube around the focuser.

Cheers

[Robculm]

Hi Varavall,

That sure is a fully loaded rig! 👍