Need advice on super-portable frac

[BGazing]

12 hours ago, Louis D said:

As long as you're minimizing the weight of what gets mounted to it to 4 kg or less, that tripod should suffice.  Remember that you should avoid extending the center column to avoid adding additional damping time.  I'll admit I extend mine rather than the legs when temporarily observing near zenith.  The geared column is really handy for that.  Once locked down, the extra damping time is tolerable.

I would think carbon fiber would damp more quickly than metal, but you'll still want to make some vibration damping pads.  To make mine on the cheap, I bought some new old stock 1980s era 100% Sorbothane insoles off ebay for $6 and cut them into squares that I put under each tripod foot.  I tend to forget they're there when I pack up in the dark, and it can be a pain to find them because they are dark blue and tend to stick to the feet temporarily.  I need to spray paint them fluorescent orange.

I also wanted to mention that I've read that FPL-51 in smaller apertures (as in the scope you're interested in) is equivalent to FPL-53 in larger apertures as far as color correction.

Lastly, watch out for field curvature in a small refractor if you're used to using a 1000mm or longer Newtonian.  I ended up adding this field flattener to the front of my 2" diagonal with a spacer ring to improve things.  It's not perfect, but it is much improved.  Probably the most expensive part of my rig.  It would have to be threaded onto the front of the 2" to 1.25" adapter when using it with a 1.25" diagonal.  It's not that I'm OCD about having perfect edges, it's that the edges were way out of focus in wide field 2" eyepieces.  I don't think it matters as much at higher powers.  You'll have to see if it bothers you in your observing.  I don't drop $200 lightly.  It really bugged me.

Let us know how your rig turns out.  I went the cheaper but heavier route (except for the field flattener).

 

thanks, i already have the vibration dampeners, i was prepared that they would be needed for this setup as well.

interesting thing about the field flattener...i am not surprised you wanted it for wide 2 inch eps, field curvature really gets strong in them in short fl scopes.

[iPeace]

On 08/06/2016 at 23:26, BGazing said:

Would this frac satisfy my needs 

http://www.teleskop-express.de/shop/product_info.php/info/p1151_TS-ED-70mm-f-6-Travel-Apochromat-with-carbon-tube-and-2--Crayford-focuser.html

The OP tracked me down and contacted me by PM, so more for the benefit of other readers:

I have this scope and love it. Compact and lightweight with a very nice 2" focuser, very nice build quality, I am sure many would be surprised at how nice the views are.

It must be the ultimate grab-and-go scope, the ultimate travel scope, and the ultimate finder scope all in one. Stick it on any mount, it won't complain, and off you go.

It has never let me down. It gave me my first looks at Jupiter, Mars and Saturn. I regularly use it as a finder, and often have to tear myself away from it to switch to the main scope.

I am by no means a connoisseur of fine optics, so this is all for what it's worth. I have used the TS INED70 alongside a TV85 and a SW ED120 and it only makes me more impressed with what this small APO frac does. It was my first scope, and I honestly feel it should be everyone's first scope. Heresy, of course, because it's not a cheap light bucket, but I will stand up and say that this is the scope guaranteed to keep you in this business; always ready at a moment's notice, take it anywhere, no faff, pure joy to use.

I give you the TS INED70.

[BGazing]

On 12/09/2016 at 08:52, iPeace said:

The OP tracked me down

This makes me look positively...sane. 

Even more disturbing is that I have started looking at the Borg options...in absence of a facepalm smiley this one will do...

[iPeace]

5 minutes ago, BGazing said:

This makes me look positively...sane. 

Even more disturbing is that I have started looking at the Borg options...in absence of a facepalm smiley this one will do...

Have at it!

We all want to, so keep us informed!

[BGazing]

okay, your chance to spend other people's money. all are with the light FTF focuser, weight without those light rings and dovetail

77 SW ED II, f/6.6 the least expensive (but still expensive...argh). 1.6 kgs

71 FL, f/5.6, precious thing Cannon coated with unobtanium. (around 250 EUR more) 1.1 kg

and just for the heck of it

89 SWII ED, f/6.6, 300 EUR on top of the fluorite. 2kg (I guess)

What gives? Every milimeter counts, or fluorite counts? I have no idea. Let me know what you think and the reasoning behind it. 

[Louis D]

5 hours ago, BGazing said:

What gives? Every milimeter counts, or fluorite counts? I have no idea. Let me know what you think and the reasoning behind it.

Refractors have never been priced in anything close to a linear manner by aperture.  Glass types also seem to increase exponentially rather than linearly as well for each increment in color corrected improvement.  Check out the prices on APM's site for APO refractors.  A 180mm one goes for 25.749,00 EUR.  Markus doesn't even list prices for the special order ones that are even larger.  By contrast, pretty much all of the reflector/catadioptric builders quote prices on their websites for even their largest sizes of Dobs, SCTs, Maks, Cassegrains, RCs, or Dall-Kirkhams.

[BGazing]

exactly, that is why i ask whether a 71 mm FL is a better long-term partner than a 77 mm ED for visual...

[iPeace]

Well I shouldn't really, for I lack the experience. I will say that this is an interesting question, for so often does one read about another's preference for a smaller fluorite over a slightly larger doublet, etc. whilst maintaining that aperture is king. Yes the image is darker, they say, but the improved contrast sways them. Or something intangible...

Mind you, I notice this a lot in the 100mm - 120mm realm, so perhaps there's a lower limit?

I'm enjoying this particular bout of unhelpfulness...

Maybe you have found your niche in which to become an authority. That does mean buying and comparing them all, of course.

[Stu]

1 hour ago, BGazing said:

exactly, that is why i ask whether a 71 mm FL is a better long-term partner than a 77 mm ED for visual...

It's not an exact science, and even very lovely fracs can have annoying issues associated with them. The most likely one being field curvature which is a problem been with fast fracs, when the centre and edge focus point is not the same. This has alway struck me as a fundamental flaw which should be corrected via the eyepiece but is not. Using widefield eyepieces in fast fracs can leave you with this problem. Fluorite objectives in my experience, if nicely figured are worth having over a standard ED doublet, and possibly even a triplet for visual. The benefits seem to be the very low light scatter and better contrast.

My personal view is that a frac is giving up aperture to other options, so had better be perfect on as many other ways as possible to make up for it  

As you say in your first post though, if the scope is too big, you won't take it with you when you travel, and that's the key.

I've tried a few below 100mm....

William Optics ZS SD 66mm - lovely, tiny scope. Nice for visual although some CA

Tak FS-60C - sharper optically than the 66mm, similar size but heavier due to the clamshell.

Televue 76 - Built like a tank, a bit too heavy for air travel in my opinion. Nice optically, not quite as good as the 76mm Tak

Tak FC-76DC - Lovely , very sharp and well corrected.

Stellarvue ED80 - good scope, split tube so quite portable. Optics similar to SW ED80, which means pretty good.

Televue 85 - Aperture benefit shows over the 76mm scopes but too long and heavy. Very lovely scope in all other respects,.

Tak Sky 90 - didn't like! Too fast and too many abberations visually, not as portable as you would think. Might have had a poor one though.

 

Out of these, if buying again I would get the Tak FS-60Q which is the 60mm f5.9 with extender to f10, plus the 76mm objective upgrade kit. This is not a cheap option, but gives you lovely fluorite objectives in a package which covers most travel scenarios; you will always be able to squeeze the 60mm in, and sometimes the 76. If I remember correctly the 76mm is f7.5 and doesn't suffer from field curvature, whilst the 60mm is very acceptable too. Given the split tube design they are highly portable still.

http://www.trutek-uk.com/takahashi/fc-76.html

If you can find one, the TMB 92mm f5.5 is supposed to be excellent, but I expect it too would show field curvature. I'd like to try one some day.

[Louis D]

9 hours ago, BGazing said:

exactly, that is why i ask whether a 71 mm FL is a better long-term partner than a 77 mm ED for visual...

My recollections on flourite vs high end glass:

1. The difference in correction between FPL-53 and flourite is said to be rather small.  I'm not sure if you have to go to a triplet in the former to match a doublet in the latter, though.

2. A flourite doublet objective should have the flourite element as the second element to protect it since flourite is a rather soft material that can be damaged easily.  This should be a consideration if regular transport of the OTA is a possibility.

Stu, correct me if I'm mistaken on these two points.

[Stu]

3 minutes ago, Louis D said:

My recollections on flourite vs high end glass:

1. The difference in correction between FPL-53 and flourite is said to be rather small.  I'm not sure if you have to go to a triplet in the former to match a doublet in the latter, though.

2. A flourite doublet objective should have the flourite element as the second element to protect it since flourite is a rather soft material that can be damaged easily.  This should be a consideration if regular transport of the OTA is a possibility.

Stu, correct me if I'm mistaken on these two points.

Louis, with my sparse knowledge, I am in no position to correct anyone!!

On point 1) I think you are probably correct, but for me it is about other issues such as light scatter and contrast. My belief is that in a doublet, a fluorite element gives you better performance in those aspects. It is a hugely complex and subjective area though, just have a look on CN for the different view points on it. I'm not even going to pretend I'm right, but I have a view and it seems to fit my (visual) experience.

Point 2), I believe used to be a concern, but hardened modern coatings have put this in the past. My FC-100 has the fluorite element at the rear, so this is no longer a problem.

[iPeace]

Going out on my particular limb here, and strictly for what it's worth.

You haven't used a frac for... Well, it's been a while. Don't know how often you travel below the equator; by some strange reckoning it seems to me that it should be less often than trips to the park with the kids to see the Moon.

I have no illusions that a INED70 is equal to the alternatives you are considering. And I appreciate the attempt to get things right the first time.

All things considered, I can't help but think you would not regret getting one, even if you decide to upgrade later on. I have not actively sought out its faults, and they have not slapped me in the face at any time. For the money, I can only recommend it highly, either as a starting point, or, as in my case, as a multi-talented friend for the duration.

Good luck with your decision.

[Piero]

My TV60 weighs 1.5 kg, so it can be fitted on a 1.1 kg light tripod if light eyepieces are used. Quite expensive after TV increased the price of its items. 

Tak60 might be a better option economically. With small fracs you don't really need a finder so, that is something you can save some cash. 

Other cheaper options are by Altair and William Optics as far as I know. Never looked through them though.  Plus one for the WO ZS66 if you find one in the s/h market. It was well received by many.

[iPeace]

5 minutes ago, Piero said:

My TV60 weighs 1.5 kg, so it can be fitted on a 1.1 kg light tripod if light eyepieces are used. Quite expensive after TV increased the price of its items. 

Tak60 might be a better option economically. With small fracs you don't really need a finder so, that is something you can save some cash. 

Other cheaper options are by Altair and William Optics as far as I know. Never looked through them though.  Plus one for the WO ZS66 if you find one in the s/h market. It was well received by many.

I will certainly own a TV60 someday.

[Chris]

On 08/08/2016 at 15:59, BGazing said:

suppose i get a 70ish mm one which is f/6...and given that the mount is a fairly simple one with no tracking, what should be the ep lineup? i could use my es 24 68mm for scanning widefield at 4mm exit pupil at about 20x, what else? 15mm synta gold is, i reckon, unuseable. two quality widefield eps with shorter lenghts and a good barlow? would hyperion zoom be useful at all?

Just to give you fair warning, when I used my es68 24mm with an ed70 f6 it was warp drive time, not very usable. These ep,''s work better with longer tubes I found. 

I've often fancied the tak 60 for ultra portability but it would need different ep,'s

[Piero]

34 minutes ago, iPeace said:

I will certainly own a TV60 someday.

 

 

Inside the bag: TV60, Everbrite, 24Pan, Vixen SLV 9mm, 5mm. I either put my PM2.5x inside the bag or I carry it inside my pocket.

Total weight is around 3kg.    

 

[BGazing]

23 minutes ago, Chris Lock said:

Just to give you fair warning, when I used my es68 24mm with an ed70 f6 it was warp drive time, not very usable. These ep,''s work better with longer tubes I found. 

 

uh-huh. but i know FC is murderous at 420mm focal length...

[iPeace]

11 minutes ago, Piero said:

 

 

Inside the bag: TV60, Everbrite, 24Pan, Vixen SLV 9mm, 5mm. I either put my PM2.5x inside the bag or I carry it inside my pocket.

Total weight is around 3kg.    

 

Love.

[BGazing]

1 hour ago, iPeace said:

Going out on my particular limb here, and strictly for what it's worth.

You haven't used a frac for... Well, it's been a while. Don't know how often you travel below the equator; by some strange reckoning it seems to me that it should be less often than trips to the park with the kids to see the Moon.

I have no illusions that a INED70 is equal to the alternatives you are considering. And I appreciate the attempt to get things right the first time.

All things considered, I can't help but think you would not regret getting one, even if you decide to upgrade later on. I have not actively sought out its faults, and they have not slapped me in the face at any time. For the money, I can only recommend it highly, either as a starting point, or, as in my case, as a multi-talented friend for the duration.

Good luck with your decision.

nothing like a dose of dutch rationality to curb one's enthusiasm/hotheadedness 

[Chris]

45 minutes ago, BGazing said:

uh-huh. but i know FC is murderous at 420mm focal length...

That's good then, as long as you know already you wont be surprised. I got a shock thinking the ES68 24mm would be ok at f/6 but when I looked through it for the first time I felt like quoting Star Wars "Punch it Chewie!". The same EP was great with my f/5 200p. 

[iPeace]

1 hour ago, BGazing said:

nothing like a dose of dutch rationality to curb one's enthusiasm/hotheadedness 

As I said before, have at it! Consider it a way out if the can of worms seems... Well, etc.

[Louis D]

3 hours ago, Chris Lock said:

Just to give you fair warning, when I used my es68 24mm with an ed70 f6 it was warp drive time, not very usable.

That's why I put one of these on the front of my 2" diagonal.

[Stu]

24 minutes ago, Louis D said:

That's why I put one of these on the front of my 2" diagonal.

I had some success using a similar flattened from TS, although I never felt I got the best out of it. It certainly reduced the FC, but I guess you need to get spacing correct for it to be ideal?

There is a case for someone developing widefield eyepieces which counter the FC found in faster fracs I think...

[Chris]

26 minutes ago, Louis D said:

That's why I put one of these on the front of my 2" diagonal.

Nice! I like how it doesn't add distance to the optical train, very neat!

[Louis D]

13 minutes ago, Chris Lock said:

Nice! I like how it doesn't add distance to the optical train, very neat!

It took some fiddling to get it to work.  Technically, it should just insert up into the focuser tube and not change back-focus, but the AT72ED focuser's baffling is narrower than the normal 2" opening, so even my 2" diagonal doesn't insert all the way in.  For the specifics of my setup:

This particular FF is M48 threaded on both ends and does not change the focal length, so the focus point doesn't change.  Since it needs about 128mm of backfocus for the 72ED scopes, I put it on the front of my 2" diagonal's filter threads.  However, this proved to be a bit too much and resulted in overcorrection.  Because I have a GSO 2" star diagonal with SCT threads on the input barrel side, I was able to get a SCT to M48 adapter from Agena Astro that replaces the front barrel.  I then screw the 15mm extension that came with the field flattener into the M48 threads of the adapter and then thread the flattener in to the extension.  It's not quite perfect, but it's darn close.  It does require another 20mm or so of back-focus over the GSO diagonal alone.  If your focuser allows 2" tubes to be inserted into it without limitation, there won't be any change to your back-focus.

The spacing between the FF and the eyepiece fieldstop must remain constant across eyepieces, so you'll need to parfocalize your eyepieces for best correction.  Most of my eyepieces have the fieldstop at, or very close, to the shoulder, so this isn't much of an issue.