Astrophotography - 8" f/5.9 newt Vs 120ED f/7.5

[cybereye]

OK,

I'm still trying to decide which would be the next best scope to upgrade to and now I'm looking at an ED120 APO refractor, in particular the Skywatcher Black Diamond or the Megrez 120 f/7.5.

I guess my main question is: if I take a 5 minute image using my 8" f/5.9 newt do I need to take an image 4 times as long (double the exposure time for the roughly one f stop difference and double again for the roughly half decrease in aperture) to get the same image?

If so, what is the advantage of going to a refractor?

Cheers,
Mario

[Gem]

I understand that since the refractor has no secondary mirror, it enables you to image over multiple nights without worrying about spikes not lining up. I image over multiple nights with my SCT since it corrector holds the secondary in place rather than the cross hair holder of a reflector.

[Peter.M]

The F number of a telescope takes into account the aperture. So to get an image of the same brightness you only need to compare the F number not the aperture.

This is where it gets a little more complex though, if an object has a brightness of 1 mag per arc second. The lower focal length scope (the refactor) would have a scale of 1.238 arc seconds per pixel (kaf8300) and the reflector would have a scale of 0.928 arc seconds per pixel.

because the refractors pixels will see more of the sky than the reflectors, and objects have a fixed magnitude per arc second you can't directly compare the images from the two scopes.

The reflector also has the secondary mirror that obstructs the primary, which is not taken into account when they quote an F number.

What you need to compare when looking for a new scope is the focal length, especially as a beginner as this will impact dramatically on the way your images look (far more than an F number).

[gregbradley]

Interesting point.

My CDK17 does not have many diffraction spikes, mainly only on the brightest of stars.

I can't say that spikes not lining up has ever been an issue over time though.

Perhaps the combining process tends to discard the misaligned ones. Not sure but I have taken images over 2 months of the same object and it did not cause a problem. I also don't use a rotator.

Greg.

[DJ N]

Hi Mario,

Hopefully I can give you at least an insight to the ED120, especially with regards that we are both using a QHY8L. I have had mine now for around 3 years and I absolutely love the scope. I have added a Moonlite focuser (with DC motor option) to it. For imaging, you really do need a flattener and I use both the Hotech SCA Flattener or the latest revised William Optics 0.8x Reducer/Flattener III such that I can "speed" up the rig from f7.5 to f6.


Here are a couple of recent examples of images. The first set from a dark sky location and the second from metroploitan Adelaide.

http://www.iceinspace.com.au/forum/showthread.php?t=88312

http://www.iceinspace.com.au/forum/showthread.php?t=94019


For a while now I have been seriously contemplating purchasing the new Skywatcher Black Diamond f4 imaging newt to replace the ED120. The only reason behind this is to image at F4 (at a similar focal length), which would mean 3.5x quicker imaging over the native f7.5 of the ED120. But, I am very unsure to how much more "work" would be involved in using a very fast newt over the ED120. I hear a lot about critical collimation and continual collimation tweaking, mirror flop etc with a lot of these f4 newts.

From an ED120 perspective, every time I image, I am truly amazed to how truly "plug and play" a refractor is. I recheck focus about once every hour and that is it!

So not sure if I have helped in any way, but please do not hesitate to send me a PM if you would like any additional info etc.

All the best!

Daniel

[cybereye]

This is just what I was after!

So basically the points that I can take from this is:

1. My exposures will take roughly half the time again since there is one f stop difference but the f ratio of the 8" newt does not take into account the central obstruction of the secondary mirror.

2. If I want to take several nights worth of data it's probably better to use a refractor so that I don't have problems with trying to line up diffraction spikes. This however may not be such a big drama....

3. No collimation needed on the refractor - plug & play.

4. I'll still need some type of field flattener if I go to a refractor.

Have I missed anything?

Thanks for your help gents,

Cheers,
Mario

[RickS]

Movement in diffraction spikes from night to night is only an issue if the OTA rotates on the mount. My RC10 sits on a fixed Losmandy D plate so the spider always has the same orientation wrt the mount and this is never a problem. A newt mounted with rings and a D plate would be fine too so long as you leave it in the rings.

Cheers,
Rick.

[cybereye]

But wouldn't you need to ensure your camera was always in the same orientation?

[RickS]

No! The diffraction spikes will always have the same orientation with respect to the sky. Image registration will take care of any camera rotation and realign the spikes along with the rest of the sky.

Cheers,
Rick.

[cybereye]

Yep, you're right. I had a think about it on the drive home from work and came to the same conclusion.


I'm now finding it hard to justify buying a new scope!

There must be some reason to get a new one....

Mario

[RickS]

You can never have too many scopes, but if you can't justify one then there's always a new camera or a new mount ;)

[strongmanmike]

Yep aperture!

If you place the OTA in a different orientation in your rings (ie in relation to the sky) between sessions the spikes will not match, just removing the camera and replacing it slightly differently is ok. May need new flats though?

Mike