Trying to decide between 8"f/4 newt and RC8

[Gunsnroses]

Hi everyone. First post here and I'm looking for insight between the 8" f/4 newtonians and the RC8 astrographs.

At the moment I have AZ-EQG mount and a Orion ED-80.

I also have a ST-80 and ASI120MM to use for guiding. But haven't set it up yet since my AZ-EQG has been doing 3 min unguided shots so far. That seems to work well in my light polluted area with the 600mm ED80. 

But once I go up in focal length, I'm sure the guide scope will come in handy.

Anyway, I know the 8" f4 newts need a few mods out of the box. Focuser, reinforced ota around the focuser and mirror locks as well as new collimation knobs.

I haven't really read that the RC's need much modifications out of the box.

The 8"f4's seem to be critical for collimation. Much more than the RC's.

But with good collimation tools, that shouldn't be an issue I would guess.

And a Baader Coma Corrector is a must.

The only down sides I can see in the 8"f4 is that it's only 800mm, which is only 200 mm more than my ED80. So it'd still going to be pretty wide on planetary nebula and star clusters. 

And the fact that balance will be thrown off each time the mount adjusts to a new target since the focuser is on the side of the ota. Wouldn't stress that to much if I had a larger mount, but after all the equipment is mounted on the AZ-EQG, it's going to be at its limit for astrophotography and a shift in balance could throw off the tracking.

The only down sides I can see to theRC'8 is the 1600mm focal length and the fact that is an f/8.

F/8 is pretty slow. So longer exposures will be necessary. And at 1600mm, tracking will be more critical. 

Soooooo, I'm looking for insight on these two ota's from those who have used them or are using them now.

Do use barlows effectively with the 8"f/4 to magnify objects like planetary nebula?

Do you use focal reducers effectively with the RC8's to get a wider field of view and a faster f/stop?

Any insight on these ota's for imaging would be much appreciated. 

Sorry for the long post and thanks so much in advanced.

Clear skies

[DHEB]

6 hours ago, Gunsnroses said:

Hi everyone. First post here and I'm looking for insight between the 8" f/4 newtonians and the RC8 astrographs.

At the moment I have AZ-EQG mount and a Orion ED-80.

I also have a ST-80 and ASI120MM to use for guiding. But haven't set it up yet since my AZ-EQG has been doing 3 min unguided shots so far. That seems to work well in my light polluted area with the 600mm ED80. 

But once I go up in focal length, I'm sure the guide scope will come in handy.

Anyway, I know the 8" f4 newts need a few mods out of the box. Focuser, reinforced ota around the focuser and mirror locks as well as new collimation knobs.

I haven't really read that the RC's need much modifications out of the box.

The 8"f4's seem to be critical for collimation. Much more than the RC's.

But with good collimation tools, that shouldn't be an issue I would guess.

And a Baader Coma Corrector is a must.

The only down sides I can see in the 8"f4 is that it's only 800mm, which is only 200 mm more than my ED80. So it'd still going to be pretty wide on planetary nebula and star clusters. 

And the fact that balance will be thrown off each time the mount adjusts to a new target since the focuser is on the side of the ota. Wouldn't stress that to much if I had a larger mount, but after all the equipment is mounted on the AZ-EQG, it's going to be at its limit for astrophotography and a shift in balance could throw off the tracking.

The only down sides I can see to theRC'8 is the 1600mm focal length and the fact that is an f/8.

F/8 is pretty slow. So longer exposures will be necessary. And at 1600mm, tracking will be more critical. 

Soooooo, I'm looking for insight on these two ota's from those who have used them or are using them now.

Do use barlows effectively with the 8"f/4 to magnify objects like planetary nebula?

Do you use focal reducers effectively with the RC8's to get a wider field of view and a faster f/stop?

Any insight on these ota's for imaging would be much appreciated. 

Sorry for the long post and thanks so much in advanced.

Clear skies

Hi and welcome! You already went through much of the info necessary for the decision, which actually hangs on your preferences. My two cents here:

- for planetary imaging take the RC8, with longer focal length you will reach a larger image scale at the focus, even more if you use a Barlow. You will be able to work out a lot of detail in planets and moon with this.

- for imaging extended objects go with the f/4, this will give you a wider field and, when fitted with coma corrector, you will achieve near perfect images of large star fields, nebulae, galaxy clusters, etc. Even for discovering comets and variable star work this will be an excellent decision.

Good luck and clear skies!

 

[MattJenko]

My view would be that you already have an ED80 for widefield. Get the RC. You can get a reducer to bring the FL down should you so wish. For galaxies and other little objects, the F-ratio matters little in reality as you are going to be gathering ALL the photons from the galaxy on both the native FL as well as the reduced FL on your sensor for all but the biggest galaxies, so fast/slow, makes no real difference. Also, you are going to have to guide I would suggest with either option.

[DHEB]

15 minutes ago, MattJenko said:

My view would be that you already have an ED80 for widefield. Get the RC. You can get a reducer to bring the FL down should you so wish. For galaxies and other little objects, the F-ratio matters little in reality as you are going to be gathering ALL the photons from the galaxy on both the native FL as well as the reduced FL on your sensor for all but the biggest galaxies, so fast/slow, makes no real difference. Also, you are going to have to guide I would suggest with either option.

Good advice!

[ollypenrice]

From what I've seen of them, and I've worked on one recently, there is nothing remotely easy about the 8 inch RC. Frankly I thought it was a pig. Every time we adjusted one parameter it changed another. In fact it's a pretty difficult beast to collimate and lots of people never manage it. When it's good it's good, though. Trouble is, that can be said about all sorts of difficult scopes.

The 8 inch F4 is another difficult scope. When sorted they seem pretty good but only when sorted.

So why, if you think the 8 inch F4 will have insufficient FL, do you want an F4 at all? Why not have an F5, far easier to collimate and with a considerably longer FL? I think it's a better bet all round.

What camera are you using? If it's a DSLR you are going to be working at a very fine pixel scale at 1600mm. Just as an example, an EOS1100D would be at 0.66 arcsecs per pixel. That would require phenomenal guiding and phenomenal seeing. If you didn't achieve the necessary level of guiding or have the necessary seeing the long focal length would be a complete waste of time. You would resolve as much detail, faster, and have a wider field of view, in the shorter FL scope. An 8 inch F5 would give you 0.97"PP. I would put a lot of money on the 8 inch F5 giving the same detail, in a wider field and in a fraction of the time required by the RC. Remember that you cannot bin a one shot colour camera 2X2 so you would be stuck with about 0.6"PP.  I've imaged at that scale but I live at an elevated dark site, was using a Mesu 200 mount and taking 20 hours on an image. 

To check out your pixel scale there's a calculator here: http://www.12dstring.me.uk/fov.htm  Once you go below a second or so everything has to be going your way - absolutely everything.

Olly

[Chris]

I agree with Olly, get a 200mm f/5 newt with 1000mm focal length and have a relatively easy life.

[Gunsnroses]

Not sure about all that "arc second" stuff or "PP" stuff, but I can answer a few of the other questions.

I am imaging with a DSLR. It's a70-D crop camera. Unmodded.

The reason I am/was considering a f4 Is the shorter tube length. The f/4 and RC are short. The f/4 is only 30 inches and the f/8 is 22 inches.

The F/5 is almost 40 inches.

The weight of the three are pretty close at around 16/18 lbs.

I am looking at a focal length of around 1000mm to 1200mm.

That gives me a pretty good magnification to allow me to frame things like the Orion Nebula and Horsehead, but also enough magnification to get detail on objects like the Dumbell and Ring Nebula as well as galaxies.

The F/8 RC is way to long at 1600mm.

And I can't find any info on what a focal reducer brings the f8 down to.

The attractive things about the f/8 is how well it will hold collimation, short tube and the focuser being on the back. Unattractive thing is the focal length of 1600mm. If a focal reducer will bring that down to 1200mm or so, then that's the way to go cause I could use the 100mm for planets and the moon.

The 8 inch f/4 is only 800mm. I would like a bit more magnification that the f/5 will provide. But that extends my ota almost 10 inches in length. But puts me at 1000mm.

Soooooo....I guess the main question I need answered is where a focal reducer puts me with the RC. What focal length will I be working at with a good focal reducer.

 

[Gunsnroses]

This is what I'm getting now. But I want to get a bit closer to my subject to resolve more detail.

[ollypenrice]

9 hours ago, Gunsnroses said:

Not sure about all that "arc second" stuff or "PP" stuff, but I can answer a few of the other questions.

I am imaging with a DSLR. It's a70-D crop camera. Unmodded.

 

It's very important to understand resolution in arcseconds per pixel when deciding what to buy. It's also easy to understand: the measurement arcsecs per pixel just tells you how large a piece of sky lands on each pixel. If a lot of sky lands on one pixel you have low resolution (low detail) and if only a little piece of sky lands on each pixel then you have high resolution (high detail.) The trouble is that high resolution comes at a cost. First of all the atmosphere causes incoming light to move around, so when this effect (the 'seeing') is bad it is impossible to resolve below a certain number of arcseconds per pixel whatever your setup can do in theory. Opinion varies as to what is a useful minimum number of arcseconds per pixel to aim for but I think that around 1"PP is going to give lots of detail that you really do have a hope of capturing for real. Below that I honestly doubt that you will resolve anything real. This puts your maximum useful focal length at something like a metre. If you took the Trifid at a metre FL and at 1.5 metre FL and compared the images my bet is that the 1.5m image would contain no more detail than the 1.0m image. The Trifid would be bigger but you would be able crop and enlarge the smaller one and it would look the same.

Above all DSLRs thrive on fast F ratios. I can't see the RC doing you any favours even wih a reducer. I'd be looking at the Newts but I still think you'd probably have more fun at F5 than F4.

Olly

 

[acey]

On 20/05/2016 at 02:26, Gunsnroses said:

 F/8 is pretty slow. So longer exposures will be necessary.

Light capture is a function of aperture, nothing else. Cameras have fixed focal length and variable aperture, which is why "fast" and "slow" have come to be used in the way they have in photography. Applied to telescopes, the terms are purely metaphorical. Two telescopes with the same unobstructed aperture and transmission capture exactly the same amount of light over equal exposure times.

[toxic]

i use the Skywatcher 200 pds 200mm/1000mm-F5 and i love this scope i have used the eos 11ood with it and it was great now i use the atik 383l mono with it and the sky watcher coma corrector and it is great. i also had the 80 ed but i sold that and got an altair lightwave 60edt for a wider field of view.

but getting back to the 200 pds the detail you can get with this scope is just astounding to me, it is only £290 from flo and the cheapest rc8 is around £800

eg this is a crop of the center of the Pacman nebula from last year 1/8th of the frame. there are lots of images on here that are better so this is not the best you can expect from a 200 pds.

[Epicycle]

15 hours ago, Gunsnroses said:

Not sure about all that "arc second" stuff or "PP" stuff, but I can answer a few of the other questions.

I am imaging with a DSLR. It's a70-D crop camera. Unmodded.

The reason I am/was considering a f4 Is the shorter tube length. The f/4 and RC are short. The f/4 is only 30 inches and the f/8 is 22 inches.

The F/5 is almost 40 inches.

The weight of the three are pretty close at around 16/18 lbs.

I am looking at a focal length of around 1000mm to 1200mm.

That gives me a pretty good magnification to allow me to frame things like the Orion Nebula and Horsehead, but also enough magnification to get detail on objects like the Dumbell and Ring Nebula as well as galaxies.

The F/8 RC is way to long at 1600mm.

And I can't find any info on what a focal reducer brings the f8 down to.

The attractive things about the f/8 is how well it will hold collimation, short tube and the focuser being on the back. Unattractive thing is the focal length of 1600mm. If a focal reducer will bring that down to 1200mm or so, then that's the way to go cause I could use the 100mm for planets and the moon.

The 8 inch f/4 is only 800mm. I would like a bit more magnification that the f/5 will provide. But that extends my ota almost 10 inches in length. But puts me at 1000mm.

Soooooo....I guess the main question I need answered is where a focal reducer puts me with the RC. What focal length will I be working at with a good focal reducer.

 

The most common reducer for the RCs is the astrophysics telecompressor ccdt 67, you can happily make the RC an f6,

 

e.

[ollypenrice]

47 minutes ago, toxic said:

i use the Skywatcher 200 pds 200mm/1000mm-F5 and i love this scope i have used the eos 11ood with it and it was great now i use the atik 383l mono with it and the sky watcher coma corrector and it is great. i also had the 80 ed but i sold that and got an altair lightwave 60edt for a wider field of view.

but getting back to the 200 pds the detail you can get with this scope is just astounding to me, it is only £290 from flo and the cheapest rc8 is around £800

eg this is a crop of the center of the Pacman nebula from last year 1/8th of the frame. there are lots of images on here that are better so this is not the best you can expect from a 200 pds.

That's pretty convincing! To my mind this gives a 'real world' resolution and operates at a 'real world' F ratio, fast but not too fast.

4 hours ago, acey said:

Two telescopes with the same unobstructed aperture and transmission capture exactly the same amount of light over equal exposure times.

They do, but the longer focal length puts that light onto more pixels meaning less light per pixel, so it then becomes important to be able to expose for long enough to get over the read noise. 

Olly

 

[Gunsnroses]

Soooo, I guess I'm going to try out the 8" f/5. It's relatively inexpensive so it won't be a big loss if it doesn't turn out to be what I want.

Going to have to get tube rings, Losmandy style dovetail, Baader MPCC III and a Moonlite focuser.

Also have to invest into sone collimation tools. Not sure on those just yet.

Maybe Hotech or Glatter.

But I do want a Cheshire to start off with so I can center the secondary once I put the new moonlite on.

Guess we shall see how it goes.

[ollypenrice]

Dead set on Moonlite? I don't think much of the one I've just acquired on a used Meade ACF. It's very pretty to look at but has the fatal flaw that the friction roller runs on the smooth surface of the draw tube. This is a big mistake. Where is the friction? The Baader Diamant or FT Crayfords use a grippy strip on the drawtube. On a Dob, for visual, the focuser is always horizontal but the best place on a Newt is like this, which means the Drawtube is often under tension:

https://pietervandevelde.smugmug.com/Pics/Equipm/i-gStw3gg/A

A friend bought three Moonlites for his triple rig and had to get rid of them all, though that was in a robotic application. Mine is also proving unconvincing so far in robotic use, but it is too early to say for sure that it won't work. Still, there are far better focusers.

Olly

[Uranium235]

The tube rings that come with the 8" newt are perfectly adequate, and there is no need for fancy collimation tools - a cheshire will do the job nicely. The only thing you need to fork out for is a losmandy dovetail long enough to maintain a decent spacing of the tube rings.... even so, I'd probably find a solid (ADM style) Vixen bar being up to the job. You may also need a vixen bar for the top of the tube too, firstly for more ridgity, and secondy to allow you mount a small guidescope or finderguider (preferably the latter).

[DHEB]

5 hours ago, Gunsnroses said:

Soooo, I guess I'm going to try out the 8" f/5. It's relatively inexpensive so it won't be a big loss if it doesn't turn out to be what I want.

Going to have to get tube rings, Losmandy style dovetail, Baader MPCC III and a Moonlite focuser.

Also have to invest into sone collimation tools. Not sure on those just yet.

Maybe Hotech or Glatter.

But I do want a Cheshire to start off with so I can center the secondary once I put the new moonlite on.

Guess we shall see how it goes.

I believe this is a good decision. I have a Skywatcher 200 PDS, 200 mm f/5, with a Baader MKIII coma corrector, on a EQ5 dual axis eq mount and I am very happy with it. It is a fairly affordable and very good instrument. As for collimation, I believe that you can teach yourself to do it perfectly without any instrument other than your eyes and the stars. I just drilled a 1mm hole in the plastic cap of the 1.25" adapter and do the first rough collimation visually through it. Then, at night, I do the final star collimation by first centering the diffraction rings seen of a bright star, and then, if seeing allows, doing the same on the rings of a second magnitude star seen through a barlowed short (6mm) eyepiece. I check collimation often, but I find that I seldom have to readjust collimation in a substantial manner, just tiny bits every some weeks. Good luck!

[Uranium235]

11 minutes ago, ollypenrice said:

Dead set on Moonlite? I don't think much of the one I've just acquired on a used Meade ACF. It's very pretty to look at but has the fatal flaw that the friction roller runs on the smooth surface of the draw tube. This is a big mistake. Where is the friction? The Baader Diamant or FT Crayfords use a grippy strip on the drawtube. On a Dob, for visual, the focuser is always horizontal but the best place on a Newt is like this, which means the Drawtube is often under tension:

https://pietervandevelde.smugmug.com/Pics/Equipm/i-gStw3gg/A

A friend bought three Moonlites for his triple rig and had to get rid of them all, though that was in a robotic application. Mine is also proving unconvincing so far in robotic use, but it is too early to say for sure that it won't work. Still, there are far better focusers.

Olly

Olly, Im not even sure whether its worth him upgrading the focuser until he's actually tried the stock one. It's fine for a DSLR, or with a little tweaking - a lightweight CCD and 1.25" filter wheel. No point in fixing something that isnt broke   (yet)

Have to agree in regard to mounting the OTA (with the drawtube parallel to the counterweight bar), thats how Ive always had mine.

[ollypenrice]

3 minutes ago, Uranium235 said:

Olly, Im not even sure whether its worth him upgrading the focuser until he's actually tried the stock one. It's fine for a DSLR, or with a little tweaking - a lightweight CCD and 1.25" filter wheel. No point in fixing something that isnt broke   (yet)

Have to agree in regard to mounting the OTA (with the drawtube parallel to the counterweight bar), thats how Ive always had mine.

Good point. Try the standard one first. 

The scope in my link above is Pieter Vandevelde's self-built fast corrected Newt. If I were to image with a Newt I'd just do what Pieter does because his images speak for themselves - as do yours.

Olly

 

[Uranium235]

Yep I did take a look at that newt, looks really nice... precision built from scratch, reminds me of a pumped-up AG10.

[toxic]

there is nothing wrong with the stock focuser all i did with mine is shorten the draw tube by 25mm so it didnt protrude to much into the scope. you don't need to spend loads on a scope just a few little tweaks and you have a great scope.but you don't even need to do these mods at first.

 

[Gunsnroses]

Uranium235, the 8 inch f/5 doesn't come with rings. Only the f/4. But your right, those cheap Orion rings do work good.

And I already have one of the  Orion universal wide dovetails. And it's rock solid. But everyone is out of them. I had forgotten about Anthonys dovetails though. I'll have to give him a call.

As for the Moonlites..6 years ago, and for many years before that, I bought Moonlites for all my scopes.

I always enjoyed them. But back then, I only done visual observing. And only dabbled in astrophotography. 

I talked to Ron again yesterday and we discussed a CR2 with 2" compression ring drawtube with tri-knob reduction and a shaft lock.

The shaft lock feature will be nice cause it will lock my focus without shifting the drawtube.

But I will check out the other focusers you posted Olly.

And just to be clear...I have been in visual astronomy for over 20 years. I had to sell off all my equipment when my wide and I bought our house. We needed that down payment. But all my gear was for visual only.

I hade a CPC 1100 with Earth win Binoviewers and a CGE withan Orion 10 inch newtonian. 

But since I've gotten back around to astronomy again, this urge to image has arisen. 

So I want to give it a try. So far with the ED80, visually, it's been a lot of fun.

Mechanically, it's been a lot of work. And the sit and wait for the camera to finish has been a bore. Lol

So after I get the imaging thing going good, I'm going to have to pick up another CPC 1100 so I can do some visual stuff while I wait.

[Gunsnroses]

Custom painted CGE and 10 inch ota with Orion 80-ED. 

[Gunsnroses]

O.k. 

Just ordered....

1- 203mm f/5 ota

1- set of 235mm rings

1- cooling fan

1- Baader MPCC III Coma Corrector

1- ADM DUP 15" universal plate

Next up will be Scopestuff things.

New collimation knobs and a Cheshire collimation tube.

Still haven't decided on the focuser yet.

Also looking for a 120v plug in converter for the Dew Not controller.

Any suggestions? 

 

 

[Gunsnroses]

Just to update, if anyone is still folowing, I ordered the 8"f4 today as well.

So I now have both scopes coming.

I will test them both with my DSLR and see if there is a focus issue.