130mm F4

[Tommohawk]

So I'm sitting in the departure lounge, off to Spain for a week. Forecast is sunny with clear night skies. But I don't have a travel scope.

So, anyone know where I can get a 130mm F4 primary? I reckon I could get a 130pds, cut it down by about 100mm, swap to the F4 mirror, and it would a good fit for a cabin bag. I'm thinking here for the next trip of course, not prior to departure today! Any ideas?

[RT65CB-SWL]

Hi Tom,

There is a 130mm parabolic & secondary on sale at http://www.astroboot.co.uk

[Stu]

Hi Tom,

There is a 130mm parabolic & secondary on sale at http://www.astroboot.co.uk

All f5 or longer though

[Tommohawk]

Yep, standard mirror is F5, so unfortunately this doesn't help... Any other thoughts?

[Stu]

Keep the f5 and construct a little truss dob of some sort?

[Macavity]

A bit bigger, but if you particularly want an f/4 Newtonian:

http://www.teleskop-express.de/shop/product_info.php/info/p4762_GSO-6--Imaging-Newtonian---6--f-4---2--Monorail-focuser.html

I seem to recall our friend "Laser_Jock" uses one of those...

Darn good scopes, but substantial, more for imaging maybe. 

Visially, the larger secondary shadow of an 8" f/4 becomes

more significant - You might not be able to get a wider visual

field than a typical f/5 scope? The f/4 Newts are smaller tho'.

(Can't do the maths in my head, but something to consider!)

P.S. Just added... by the ZWO camera people. Who knows?

http://www.teleskop-express.de/shop/product_info.php/info/p8255_ZWO-153-mm-F-4-Newtonian-with-Carbon-Tube-and-2--Crayford-Focuser.html

They make scopes too! Expensive, but cool RED highlights...

(Are they using a plain glass secondary support??? Weird!)

[Moonshane]

I'd recommend either a 102 or 120 f5 frac

[Tommohawk]

Keep the f5 and construct a little truss dob of some sort?

Good idea! But a lot of work, and if I can get an F4 thats so much better for fast and probably unguided imaging.

[Moonshane]

Given this comment, I'd suggest an 80mm ED with a focal reducer to supplement what you already have.

[Tommohawk]

A bit bigger, but if you particularly want an f/4 Newtonian:

http://www.teleskop-express.de/shop/product_info.php/info/p4762_GSO-6--Imaging-Newtonian---6--f-4---2--Monorail-focuser.html

I seem to recall our friend "Laser_Jock" uses one of those...

Darn good scopes, but substantial, more for imaging maybe. 

Visially, the larger secondary shadow of an 8" f/4 becomes

more significant - You might not be able to get a wider visual

field than a typical f/5 scope? The f/4 Newts are smaller tho'.

(Can't do the maths in my head, but something to consider!)

P.S. Just added... by the ZWO camera people. Who knows?

http://www.teleskop-express.de/shop/product_info.php/info/p8255_ZWO-153-mm-F-4-Newtonian-with-Carbon-Tube-and-2--Crayford-Focuser.html

They make scopes too! Expensive, but cool RED highlights...

(Are they using a plain glass secondary support??? Weird!)

Yep there are a few F4 150mm scopes which are great - but way too big for cabin bag. You can get F4 300mm, 250mm, 200mm, and 150mm - but no 130mm F4s. And I think you are right that the smaller you go, the more the secondary becomes an issue. Maybe thats why there are no 130mm F4s. (That said, there is an F4.5 114mm Starblast.)

I've seen some of LaserJocks images - very nice, and he has ASA 0.73 reducer too. My ideal would be 130mm PDS with F4 conversion, ASA reducer, and CCD imager. I think that would give some nice widefield results unguided.

I'd recommend either a 102 or 120 f5 frac

I should have said that my main thing is imaging - pretty much never do visual. The 102mm F5 StarTravel is very compact - but everyone tells me the CA is unacceptable for imaging. I've considered using this or some other achromatic for narrowband imaging - but I think it would be necessary to refocus between filters. Also if I have to go narrowband to avoid the CA, thats ups the exposure time so then guiding becomes an issue.

Given this comment, I'd suggest an 80mm ED with a focal reducer to supplement what you already have.

Yes thanks, I've considered 80mm ED. The SW Evostar is F7.5 so too long at 600mm - it has to be <550mm for cabin bag. The Espirit F5 is OK for lenght but £929 - all the F5 80mm APOs are £750 ish.

The Newtonian design is more compact and the standard 130PDS is almost OK at 600mm tube length and a mere £159 at FLO. The Quattro 200S is £395. So in theory a 130mm F4 should be about £250-£300, but nobody seems to make one. Shame!

[Moonshane]

I have a Meade 80mm f6 triplet which with the dewshield retracted is 390mm long. worth looking out for used. Got mine for £200 - have added a couple of bits since though - cnc rings and wo dovetail bar. lovely for visual and wl solar.

[Chris]

Just get a ED66 or ED70, that will be short enough and great for imaging. 

You can pick them up second hand for around 200-250 or 350-400 new.

The only downside I see is they are f/6, but I've found they reduce down to F/4.7 nicely with a reducer/flattener.

[kev]

http://www.firstlightoptics.com/heritage/skywatcher-heritage-114p-virtuoso.html

Is a 114mm f4.38 Parabolic

http://www.firstlightoptics.com/dobsonians/skywatcher-heritage-100p-tabletop-dobsonian.html

Is a 100mm f4 parabolic.

However, I suspect that neither would allow you to get to prime focus without substantial modding.

I also suspect that coverage of the camera would be an issue too.

[Chris]

http://www.firstlightoptics.com/dobsonians/skywatcher-heritage-100p-tabletop-dobsonian.html

Is a 100mm f4 parabolic.

However, I suspect that neither would allow you to get to prime focus without substantial modding.

I also suspect that coverage of the camera would be an issue too.

I can confirm this, I tried with my former 100p f4 Dob and sadly it doesn't come close to reaching focus with a DLSR. Shame because it's a lovely little mirror set, gives very nice wide views for visual! 

Actually, I think it was a CCD I tried it with so it defo wouldn't work with a DLSR.

[Atreta]

http://www.firstlightoptics.com/heritage/skywatcher-heritage-114p-virtuoso.html

Is a 114mm f4.38 Parabolic

http://www.firstlightoptics.com/dobsonians/skywatcher-heritage-100p-tabletop-dobsonian.html

Is a 100mm f4 parabolic.

However, I suspect that neither would allow you to get to prime focus without substantial modding.

I also suspect that coverage of the camera would be an issue too.

i have the 114p virtuoso and it won't reach focus with a dslr, it will focus only with a 2x barlow i've got.

some guys from a brazilian store made a shorter tube for it and said they were able to use it for imaging. it's a nice little scope, very portable.

[Astrobits]

Have you looked at the ASTRONOMERS WITHOUT BORDERS  ONESKY telescope?

F/5 with a collapsable tube.

Nigel

[Tommohawk]

I have a Meade 80mm f6 triplet which with the dewshield retracted is 390mm long. worth looking out for used. Got mine for £200 - have added a couple of bits since though - cnc rings and wo dovetail bar. lovely for visual and wl solar.

Sounds like a good deal, but theoretical F4 130mm newt would be faster especially with either SW x0.9 CC for F3.6 or with ASA 0.73 reducer for F2.9. And the 130 would be cheaper, given the CC/reducer would double with my 200PDS

Just get a ED66 or ED70, that will be short enough and great for imaging. 

You can pick them up second hand for around 200-250 or 350-400 new.

The only downside I see is they are f/6, but I've found they reduce down to F/4.7 nicely with a reducer/flattener.

Same comment as above - 130mm newt with CC  would be way faster.

http://www.firstlightoptics.com/heritage/skywatcher-heritage-114p-virtuoso.html

Is a 114mm f4.38 Parabolic

http://www.firstlightoptics.com/dobsonians/skywatcher-heritage-100p-tabletop-dobsonian.html

Is a 100mm f4 parabolic.

However, I suspect that neither would allow you to get to prime focus without substantial modding.

I also suspect that coverage of the camera would be an issue too.

Thanks for the idea, but looks like focus is a problem.

Have you looked at the ASTRONOMERS WITHOUT BORDERS  ONESKY telescope?

F/5 with a collapsable tube.

Nigel

Thanks Nigel - I think thats the same as the SW Heritage offering which is more readily available. Certainly looks portable enough - but I'm not sure the focuser would support a DSLR.

I still come back to the 130PDS with an F4 conversion.

Another crystal clear moon-free night here in Spain! Wish I had my regular rig here!

[Moonshane]

That's all well and good but where will you get one?

[Tommohawk]

That's all well and good but where will you get one?

Details, details..... Well of course you are quite right there's no point banging on about how good it might be if there isn't one out there. But if I could get an F4 mirror made for the 130 PDS for about £150, I'd have a really useful fast widefield for about £300.

I'm trying to source such a mirror, so if anyone has any ideas I'd be grateful.

[JED-E3]

I think you need to ray trace this f4 scope to see if it is practical, there might be a reason why you can't buy one.

For it to work you'll need a very low profile focuser and even then the secondary mirror will have to be huge.

[Tommohawk]

I think you need to ray trace this f4 scope to see if it is practical, there might be a reason why you can't buy one.

For it to work you'll need a very low profile focuser and even then the secondary mirror will have to be huge.

Thanks for that, and I've crunched the numbers with Newt for Web best I can, and here's the result.

Unit of Measure mm  

Primary Mirror Diameter 130.000  

Focal Length 520.000   Focal Ratio 4.000  

Tube Inside Diameter 159.000 ?

Tube Thickness 1.000  

Focuser Minimum Height 52.000 ?

Focuser Inside Diameter 52.000 ? Focuser Extra Travel 19.000 ?

Focuser Camera Travel 17.500 ?

Diagonal Minor Axis 50.000 ?

Diagonal Offset 3.125  

100% Illumination Diameter 11.481 ?

75% Illumination Diameter 35.870 ?

Front Aperture Diameter 160.618 ?

Mirror Face to Focuser Hole 351.000  

Focuser to Front End of Tube 100.000  

Mirror Face to Back of Tube 50.000  

Tube Length 501.000

So, all I have to do is buy a 130PDS, replace the stock 47mm secondary with a 50mm job, shorten the bottom of the tube by about 90mm, replace the stock F5 with a custom F4 (current quote £300 without coating), replace the focuser with a low profile type, ditch the DSLR in favour of a ZWO 174 with filter wheel, shorten the front of the tube by 20mm to avoid vignetting at the front aperture, which would mean reworking the front ring to fit outside of the spider screws. .... and it's all good!

Well, I think the answer is it's not a realistic proposition. Shame, because the design above gives 100% illumination for a mid-size chip, and would be super-fast, and portable at only 501mm length.

Coudnt be used with a DSLR though, but I've been looking to move on anyhow. Would be tight for a CC too I guess.

So, looks like I'll be looking for an 80mm Apo. (Yes, you did tell me so   )