Advice Needed - Sky-Watcher Quattro f4 Imaging Newtonian

[Guest]

Hi fellow enthusiasts,

I have been using a SW Evostar 72ED for the start of my imaging journey and have been very pleased with it. I have used this on an EQM-35 mount. I have just taken delivery of a EQ6-R Pro mount and would like to start planning for a much narrower FOV setup for some galaxy imaging to compliment my widefield setup.

Various research has lead me to consider the Sky-Watcher Quattro f4 Imaging Newtonian in either 8" or 10" options. I think I could potentially use the Evostar 72ED for guiding for this setup having looked at a guide scope suitability calculator.

I have read reports that these scopes should only be condiered by those with experience due to the precise collimation required but I have experience of collimation of Newtonians and would consider a Cheshire collimator an essential purchase due to the accuracy.

Does anyone have any experience, thoughts, views and pros/cons on my options given this is a relatively cost effective route to narrow FOV imaging.

Many thanks

Chris

[MarkAR]

If the 10" is anything like my 250P-DS then it's a real handful to mount. Also a bit of a wind catcher.

I'd be tempted to say save up a bit and get a refractor secondhand.

[almcl]

I image with an 8" reflector. 

Started out guiding with a finder scope, switched to an ST80, but finally ended up with an OAG.  This was a game changer as it dramatically improved the roundness of stars and guiding accuracy, generally. 

My back garden is fairly wind protected but if wind strength is above 10 mph from the east or 12 from the west, a lot of subs get lost...

[Martin Meredith]

The 8" Quattro with the carbon fibre tube is my main scope and has been for 6 years. I collimate with the CatsEye autocollimator tools using the hotspot symbol. This is not too onerous. The most frustrating aspect is secondary adjustment. Even though I replaced the adjustment screws with Bob's Knobs and added some home-made plastic washers, it is not as smooth as I would like. The flip side is that -- once adjusted -- the secondary holds collimation well and it is usually just the primary that needs a quick 30s adjustment. 

As for wind, the 8" at f4 is quite short at 75 cm so not too bad I find. I only notice wind effects when I add the dew/light shield, which adds another 30cm to the length. This is mounted on an Eq6 class mount (Az-EQ6 to be precise).

Caveat: I use my scope solely for EEA with a small sensor. If I were to use it for AP I would definitely use a coma corrector.

cheers

Martin

 

[johneta]

I use the Orion 8" F3.9 which looks identical to the quattro. I wouldn't be surprised if they are made in the same factory.
Im happy with it.
Collimation is easy with a Laser /Barlow combination.
F4 is great for gathering light. I use a 300mm F5.6 lens for guiding which works well. I'm not sure about Galaxy imaging with 800mm though. Ive done quite a few Galaxies but it would be better to have a longer focal length if Galaxies are important to you.

[ollypenrice]

When these scopes are good they are very good. There was a long and detailed post by Singlin on here a few years ago in which he went through all he'd done to get his into good shape. I can't find that thread but there is good info on this one. Laser Jock runs through an interesting list of mods. 

I think any reflector is best guided by OAG because of potential mirror movement.

Olly

[johninderby]

Is this the thread?

 

[Guest]

All really useful feedback thank you all. I am still somewhat on the fence though may delay getting a larger Newtonian until such time as I have a permanent setup and an obsy is certainly on the cards but perhaps not for a year or two. I might consider the SW 120ED in the shorter term whilst I setup each night for imaging.

[SamAndrew]

Great scopes, excellent value for money, 8s is the sweet spot if you haven't got an observatory.

[ollypenrice]

On 16/08/2020 at 12:08, johneta said:

F4 is great for gathering light. 

No, and this is important, it isn't.  Aperture is great for gathering light. To give an extreme example, you have a tiny telescope with 10mm of aperture - but it's F2. Is it great at gathering light? Clearly not. Aperture, and only aperture, gathers light. The focal length behind the aperture has precisely no effect whatever on how much light is gathered.

Olly

 

[johneta]

On 21/08/2020 at 09:57, ollypenrice said:

No, and this is important, it isn't.  Aperture is great for gathering light. To give an extreme example, you have a tiny telescope with 10mm of aperture - but it's F2. Is it great at gathering light? Clearly not. Aperture, and only aperture, gathers light. The focal length behind the aperture has precisely no effect whatever on how much light is gathered.

Hmm, I'd like to hear more on that.
Makes sense that its the aperture that is gathering the light, but if you spread that light over a bigger area of the sensor (ie a longer focal length) then you have effectively got less light on the sensor and have to expose longer?

[ollypenrice]

10 minutes ago, johneta said:

Hmm, I'd like to hear more on that.
Makes sense that its the aperture that is gathering the light, but if you spread that light over a bigger area of the sensor (ie a longer focal length) then you have effectively got less light on the sensor and have to expose longer?

For a given aperture you have the same number of object photons whatever the focal length or F ratio. And yes, if you shorten the FL you make a smaller, brighter, less resolved image in the time - just as you do if you capture in bin 2 or bin 3 etc., or if you software bin the longer FL version.

The camera lens notion that exposure time goes as the square of the F ratio is straightforward and true when the aperture is the variable, as it is with camera lenses. (You open or close the diaphragm.) It ceases to be meaningful and becomes very misleading when focal length is the variable.  Changing the aperture and changing the focal length are very clearly not equivalent actions.

Olly

 

[Endolf]

The F ratio is useful for gauging how long it will take to gather enough photons for the frame being shot. The key is that the f ratio doesn't tell you which part of the ratio has changed. If the target you are shooting fills the field of view of the camera combination with say a 1000mm focal length and say 200mm aperture, then shooting the same target at 200mm aperture at f4 will mean the target is smaller on the sensor, if you then crop and upscale to produce the same final image, then you've just lost all the extra photons. If on the other hand, you go to 1000mm but with an f4 (increasing the aperture rather than reducing the focal length) then you will be gathering more photons on the same target.