130mm F4 scope?

[Tommohawk]

Hi all.

So, I've been imaging using a 200P for some time, and would now like something with a wider field - I reckon a 400 or 500mm FL would be about right.

The alternatives seem to be

Canon lens eg 400mm FL f2.8 @ £5700 or  

Apo/Ed refractor eg 500mm FL Equinox f5 @£515, or

Newt eg 600mm f4 TS optics @ £349, or SW 130pds with 650mm FL for £159.

Or, I guess an ASA 0.75 reducer for the 200P

My assessment of this is that the Canon lens is lovely but too expensive, the refractor isn't very fast, and the newts are still too long, as is the ASA reducer.

Now what would be really good is an F4 version of the 130PDS newtonian giving 520mm FL. Does anyone know if such a thing exists? 

The other consideration I suppose is that currently I'm using a DSLR, but at some point I may go for CCD, in which case I guess an F5 frac would be fast enough. But an F4 130PDS would still be faster and probably cheaper. 

Also, just as a general point, am I right in thinking that the size of the secondary on a smaller imaging newt needs to be maintained at a reasonable diameter, so that is has a greater proportionate obstruction?

Any ideas?

[Alien 13]

The WO Star 71 is a good option.

Alan

[ollypenrice]

The WO Star 71 is a good option.

Alan

Agreed.

I think it would be tough to use the Canon wide open.

My general feeling about very fast optics is that they are often very slow to get into working order... On the other hand very fast cameras are no harder to use then very slow ones....

Olly

[Alan64]

There is this option: a 114mm f/4, and with a parabolic primary... http://www.scsastro.co.uk/catalogue/orion-starblast-astro-telescope.htm?term=starblast+4.5&gclid=Cj0KEQjwkeiwBRCzmo-wiKL49pEBEiQAhvGKYbaBhqJchv3UO_eKSvMWd8syEu7dzh6GcA6yxdEaxeQaAk_q8P8HAQ

It should require a coma-corrector, and may require a focusser-mod or a replacement entirely in order for the camera to reach focus.

I have the 150mm f/5 "StarBlast", and the mirrors are of surprisingly-good quality...

I removed the 6" f/5 from its stock mount, then placed it on a traditional alt-azimuth, and for visual use.

[miafey]

GSO has a fast image Newt, 6inch F4, 600mm, available from TS website

[Roy Challen]

Takahashi Epsilon 130 f/3.3 sounds right up your street.

[Tommohawk]

Hey thanks all for the helpful responses.

The WO Star 71 is a good option.

Alan

It looks lovely, but £950 is pretty stiff

Agreed.

I think it would be tough to use the Canon wide open.

My general feeling about very fast optics is that they are often very slow to get into working order... On the other hand very fast cameras are no harder to use then very slow ones....

Olly

Hi Olly - I honestly dont get quite so twitchy about fast scopes, but your point is of course a very good one. There's no point in going crazy for a fast scope when a fast camera is easier. TBH I will probably switch to CCD at some point, but if doing so would probably want quite a decent sized sensor and cant really justify the cost of this. Hmmmm, now youve got me thinking that maybe I should switch cameras first. 

Decisions, decisions.

There is this option: a 114mm f/4, and with a parabolic primary... http://www.scsastro.co.uk/catalogue/orion-starblast-astro-telescope.htm?term=starblast+4.5&gclid=Cj0KEQjwkeiwBRCzmo-wiKL49pEBEiQAhvGKYbaBhqJchv3UO_eKSvMWd8syEu7dzh6GcA6yxdEaxeQaAk_q8P8HAQ

It should require a coma-corrector, and may require a focusser-mod or a replacement entirely in order for the camera to reach focus.

I have the 150mm f/5 "StarBlast", and the mirrors are of surprisingly-good quality...

6 f5 mounts.jpg

I removed the 6" f/5 from its stock mount, then placed it on a traditional alt-azimuth, and for visual use.

Now that seems like a bargain! F4 114mm giving FL 450mm. Not so sure about the 1.25" focusser though - also, is there a 1.25" coma corrector?

GSO has a fast image Newt, 6inch F4, 600mm, available from TS website

Yep, and that does look tempting too. But 600mm FL still a bit long though.

Still wondering where the 130mm F4 is!! Something for SW to consider maybe - a quattro 130mm with 2" focusser.

Takahashi Epsilon 130 f/3.3 sounds right up your street.

It would be right up my street but at that price I would need to sell my street in order to fund it!  

All of which has somehow left me wondering about cameras, rather than scopes!!

Thanks all. 

[spaceboy]

If I remember correctly Peter Lawrence has a 130mm f/3 or 4 but couldn't tell you what brand etc it is. Maybe a quick search on the net might bring up some results with him being a public figure.

[laser_jock99]

GSO has a fast image Newt, 6inch F4, 600mm, available from TS website

I use the GSO 6" F4 Newtonian + ASA 0.7x corrector/reducer which gives me a focal length of 420mm which, importantly, is also at F2.9

As Olly points out- such a fast system will require some application of thought to get the most from (it's not for numpties). The ASA corrector is expensive but is a whole lot cheaper than an a Takahashi Epsilon and you can also use the ASA corrector/reducer with any other Newts you might own (I have 5!!). Additionally you can use the GSO 6" at it's native F4/600mm - the Tak is fixed focal length.

The 'halfway house' where someone else has done the tweaking for you is a Boren-Simon 6" f/2,8 Powernewton Astrograph . Again- not a cheap option at £1100 (vs £4300 for the Takahashi) but it overcomes some of the problems of the budget route in an 'out of the box' package.

The sort of FOV you get at 420mm with an APS-C sized CCD - M8 & M20

[Alan64]

Now that seems like a bargain! F4 114mm giving FL 450mm. Not so sure about the 1.25" focusser though - also, is there a 1.25" coma corrector?

Not ideal, as it multiplies... https://www.astromarket.org/astrofotografie/coma-corrector/27x-coma-correcting-barlow-125---t2?language=en

My own UO 2.8x Klee barlow is also a coma-corrector.  There's this 1.25" corrector, but unreasonably priced... http://khanscope.com/accessDetails.cfm?productID=4802

There is the possibilty of custom-ordering one from Harry Siebert of Siebert Optics.  The price was US$125 in 2012... http://www.siebertoptics.bizland.com/

It's strange that 1.25" coma-correctors are not as prolific as they should be.

[Uranium235]

Unfortunately, no such thing as a 130 f4. However I have discussed this with a few people, and what could be done is a bit of "DIY" on a 130 f5.

Get a 5" f4 mirror ground to the same dimension as your existing mirror (so you can use the same holder), shorten the tube, then install the mirror. However, you might need a larger secondary to cope with the stubbier light cone.

Check here for various aspects of newt design:

http://stellafane.org/tm/newt-web/newt-web.html

[laser_jock99]

I (very occasionally) try the 0.7X ASA corrector with my Skywatcher 6" F5 (150PDS) which gives a 525mm FL / F3.5 instrument. It works but is on the borderline of the design parameters for the corrector.

Used with a 130PDS you'd get a 455mm FL / F3.5 scope (which technically might be more forgiving in collimation than a 420mm F2.9 ?) 

[Uranium235]

I know of only one person who has used the ASA with the 130, and from what I remember it definitely wasnt an easy thing to do. I wouldnt like to try it, and its a pretty expensive thing to do.

Probably the easiest/cheapest route would be to use the SW 0.9x corrector and operate at f4.5 - which is still pretty fast in anyones book.

[Tommohawk]

If I remember correctly Peter Lawrence has a 130mm f/3 or 4 but couldn't tell you what brand etc it is. Maybe a quick search on the net might bring up some results with him being a public figure.

Ta for the idea, but had a look and cant find it - I'm guessing its the Tak though.

Not ideal, as it multiplies... https://www.astromarket.org/astrofotografie/coma-corrector/27x-coma-correcting-barlow-125---t2?language=en

My own UO 2.8x Klee barlow is also a coma-corrector.  There's this 1.25" corrector, but unreasonably priced... http://khanscope.com/accessDetails.cfm?productID=4802

There is the possibilty of custom-ordering one from Harry Siebert of Siebert Optics.  The price was US$125 in 2012... http://www.siebertoptics.bizland.com/

It's strange that 1.25" coma-correctors are not as prolific as they should be.

As you say barlow defeats the object rather - nice to know it exists though.

I use the GSO 6" F4 Newtonian + ASA 0.7x corrector/reducer which gives me a focal length of 420mm which, importantly, is also at F2.9

As Olly points out- such a fast system will require some application of thought to get the most from (it's not for numpties). The ASA corrector is expensive but is a whole lot cheaper than an a Takahashi Epsilon and you can also use the ASA corrector/reducer with any other Newts you might own (I have 5!!). Additionally you can use the GSO 6" at it's native F4/600mm - the Tak is fixed focal length.

The 'halfway house' where someone else has done the tweaking for you is a Boren-Simon 6" f/2,8 Powernewton Astrograph . Again- not a cheap option at £1100 (vs £4300 for the Takahashi) but it overcomes some of the problems of the budget route in an 'out of the box' package.

Yes I like the idea of the ASA, but I'm thinking at £650 ish thats kind of the cost of a very good small newt. But as you say, can you be used with any other Newt with 2" focusser, so quite attractive idea. 

I also like the looks of the Boren-Simon scopes, and I think the 6" one at £1200 include the ASA x0.73 corrector, so thats £650 for the corrector and £550 for the carbon scope - not a bad package. I'm toying with the idea of a 250 Quattro, so that would give me Quattro at 1000mmFL, or 730mm with the ASA, and BS at 600mm FL, or 438mm FL. Pretty good range.

I bet they dont come up second hand too often - same for the ASA 0.75 reducer.

Unfortunately, no such thing as a 130 f4. However I have discussed this with a few people, and what could be done is a bit of "DIY" on a 130 f5.

Get a 5" f4 mirror ground to the same dimension as your existing mirror (so you can use the same holder), shorten the tube, then install the mirror. However, you might need a larger secondary to cope with the stubbier light cone.

Check here for various aspects of newt design:

http://stellafane.org/tm/newt-web/newt-web.html

Yes, I alos though about the DIY option too. But TBH by the time I've bought a 130, got a new mirror and shortened the tube etc I might as well just spend the money on a purpose built one. Worth thinking about though.

Thanks again to all for the input 

[kev]

Re coma correctors. I used to use a lumicon one on my 14" f/5 when i had it. Worked nicely.

Sent from my iPhone so excuse the typos!

[kev]

Ps. Whatever happened to "comet catchers" ?!

Sent from my iPhone so excuse the typos!