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Best high speed telescope under £1500

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#1 Henry_Hird

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Posted 24 May 2020 - 04:47 PM

Hi all,

I'm fed up with my f/7 refractor and would like something faster to combat the British cloudy weather. What would be the fastest/best scope I could buy for (or under) £1500. Note: I've never had a newt so a particularly tricky collimation may be bad.

Thanks
Henry

#2 Kevin Ross

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Posted 24 May 2020 - 05:47 PM

For 1007 euros, the TSO 76EDPH 76mm at f/4.6 looks like a nice scope (I've never used it). Flat field (no extra flattener needed), APO, FPL-53 glass. At only 342mm focal length, it might be a bit too widefield for you. You don't say what focal length you're looking for.

 

https://www.teleskop...ture-F-4-5.html

 

The RASA 8 is incredibly fast at f/2.0, but is slightly above your budget at £1,839 from First Light Optics.

 

The TS ONTC series of Newtonians are cheaper, but they will require purchasing some add-ons (coma corrector, collimation tools) that will probably go above your budget as well.


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#3 Stelios

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Posted 24 May 2020 - 06:54 PM

If you have never had a Newt, they tend to have their own problems (not to mention diffraction spikes) unless  you buy a quality one like the UNC and ONTC Newts which are not cheap. With most Newts you have to figure on coma correctors and collimation tools at a minimum, possibly focuser upgrades.

 

What size targets are you thinking of? For widefield, Kevin made a great suggestion. Your best bet in other focal lengths will be native F/6 scopes with available reducers to bring them down to F/4.8. At F/4.8 you will be getting signal 2.13 times faster than at F/7.



#4 OhmEye

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Posted 24 May 2020 - 07:34 PM

I bought my F/7 refractor with the intention of using a reducer to use it at F/5.5. I could go faster with more reduction but am happy enough with F/5.5

 

For relatively modest cost you could use a reducer on your ED102, especially with your small sensor camera. The TS-Optics 0.74x reducer has a circle of 44mm for full frame sensors and should give you a nice flat field 530mm at F/5.2. Without knowing what focal length you want or how fast "faster" means to you it's hard to say. In that price range everything I know of involves something with longer reach using a reducer.


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#5 ks__observer

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Posted 24 May 2020 - 08:15 PM

I think the top telescope you can own is an 8in f/4 Orion Newt :)

Mine is upgraded with a Moonlite focuser.

IMO collimation difficulty is over-blown.

I highly recommend just the $5 collimation cap over a laser -- I think you can get higher precision with a cap -- and very easy to use -- just look in the tube and adjust the knobs by hand.  I also have a Farpoint autocollimator that i use as a fancy collimation cap for higher precision collimation -- but still easy.

The secondary mirror you may have to tweak once a year -- and when you first get it.

I found that the difficulty with my Newt came from adjusting for mirror-flop on the meridian shift and change of position -- it took me a little time to find the right balance of tightening the mirror clips and avoiding pinching.  Knock-on-wood I have not an issue in a while.

Once you have things running it is very easy to operate.



#6 Henry_Hird

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Posted 25 May 2020 - 02:46 AM

I think the top telescope you can own is an 8in f/4 Orion Newt smile.gif

Mine is upgraded with a Moonlite focuser.

IMO collimation difficulty is over-blown.

I highly recommend just the $5 collimation cap over a laser -- I think you can get higher precision with a cap -- and very easy to use -- just look in the tube and adjust the knobs by hand.  I also have a Farpoint autocollimator that i use as a fancy collimation cap for higher precision collimation -- but still easy.

The secondary mirror you may have to tweak once a year -- and when you first get it.

I found that the difficulty with my Newt came from adjusting for mirror-flop on the meridian shift and change of position -- it took me a little time to find the right balance of tightening the mirror clips and avoiding pinching.  Knock-on-wood I have not an issue in a while.

Once you have things running it is very easy to operate.

 

 

I bought my F/7 refractor with the intention of using a reducer to use it at F/5.5. I could go faster with more reduction but am happy enough with F/5.5

 

For relatively modest cost you could use a reducer on your ED102, especially with your small sensor camera. The TS-Optics 0.74x reducer has a circle of 44mm for full frame sensors and should give you a nice flat field 530mm at F/5.2. Without knowing what focal length you want or how fast "faster" means to you it's hard to say. In that price range everything I know of involves something with longer reach using a reducer.

 

 

If you have never had a Newt, they tend to have their own problems (not to mention diffraction spikes) unless  you buy a quality one like the UNC and ONTC Newts which are not cheap. With most Newts you have to figure on coma correctors and collimation tools at a minimum, possibly focuser upgrades.

 

What size targets are you thinking of? For widefield, Kevin made a great suggestion. Your best bet in other focal lengths will be native F/6 scopes with available reducers to bring them down to F/4.8. At F/4.8 you will be getting signal 2.13 times faster than at F/7.

 

 

For 1007 euros, the TSO 76EDPH 76mm at f/4.6 looks like a nice scope (I've never used it). Flat field (no extra flattener needed), APO, FPL-53 glass. At only 342mm focal length, it might be a bit too widefield for you. You don't say what focal length you're looking for.

 

https://www.teleskop...ture-F-4-5.html

 

The RASA 8 is incredibly fast at f/2.0, but is slightly above your budget at £1,839 from First Light Optics.

 

The TS ONTC series of Newtonians are cheaper, but they will require purchasing some add-ons (coma corrector, collimation tools) that will probably go above your budget as well.

Hi all,

 

Sorry to not mention focal length! I'm looking for something around 400 - 800. I'm not too bothered as I can always use barlows etc. 



#7 John Tucker

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Posted 25 May 2020 - 05:13 AM

Hi all,

Sorry to not mention focal length! I'm looking for something around 400 - 800. I'm not too bothered as I can always use barlows etc.

In that focal length, speed, and price range you are pretty much looking at a Newt. I bought one of the TS units for reasons similar to yours. You’ll hear lots of conflicting commentary on collimation difficulty. It took me about 20 outings to get mine tamed and I definitely collimate every time I go out. One thing that seems key is to use an extension tube to keep the distance to the autocollimatot roughly similar to the mirror to camera distance

A reasonable compromise might be an F5 Newt. Should be almost twice as fast as F7 with easier collimation. Get a carbon tube if you can and be careful not to overload your mount. Even the 6 inch Newts run a bit heavy.

F4 is OK if you have a local mentor to help you out. But most of the posts I’ve made to CN that got zero responses were collimation related. It doesn’t seem to be a topic people have much patience with.

Edited by John Tucker, 25 May 2020 - 05:19 AM.


#8 John Tucker

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Posted 25 May 2020 - 05:46 AM

Another point.  I just noticed that your camera is a QHY183.  That baby has 2.2 uM pixels.  I'm not an expert on this sort of thing, but my understanding is that you can increase your speed about 3x by going to something like the ZWO ASI1600 with 3.8 uM pixels. 

 

Save yourself a few hundred pounds by selling the 183 and buying a 1600.  And avoid the whole collimation business.

 

I'm not one of the AP heavies around here.  But in my experience 5uM-ish pixel size is quite adequate at focal lengths of 600 or higher.  At 400 and below the smaller pixels really come into their own.  And it either case you can pick up some resolution by drizzling.


Edited by John Tucker, 25 May 2020 - 05:50 AM.


#9 ks__observer

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Posted 25 May 2020 - 06:32 AM

Re 183:

It has higher QE than the 1600.

You can bin / downsample the 183 and get whatever pixel size you want.


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#10 John Tucker

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Posted 25 May 2020 - 06:52 AM

Re 183:

It has higher QE than the 1600.

You can bin / downsample the 183 and get whatever pixel size you want.

I don't understand all the issues here.  But my impression from the little bit of reading I did on this is that sensitivity is a function of more than QE alone.  If Sony stumbled onto the magic formula for making more sensitive pixels, wouldn't they incorporate it in all their sensors?

 

Also, that binning on a CMOS camera is virtual, not physical, and as such the sensitivity goes up as the square root of binning, e.g. the sensitivity of a 2 uM pixel array with 2x binning is 2x that of the native 2uM pixels, not 4x as you would get with physical 4uM pixels. 

 

Maybe we need input from someone  with greater expertise, but I still come up with roughly 1.5x speed advantage for a 3.8 uM pixel over binned 2.2uM pixels.



#11 OhmEye

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Posted 25 May 2020 - 06:56 AM

Re 183:

It has higher QE than the 1600.

You can bin / downsample the 183 and get whatever pixel size you want.

I agree, with a caveat. If your typical seeing conditions are rarely better than 2"/pixel or you rarely use focal lengths under 600mm, a sensor with larger pixels is likely to be a better choice across the board. Otherwise the 183 can be a good choice, with the main disadvantage being a somewhat shallow full well capacity (FWC.)

 

I have no issues processing out amp glow, but I do find the FWC challenging at times to avoid clipping a lot of stars.


Edited by OhmEye, 25 May 2020 - 06:57 AM.


#12 ks__observer

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Posted 25 May 2020 - 08:06 AM

This subject re binning seems to keep popping up.

 

SNR = T/sqrt(T + Sky + temp + RN^2)

T= target photons

Sky = sky photons

temp = temperature related electrons that look like photons

RN = read noise

 

If you double the size of each side of your pixel, your area is 4x greater, and hence you collect 4x more data.

4x more data collection yields 2x SNR with a caveat re read noise.

So a 1 hour shot now turns into a 4 hour shot with a press of a button.

Respecting read noise:

CCD -- hardware binning you get benefit of 1 RN for the 4 pixels

CMOS -- software binning = sqrt(4RN^2) = 2RN

 

Re seeing:

Check your FWHM and how many pixels cross your FWHM.

I generally think you don't need more than 2 pixels for full resolution of data.

But it is easy to experiment to see what you like.

 

Re FWC and saturation:

Star saturation speed is a function of (app / f-ratio)^2 -- so my 200mm f/4 Newt will saturate stars a lot faster than my 102mm f5.6 APO.

You can reduce star saturation by reducing gain and reducing exposure time.

I think you can't avoid saturating the brighter stars.


Edited by ks__observer, 25 May 2020 - 08:07 AM.

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#13 John Tucker

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Posted 25 May 2020 - 12:27 PM

This subject re binning seems to keep popping up.

 

SNR = T/sqrt(T + Sky + temp + RN^2)

T= target photons

Sky = sky photons

temp = temperature related electrons that look like photons

RN = read noise

 

If you double the size of each side of your pixel, your area is 4x greater, and hence you collect 4x more data.

4x more data collection yields 2x SNR with a caveat re read noise.

So a 1 hour shot now turns into a 4 hour shot with a press of a button.

Respecting read noise:

CCD -- hardware binning you get benefit of 1 RN for the 4 pixels

CMOS -- software binning = sqrt(4RN^2) = 2RN

 

Re seeing:

Check your FWHM and how many pixels cross your FWHM.

I generally think you don't need more than 2 pixels for full resolution of data.

But it is easy to experiment to see what you like.

 

Re FWC and saturation:

Star saturation speed is a function of (app / f-ratio)^2 -- so my 200mm f/4 Newt will saturate stars a lot faster than my 102mm f5.6 APO.

You can reduce star saturation by reducing gain and reducing exposure time.

I think you can't avoid saturating the brighter stars.

Not sure I followed. 

 

Isn't S/N = (Signal from target)/(signal from all noise sources)?  Why the square root?

 

As you combine N subs, the "real" signals (target and sky, which are the same in each sub), should increase linearly with N, and the "random noise" signals, whose magnitude at any X,Y coordinate varies (presumably in some sort of normally distributed manner) should increase as the square root of N.

 

So reasoning from first principles and not being terribly educated on this topic, I would have come up with the following for a multi-sub stack:

 

S/N = Ns/[sqrt(N)*(t + rn) + N*sky)], where s is the signal from a single sub, t is the temperature noise from a single sub, rn is the read noise from a single sub, and sky is the sky signal from a single sub.

 

In truly dark skies this equation reduces to S/N = sqrt(N) * s/(temp + read noise), which is the analogous to the equation we use in NMR spectroscopy, which is conceptually similar.

 

Virtually combining the data from adjoining pixels would then follow the same math as increasing the number of scans: Since you are adding the data from the pixels together after the data was collected, the read and thermal noise increase as well.  But being random they increase as the square root of the number of pixels combined.  And the signal increases linearly so you get the S/N increasing as the square root of the number of pixels combined.


Edited by John Tucker, 25 May 2020 - 12:29 PM.


#14 ks__observer

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Posted 25 May 2020 - 12:46 PM

As you combine N subs, the "real" signals (target and sky, which are the same in each sub), should increase linearly with N, and the "random noise" signals, whose magnitude at any X,Y coordinate varies (presumably in some sort of normally distributed manner) should increase as the square root of N.

 

Target and sky are not the same in each sub.

A pixel looking at a spot in the sky may collect 50 photons in the first exposure, 68 photons the next, 43, the next, etc.

The more data you collect the more accuracy you get.

Check:

 https://jonrista.com...phy-basics/snr/


Edited by ks__observer, 25 May 2020 - 12:47 PM.

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#15 Stelios

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Posted 25 May 2020 - 01:48 PM

Guys, please get back on topic. The OP did not ask for signal to noise analysis or for camera options. Let's focus on getting him a scope in his budget. Thanks.


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#16 Henry_Hird

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Posted 25 May 2020 - 02:14 PM

Guys, please get back on topic. The OP did not ask for signal to noise analysis or for camera options. Let's focus on getting him a scope in his budget. Thanks.


Thank you.

#17 John Tucker

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Posted 25 May 2020 - 02:47 PM

Target and sky are not the same in each sub.

A pixel looking at a spot in the sky may collect 50 photons in the first exposure, 68 photons the next, 43, the next, etc.

The more data you collect the more accuracy you get.

Check:

 https://jonrista.com...phy-basics/snr/

Thanks.  I thought I had discussed with in a post Jon joined in on a few months back and that he said that sky background wasn't noise in the same sense because the light is actually there, as opposed to other types of noise that aren't "really there" and being stochastic in nature, gradually average out. 

 

But theory isn't my strong suite and so I'll defer to your judgment.


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#18 OhmEye

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Posted 25 May 2020 - 04:19 PM

Some of the recommendations above got me looking more deeply into a 8" newt specifically for speed at the 800mm focal length.

 

There are a few options I know of, the TS 8" F/4 ONTC is at the high end of the OP's budget with not much room for accessories. Most if not all of the 8" F/3.9 look like the same OEM scope as the Orion, and that's the one I'm looking at.

 

  • The steel tube OTA is $499, so some good headroom for upgrades and accessories (I admit if I saw a good Memorial Day sale for this I might have pulled the trigger, probably best that I didn't see a sale!) lol.gif
  • A coma corrector seems to be a must. The Baader MPCC mkIII seems popular with fair reviews for around $200.
  • The focuser reportedly should be replaced. The Moonlite CR with a mounting kit looks like around $250 without a motor. I'm not 100% sure about that but that's how I read it on the Moonlite shop though I may be misunderstanding the options.
  • I'm pretty sure I could mount a $200 ZWO EAF to the moonlite with some minor modification, not 100% sure though.
  • The OTA comes with a collimation cap, but I would add on a $30 laser collimator with the understanding the collimator itself will need to be collimated out of the box.
  • I'm assuming an existing guidescope can be used, if it worked with an ED102 it should work on the newt.
  • If there's extra budget or desire, the carbon fiber version of the OTA is available. I've read very mixed opinions about whether the cost difference is worth it for the CF version.
  • I already have a long losmandy style dovetail, it looks like that doesn't come included with the Orion OTA.

 

I'm kicking this idea around for myself as well, this rig would be more than 3x faster than my APO's native speed for a relatively low cost. For myself, I would only need to buy the OTA, coma corrector, and focuser to have a F/3.9 rig for under $900. I find myself thinking this would be fun to try.

 

Where I'm most uncertain is which Moonlite focuser is needed, which mount kit and drawtube size, and if using my ZWO EAF would be possible.


Edited by OhmEye, 25 May 2020 - 04:36 PM.


#19 ks__observer

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Posted 25 May 2020 - 04:35 PM

Get a Farpoint Autocollimator instead of a laser:

 https://farpointastr...tocollimator-2/

 

I notice very little focus shift -- so not sure a carbon fiber is worth the extra money.

 

I suggest a ZWO OAG with 290 mini over a guidescope.

 

I use a Exp Sc Coma Corrector -- and I think it does just fine.




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