Skywatcher Quattro f/4 - How much faster than f/5?

[Gib007]

Hello all. I've been using a Skywatcher Explorer 150PDS f/5 telescope very happily since about November 2012. The Moonlite focuser was a set-back because it wasn't set right. Now that new holes are drilled in the OTA, the focuser is square-on and my stars are nice and round across the FOV. With it I've learned an awful lot about collimation, which brings me to f/4 telescopes.

How much faster should I expect a Skywatcher Quattro f/4 to be over my current telescope? I would go for the 8" Carbon Fiber version. This provides an FOV extremely similar to my current telescope but with the added benefits of an f/4 optical system and the better tube. I expect the OTA not to have appropriate screw holes for my Moonlite focuser so I would most definitely start drilling into its Carbon Fiber tube ASAP as though the Quattro's focuser seems better than my Explorer's stock focuser, the Moonlite one I have is incredibly well built.

I'm going to hold off purchasing this for a few months though. I have a new car coming very soon and buying the Quattro means about £1,000 as I would buy the OTA, Skywatcher's f/4 coma corrector and FLO's recommended Catseye Collimation Kit. Remove VAT (Gibraltar), add postage and some import duty and voila, ~£1,000.

[ollypenrice]

4x4 is 16, 5x5 is 25. That is your ratio, more or less. Pixel scale complicates it a bit.

However, you then have to subtract the time taken to get fast astrographs to work. You might be lucky or it maight take you several years to catch up!!

If your F5 scope ain't broke...

Olly

[Stargazer_00]

1

OK so that's a bit of a lame answer... sorry.

I find that the range between F5 to F4 increases in steps of 0.1 about as much as other speeds increase in full integer steps. So I see the speed range something like this

F4, F4.1,.......,F4.7, F4.8, F4.9, F5, F6, F7, F8, F9, F10

This helps gauge things for eyepiece performance in my opinion. You can't look at an eyepiece and think I'm going from F5 to F4.5, that's like going from F10 to F9.5. In (my) reality that's more like going from F10 to F5 in terms of how the scope will show up abberations in the eyepiece. The same as the jump from F5 to F4.5.

[Andrew*]

The "speed" difference will be the squares of the focal ratios.

5^2/4^2 = 25/16 = 1.56, meaning the f/4 is 56% faster than f/5, or inversely, an exposure of 64 seconds in the f/4 will be equivalent to 100 seconds in the f/5. As Olly suggested, there might be differences in pixel scale but I dont' know how this is calculated.

The difference is significant, obviously, but with the increase in speed comes an increase in the accuracy required to square up all the optical components.... Expect stress!

[Macavity]

REAL Photographers (far younger than me!) only talk of: f8, f5.6, f4, f2.8 though?

(Like the "Hyperion debate", the "F4 Photo Newt" one determines to rumble on? LOL)

I think there are reasonable... reasons, for doing things. After initially "kicking myself" for indulging in a steel tube 8" / F4 Newt (over a standard f/5 Newt) I have begun to appreciate it's virtues. Mostly, it FITS inside a 6'x6' obsy turning circle! The collimation is "challenge", but only 2x worse than an f/5? I'm only aiming for VIDEO astronomy class images etc...

On the other hand, I might break a few f-number "rules" for the following:

http://www.officinas...a.php?idProd=12

Fast Newts are great fun projects? But I think this illustrates the cost / beauty of off-the-peg Astrographs?

(My Italian is not up to much, but... "Star Shed / Garage"??? - Shades of "Joe Green" (= Giussepe Verdi!)

[ollypenrice]

There is no free lunch with fast astrographs. I very much doubt that you'll plug and play with any of them. I've read with interest the trials and tribulations of all of them since, on paper, they are so attractive. For what it's worth I'm currently involved in trying to de-tilt a very up-market one indeed, so far without success. But we will succeed, I don't doubt (said he, with slightly doubtful expression!)

Not sure why my first answer has been declared lame! Seems OK to me... I'd assumed we were talking about use for imaging, since the Quattro is an astrograph.

Olly

[Andrew*]

There is no free lunch with fast astrographs. I very much doubt that you'll plug and play with any of them. I've read with interest the trials and tribulations of all of them since, on paper, they are so attractive. For what it's worth I'm currently involved in trying to de-tilt a very up-market one indeed, so far without success. But we will succeed, I don't doubt (said he, with slightly doubtful exp<b></b>ression!)

Not sure why my first answer has been declared lame! Seems OK to me... I'd assumed we were talking about use for imaging, since the Quattro is an astrograph.

Stargazer 00 was referring to his own answer of 1, not your own response, Olly. I had to do a double take!

[ollypenrice]

Stargazer 00 was referring to his own answer of 1, not your own response, Olly. I had to do a double take!

Ah, my paranoia's showing!! Cheers Andrew.

Olly

[Stargazer_00]

Sorry...my mistake. Your answer was perfect Olly, as ever. My own was lame. Even with the edit.

[blinky]

I love my F4 Newt - collimating is a pain as it's very sensitive. Does the drop from F5 to F4 not mean that to grab the same number of photons takes half the time? So a 5 min exposure at F5 is equal to a 2.5min exposure at F4?

[ollypenrice]

I love my F4 Newt - collimating is a pain as it's very sensitive. Does the drop from F5 to F4 not mean that to grab the same number of photons takes half the time? So a 5 min exposure at F5 is equal to a 2.5min exposure at F4?

Taking sampling rate out of it (in arcseconds per pixel) no, the ratio is 1.56 to one since exp goes as the square of the F ratio. So 1.56 mins at F5 =1.00 min at F4. However, I'll let someone else brooch the minefield of the effects of sampling rate! On my camera the F5 would give 1.86 arcsecs per pixel as against 2.32 for the F4 scope. That gives a ratio of 1.24. I've heard it argued that this is the real ratio of exposure times but I have no competence in this debate and will merely spectate!

On a practical note, the bigger your chip the more likely you are to have 'challenges' with a fast system. Orthogonality, field flatness and, indirectly (via flatness) focus all need to be better.

Olly

[Gib007]

Certainly interesting information to ponder over. Thank you for your replies to my question!

It gives me a very good idea of what I could expect in terms of required exposure times for similar image brightness comparing an f/4 telescope to an f/5 telescope. I have indeed read a lot about how precise collimation has to be in an f/4 telescope. First Light Optics recommends using the Catseye Collimation Kit they sell. I've read about it and watched a video guide they posted on the item page. It looks really good and probably much better than just using a single Cheshire Collimation Eyepiece. The eyepiece works great for my f/5 but I suspect the f/4 will want something more precise.

[laser_jock99]

There is no free lunch with fast astrographs. I very much doubt that you'll plug and play with any of them. I've read with interest the trials and tribulations of all of them since, on paper, they are so attractive. For what it's worth I'm currently involved in trying to de-tilt a very up-market one indeed, so far without success. But we will succeed, I don't doubt (said he, with slightly doubtful exp<b></b>ression!)

Not sure why my first answer has been declared lame! Seems OK to me... I'd assumed we were talking about use for imaging, since the Quattro is an astrograph.

Olly

The fast F4 and ultra fast F2.8 (F4 + reducer) Newtonians are interesting tools when tweaked and working. As Olly and others suggest they can be made work and hold collimation but don't expect them to do this from the box. The optics are (usually) fine just the factory mechanics are lacking. But if you are practically minded to correct any shortcomings then there is the possibility to end up with an instrument capable of taking deep exposures in less time.

[Chris]

I went from f/5 to f/4 and then quite quickly retreated back to f/5 again!

[Gib007]

I went from f/5 to f/4 and then quite quickly retreated back to f/5 again!

Haha, was it collimation issues getting you down? Surely this can be handled properly and that's it. Dion from AstronomyShed.co.uk has a great video on YouTube of just over an hour of tweaks he made to his Quattro 10" CF telescope, disassembling it and then reassembling it with perfect collimation.

[Chris]

Colllimation, coma, astigmatism and the time battling with all three. I had a steel tube Orion f/4 and the previous owner had siliconed the primary to the mirror cell to prevent mirror flop but this caused some very ugly shaped stars in the form of astigmatism. I spent a fair amount of time trying to collimate the scope with a hotech for the secondary to get the Laser dot in the centre of the primary then I switched to the cheshire for the primary mirror collimation. I think it took in the region of 20 times the time to collimate the f/4 compared to f/5 and by then it was 3 in the morning and time to pack up! I only had the one outing with the scope and could have chosen to work on the primary mounting by unsticking the silicone and making some mirror clips but I weighed it up and decided that it really wasn't worth the hassle compared to the ease of use and ultra quick setup of the f/5's. I also would have needed the cats eye collimatin tripple pack from FLO really if I wanted to keep the f/4 and I'm not willing to pay that much for collimation tools!

You are doing the right thing thinking about the CF tube as these will keep collimation much better after you've put the work in to achieve it! Remember that it took me hours to collimate without the cats eye and with a steel tube this would need tweaking regularly with expansion and contraction and flexure. I've not even got onto the problems with coma! f/4 has a lot lot more than f/5.

After selling my f/4 very cheaply the only scope I could affored was a 130pds but this was a blessing in disguise because after using it I would have the 130pds any day over the f/4, it was so easy to collimate it took me 5 minutes to get it absolutely spot on and I just stick the coma corrector in without much thought and it gave me almost perfect shaped stars straight the way!

p.s. I do love Dion's vids, he's a legend

[ollypenrice]

I think that the only fast astrograph that you can pretty well guarantee to work is the Tak Epsilon. They need to be carefully collimated but they have the engineering in place. I wouldn't buy one because I don't like diff spikes or square stars in widefield images and I wouldn't want an open tube telescope. It would mean a strip down every year for cleaning and then the attendant issues of getting it right again. However, if Mr Takahashi were to offer an Epsilon with an optical window...

If you have lots of cloudy nights and the ability to exploit them for fine tuning then a fast astrograph makes sense. I have lots of clear nights and a slower telescope that works ends up faster than a faster one that doesn't...

Olly

Edit; so I think I concur with Starfox above on this one.

[BlueAstra]

I have an OO AG10. Although some have tamed it, I found the route very arduous at F/3.8. I've reported my experiences on the shed forum, so be prepared for some work! ( I'm on my iPad at the moment, and as I am still learning how to drive it I can't supply a link!)

[ollypenrice]

I now know for certain (being quite old) that I would rather double my speed by adding another fast refractor and CCD than by trying to get one of these manic Ferrari astrographs to work.

I go out with the FSQ (grovelling along at F5) and I... take photos. Yes, I just do it. Decision made. I'm now using two old Fluorite FSQs, worth what? £2K apiece? More than a Quattro but a site less than a new (insert manic Ferrari astrograph of your choice...)

Olly

[BlueAstra]

I now know for certain (being quite old) that I would rather double my speed by adding another fast refractor and CCD than by trying to get one of these manic Ferrari astrographs to work.

I go out with the FSQ (grovelling along at F5) and I... take photos. Yes, I just do it. Decision made. I'm now using two old Fluorite FSQs, worth what? £2K apiece? More than a Quattro but a site less than a new (insert manic Ferrari astrograph of your choice...)

Olly

Wise words! I think actually its the focal length that's my problem. I still hanker for the long lengths supplied by my old C8 SCT, as it also gave a planetary option with barlows. If I had my time again I think I would follow your dual option for DSO, or perhaps a ODK for a longer focal length that's comfortably fast without being manic fast.

[laser_jock99]

My 12" F2.9 scope cost about £1300 (£690 for the F4 OTA and the rest on an ASA Keller 0.7x reducer/corrector (a remarkable piece of optical engineering)). Admittedly the tinkering has not finished on this scope (I still have the focuser plate strengthening mod to do (as already done on my 8" F4/F2.9) but it's getting there. I doubt I will ever get the crisp edge-to-edge stars that an expensive triplet apo might achieve but I'm happy enough with my progress at the present. However- it does make me smile occasionally when I see pictures posted that took 6 hours and umpteen images stacked, whilst I'm thinking you could do that in 200 seconds.........

NGC7000 The Wall in 200 seconds

Apologies for the mess DSS made combining one light frame and one flat. Also don't look too closely at the corners- technically these are outside the corrected 20mm dia circle of ASA Keller on an APS-C sized sensor!

[Gib007]

After reading all your replies, I'm wondering then what would be a very nice step up from my current Skywatcher Explorer 150PDS. Basically I'd like to move on to an astrograph, a proper imager's telescope with high quality optics, great build quality and a fast focal ratio. Numerous options would be great if possible! Did I read correctly that the Takahashi FSQ-106ED telescope does not require a coma corrector?

As always, many thanks!

[Gib007]

To add to my dilemma, the Skywatcher Explorer 150PDS telescope with my ATIK 383+ CCD camera provide a nice FOV that's quite wide (61.9 x 83.1 arcmin) but I found this to be a little too much for some galaxies in particular. The Takahashi FSQ-106ED for example, has an even wider FOV of 87.6 x 117.6 arcmin and I'm just worried this would make some galaxy imaging a bit disappointing. Adding focal extenders just increases focal ratio for slower imaging as well, which is why I've never been keen. Am I wrong to feel this way?

[ollypenrice]

COMA corrector in a Tak FSQ??? My dear Sir, you risk being burned for heresy!! ! Pass me the matches, Cardinal Biggles.

The FSQs, both 106 and 85, have very large flat fields. That of the 106 is ludicrously enormous at 88mm diameter. There is no chip distance to respect, you simply focus and shoot - unless using the focal reducers (F3.9 for the 85 ad F3.6 for the later 106) and then you do have to have the right chip spacing. Fear not, someone will make you the small adapters you need for less than the price of a modest yacht.

Olly

[Chris]

I hate to throw a spanner in the works but heres an idea! Get a small frac like an ED80 for widefield work and buy a 200pds for galaxy work. You can't beat Newts for the ratio of focal length to optical speed and the 200pds would give a 1000mm at f/5! Just an idea but you could certainly buy both scopes within the budget of a CF Quattro so it make sense in my head at least If you stick a skywatcher 0.9 coma corrector on it it would make the 200pds 900mm focal length at f/4.5, your in the 4's then and the coma corrector is only 99 pounds new and about 60 second hand.

Chris