One Scope to Rule Them All

[glend]

Here is the question: i need to find one scope for my declining years that works well for imaging and deliveres outstanding performance within the constraints outlined below. The plan is to reduce my present scope collection to One.


Must be a very capable as an astrograph, as 95%+ use will be imaging due to reducing vision capability.
Must weigh no more than 12kg for easy of carry and low stress guiding when image train equipped on NEQ6. Note i had considered replacing the mount but decided to spend the money on a scope that works within the mounts restrictions.
Mid-range focal length, say 600-1000mm,
Fast if possible, no slower than f6-7.
True colour rendition, with no fringing or bloating, i don't have a design preference other than no RCs (ie, refractor, newt, etc.).
Aperture, bigger is always better.
Must be both narrowband and broadband performer, thus no achros (i already have one of those).
Must be cost effective, not interested in paying a premium for a collectable brand name, function and performance for the $.
Budget, would like to stay under $3000 total spend, and if flatteners, etc are required they must fit with budget, and happy to source from anywhere.


I have a short list of possibles but i want other opinions.
Let the suggestions flow. Thanks for looking.

[Shiraz]

guessing the MN is too heavy?

Suggest a Skywatcher CF F4 200mm with a Paracorr and a Moonlite focuser. Use your 1600 camera so that you can guide with a guidescope and take short subs - would be a killer system. Oh, and cover that black CF dew magnet OTA with reflective tape to remove dew issues - won't be pretty, just effective.

[rally]

FSQ106N 2nd hand

[Satchmo]

Glen , the Skywatcher 190m F5.3 Mak-Newt is 11kg and would tick all the boxes. Didn't you allready own one?

[Merlin66]

Glenn,
I've been at this game for over fifty years.....
My "declining years" telescope for performance and portability will definitely be the original Genesis 4" f5. (Serial 1007)
I haven't found anything to beat it over years.

[glend]

Yes Mark, and Ray, i do have a very nice MN190 that i have just improved by installing a Moonlight focuser. Skywatcher lists the tube weight as 12.5kg on their website but it feels heavier to me, ha ha. My wish list does read as if i am describing it doesn't it. One of my options is to just rationalise to the MN190. I wish it was just a kilo or so lighter, a carbon fibre tube version would be a dream, and hard to do with all the baffling.
I did not mention it purposely, as i was after greenfield options.

[glend]

Ken according to my research the TV Genisis 4" f5 is in fact an achro doublet design, albeit with a rear Petzval corrector. Yes it may have nice glass up front but it will still have colour issues. I have been using a Bresser 152mm f5 with a Petzval corrector in the rear, for narrowband imaging but it is unsuitable for broadband.

[Satchmo]

I know a very skilled sheet metal worker who could replace the steel tube with aluminium and do all the internal baffles too and put reinforcing plate under the focuser. That might be an option if weight is the only factor. You won't get so many boxes ticked for that money in the Apo department .

[dannat]

I wasn't blown away by a genesis -heavy for a 4" too I thought -much more than a skywatcher 100 f9

intes mn (http://www.cloudynights.com/classifieds/item/83306-orion-argonautintes-mn61-reduced/) someone has already upgraded the focuser for you, weight just below 10kg I think

[Atmos]

I agree with Ray, a good 8" F/4 Newt with coma corrector would be a great place to look.
As rally suggests, a second hand FSQ 106 is also a good option but you're not likely to find one around the $3,000 in good condition.

Out of those two I'd gravitate towards the Newt given your requirements.

[glend]

Mark, i will consider that. Doesn't aluminium flex more than steel when used in that sort of construction or does it have to be build thicker to compensate? What about thermal expansion?

I have emailed Teleskop-Express to see if they can provide a carbon tube for the MN190. The stock MN190 tube appears to be the same basic steel tube as the 8" Skywatcher f5 newt but with the baffling added. TS already provide a Skywatcher 8" f5 newt carbon tube at a reasonable price of about 400 euro. If i could get a MN190 carbon tube, with baffles, with focuser bore positioning correct, it seems ideal. Lighter in weight, more thermal stability, stiffer, etc

[glend]

Colin, thanks, i would probably prefer an f5, simply for ease of collimation and longer focal length of 1000mm. The concerns i have with newts (in their pure form) are: diffraction spikes, the mandatory secondary heater, focuser/camera positioning, but i lived with those issues with my 10" f5 newt. The good thing about newt choice is that i can get carbon tubes for most of the popular imaging versions, which might allow greater aperture for the weight limit. I already have a Baader MPCC and spacers for my camera.

[Slawomir]

Hi Glen,

If you are going to use this telescope primarily for deep space astrophotography, then given that it will sit on EQ6 and that your camera has 3.8 micron pixels, I would personally not venture over 500mm fl, unless you like babysitting your mount and tweaking guiding settings during most sessions, if of course you aim for the best possible data. At 500mm fl your camera will give you a comfortable 1.57" per pixel, and honestly, I would probably even aim towards around 2" per pixel with EQ6 for enjoyable astrophotography and use drizzle in my workflow.

A second hand FSQ85 would tick all boxes for me with this mount and this camera.

Just my five cents.
Suavi

[Satchmo]

Its worth asking but I have never heard of a carbon fibre version of the MN190 and those prices only come through economies of scale . Its not hard to make an aluminium tube as stiff - just a small increase in thickness as stiffness goes up by the cube root. Also the internal baffle rings that Gary makes are like sleeves with a 90 degree bend - they add a lot of stifness.

I dont think these days that an aluminium tube is much of a hindrance as subs are usually 5 minutes or less and you can check focus every so often . Anyways I don't think baffles do anything at all in a Newtonian type design - I think they are really there to appeal to the retractor community .Flocking is all that is really necessary . The primary mirror and secondary already give you a massive amount of baffling .

Still if you can get a carbon fibre mod it would be ideal .

[Atmos]

You've just named every single reason why I went with a refractor over a newt or a MN190 :P Not a fan of diffraction spikes and I don't like the focuser position.

I tend to agree with Suavi, the closer you are to 500mm the better. At 400-800mm you are at a nice pixel scale where you can drizzle every bit of good seeing out of your data but not be as restricted on nights of less than perfect seeing.

As you say however Glenn, F/5 is a LOT easier to deal with in the way of collimation and sensor orthogonality.

[Satchmo]

While theres nothing to be done about focusser position other than some classy after-market ball bearing rotating mount rings for ergonomics , there are certainly no diffraction spikes other than any mirror clips in a Mak-Newt and the central obstruction is relatively modest on an F5.3 .

With good native off axis spot performance they should be optically superior to any 8" apo provided you are not going for a really large diameter focal plane - not that there are many 8" APOs in the world as the cost of the glass is prohibitive for even the keenest retractor buffs !

[gregbradley]

What about some of those Stellarvue scopes? The 70T in Andy's hands is producing some fine images. William Optics 110mm may be another 2nd hand with reducer.

Try the Teleskop Services website. They have some imaging Newts. A 110mm APO William Optics would be very nice. Stellarvue have some as well. Not sure if new is in your price bracket but 2nd hand some would come up.

Greg.

[Atmos]

No diffraction spikes on the MN190 but I do like the simplicity of the straight through focusing position on a refractor or SCT/CDK ect. Makes balancing in all three axis a lot easier :)

[LewisM]

Rationalised it myself fairly recently and chose a Tak Sky 90. Superb performer, flat and fast with reducer (f4.2) . Very short and very light. cools much faster than an fsq85 and is just as good a performer.

Cost me $1600 and truly very happy. Yes you need to check collimation occasionally (the second generation versions don't have an issue with this) but well worth it. The FSQ85 ranks in my top 3 of all the scopes I have owned but good luck getting a 2nd hanf one under 3000.

Or consider an Esprit 100 second hand if you can find one. Astromart will be your friend :)

[Ken]

The MN190 has a Aluminium tube or at least the Orion version does, just checked mine with a magnet.
Clear skies Ken.

[glend]

I have heard back from Wolfi at Teleskop-Express re the carbon tube for the MN190, they don't have one and no plans to make them. He also suggested that they would not do it anyway as they regard it as a closed tube scope and it would trap heat. What he doesn't know is that the MN190 has ventilation through the rear fan. Anyway that is a dead end.

Ken you are right, i never checked that until just now. The MN190 tube seems to be aluminium, it fails to interest a magnet. Boy those two pieces of glass hold some weight. Looks like i don't need to retube it, and any diet will not reduce it further. I would have hated to abandon that beautiful black diamond tube.


I have been reviewing the TS Photoline scopes like the 115mm which is in the ball park focal length and budget wise, so it could go on the short list of refractors.

Mark, all the Mak-Newts have baffle rings ( well at least the Intes ones), and i doubt it's just a styling convention to appeal to refractor fans. I suspect, having built a iStar refractor, that all front mounted glass elements require baffling to manage scatter.

Re pixel size and focal length, there seems to be all sorts of scopes being used with the ASI1600 camera, i have even seen RC images that look fine (to me). I don't know that over sampling is really an issue, under sampling is another matter i think. I still have my cooled Canon 450d DSLR, with its 5.2 micron pixels, which i will be using for OSC when i feel the need for speed. I would not want to go below my target fl range lower limit of 600mm.

I think i need to produce a chart on to track these options and features. :P

[el_draco]

I would suggest:
1/ Second hand Tak or a W.O. 110 would probably fit the budget and within the other specs.
2/Short F.L. 8" newt. with "bits"

Rom

[AstroApprentice]

The Borg 107 mm fluorite doublet f/5.6 will be released soon. It's incredibly light and slides down to a very compact size, plus the optics should be great. Hopefully $3k will buy one, but likely just the objective!

[glend]

Discussed this question with my son last night and it is clear that the family would like to retain some visual capability for the times that they visit, and the grandkids will want to have something to look through soon. So I have modified my criteria slightly to include visual capability to suit.

This creates a no-cost bias towards keeping the MN190 as it is an outstanding visual scope that takes heaps of magnification, as well as an astrograph. However, to be all it can be, I would need to look at sourcing (or making) some rotating rings, or tube slide stopper (without increasing the overall weight, hopefully). Like this:
http://i110.photobucket.com/albums/n82/tinker3480/Obby%20build/DSC_0170.jpg


I will shift my attention to 600mm focal length scopes as my wider field imaging tool, and have created a list of refractors mentioned in previous posts here, including the TS Photoline models and others that fit my budget.

[Merlin66]

Glen,
Just a comment about aging and observing/ telescopes....
Everything seems to get heavier and more awkward to use, then there's the difficulty of justifying large expenses when living on a pension...
IMHO keep it simple.

[dannat]

Borg 91FL, on a side note does peter tan have a functioning website yet?

[gregbradley]

A quick comment perhaps not entirely relevant - carbon fibre is not an appropriate material for some scopes. For example refractors its a poor match and worse than aluminium alloy which closely matches the change in focus of an APO objective.

I am not sure about a Mak Cass, it does have a corrector plate.

Carbon fibre is certainly the go for mirrored scopes like RCs, CDks, Newts.

Greg.

[LewisM]

Agreed Greg - all the research consensus is that CF is a VERY BAD idea for refractors - hence why many of the manufacturers are going AWAY from using it (not sure if OS is still using it...lord alone knows why!). There are numerous threads on CN about it, including comments from Yuri (TEC) and Roland (AP).

[glend]

That was pretty much what Wolfi at Tekeskop-Express said, that carbon fibre should not be used on closed tube scopes,, which explains why all their carbon tube offerings are newts or RCs or other open tubes. I noticed looking through their refractors that everything is alumimium tube.

[Atmos]

I was talking to Cris recently and he mentioned that they aren't making their 130mm anymore, not sure whether they are continuing with the 115 but that would be their only refractor if they are.

[Shiraz]

Glen, you have an MN, which is great for imaging and you have an achro which should be fine for the grandkids to start out with and for your occasional visual use if your eyes are not as good as they used to be. Presumably you don't do visual and imaging at the same time, so maybe you already have appropriate scopes? If you want widefield, the 450D on the MN should do a fair job.

[glend]

Your right Ray, i am more convinced that my solution lies in what i have, but needed to ask the question and get some experienced input.Thanks

[RickS]

The reason that Roland @ Astro-Physics gives for using aluminium tubes for his refractors is that the dimensional changes in an aluminium tube due to temperature change tend to compensate for changes in the focal length of the objective, giving overall better thermal stability.

This isn't going to be a significant factor for closed tube catadioptric designs where I presume the main disadvantage of a CF tube would be a longer cool down time.

Cheers,
Rick.

[Camelopardalis]

Rather than CF, just carefully cut some holes in the tube and add some fans...should keep it equilibrated on warm summer evenings...

[glend]

The MN190 already has a rear fan port behind the primary, and a ventilated focuser plug, providing a flow through path. I use a filter medium in the focuser to prevent dust getting into the tube or on the mirrors or corrector internal side, fan is oriented to exhaust air from the tube pulling air in through the focuser filter. No issues with equilibrium.
CF was a consideration for weight saving only.

[Camelopardalis]

Fair enough. The only scopes I looked at that had CF and Alu options the CF was barely any lighter.

I'd say just lift it on there and leave it! Why move it? It seems to do a great job imaging as is.

Get a little alt az to handle your achro :shrug:

[Satchmo]

I think a mak newt is basically a newtonian with a corrector plate ( albeit just a meniscus). Theres a long technical article by long term AAO telescope operator Steve Lee showing why baffle rings are superfluous in a reflecting telescope and it all made sense . The primary mirror and secondary act as main baffles. Mel Bartels demonstrated how effective the double baffle in double blind study with a flashlight.

GSO RC's also have these baffle rings and again with properly baffled primary and secondary mirror I'd say they are completely superfluous. If it meant you could go thinner aluminium and the rings would increase rigidity the rings may make sense and I suspect that the baffle rings in the MN190 are more for that , plus the perception that they do something extra .

Pretty much all you need in a Newtonian is some black velvet on the tube wall apposite the focusser and behind the secondary . I would imagine that the amount of ambient light hitting the meniscus on a mak newt that wasn't normal to the axis and then scattering inside the corrector would be pretty minimal.

Its an interesting subject and may do well to throw it open on Cloudy Nights for all the experts to mull over !

[Satchmo]

Totally :)

[glend]

Yes I would agree that straight through focusing is a real plus in so far as balancing and field rotation is concerned. Moonlight's new 'Nitecrawler' worm drive focuser and rotator is a very nice piece of gear that would be great to have.

Have a look at this video that Ron sent to me:

https://www.youtube.com/watch?v=azcv1b6u3jo

I want him to call it a Caterpillar Drive!

As to the baffle discussion, perhaps it has been discussed on CN in the past, I might do a search there.

[lazjen]

That looks like a great bit of gear. Bit of a killer price though at $US2400+ :eyepop:

[Atmos]

Looks a lot like the Optec Rotating Focuser :)

[glend]

With the help of a few folks, supplier discussions, and this thread , I have decided to go this way:



The MN190 will be retained, a no-cost option, it is a wonderful scope for visual and imaging, and gives me everything I need right now (except for a hernia).
The Bresser 152mm Corrected Achro will be kept for the time being for continued narrowband imaging in a lighter weight package, and visual use by the grandkids.
I will purchase a a TS Photoline 115mm f7 APO: as shown here:

https://www.teleskop-express.de/shop/product_info.php/info/p3041_TS-PHOTOLINE-115-mm-f-6-96-Triplet-APO-with-2-5--RP-focuser-1-11.html

which will give me a light weight (6.4kg), wider field option when equipped with a reducer/corrector (going to f5.53 at a focal length of 632mm) which is a good match for my ASI1600MM camera, and it will still give me a fl of 800mm at f7 with just the flattener when required.



This decision is within by budget (even with the reducer/flattener which are being purchased separately). My other scopes, including the 10" Newt and the iStar, will be sold over time when buyers can be found, and should go along way to paying for the TS 115mm APO.

Thanks to everyone that made suggestions, provided advice, and kept the ideas flowing.
Let's close this thread down now.
Thanks to everyone.:thanx:

[gregbradley]

That seems like a very good choice. I look forward to your review of this scope.
Greg.

[Atmos]

I agree with Greg, sounds like a very nice 4.5" visual scope :)