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Collimation of f/4 vs f/5 Newtonian for Imaging

collimation imaging reflector
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#1 DCWZ

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Posted 05 December 2019 - 10:13 AM

Hi all,

 

I am aspiring to eventually go into galaxy imaging, which would mean I would need a longer focal length for such an endeavor. For this, I am currently aiming to get a newtonian. The types of newtonian I am considering are 6" f/5 and 8" f/4, which would give me 750mm and 800mm focal lengths. I would most likely be pairing one of these with a 1" sensor astrocam. The reason why am I not going for a bigger scope is that I live in a very light-polluted city, and require long trips to rural areas for imaging, which means portability is a big factor for me. 

 

I am very much tempted to get a 8" f/4, due to its slightly longer focal length, and even more so due to the greater speed of the telescope. However, compared to the 6" f/5, I would like to know if it is much more difficult to collimate an f/4 compared to an f/5? Would an f/4 be able to hold collimation just as well? Are there any other factors I should consider in this respect?

 

Thank you!



#2 happylimpet

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Posted 05 December 2019 - 10:45 AM

The tolerances are tighter at f4 but the processes and principles are identical. I would research into scopes which hold heir collimation well, as this is more important than the absolute focal ratio. My 12" f5 holds collimation perfectly for months at a time, but then it never goes in a car.....



#3 Eddgie

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Posted 06 December 2019 - 05:05 PM

I had read things that said collimating at f/4 was hard, but using a laser, it collimated pretty much exactly as fast as my f/5 scope.   

 

My 4" also holds collimation exceptionally well.  Since first setting it, I have not had to touch it. 



#4 DCWZ

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Posted 07 December 2019 - 12:58 AM

I had read things that said collimating at f/4 was hard, but using a laser, it collimated pretty much exactly as fast as my f/5 scope.

My 4" also holds collimation exceptionally well. Since first setting it, I have not had to touch it.


I have heard such things too, which kind of unsettles me. Which laser and which Newtonian are you using?

#5 junomike

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Posted 07 December 2019 - 07:38 AM

IME my two F4's are no more troublesome than my F4.4 or previous F6. 



#6 HydrogenAlpha

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Posted 07 December 2019 - 10:09 AM

I've never tried collimation at f/5 before, but I never found it difficult to collimate at f/4. Usually just takes about 5-10 minutes before each imaging session. My first proper imaging telescope was an 8" f/4, and this was BEFORE I started using refractors (for portability rather than anything else). Collimation isn't a difficult process like people make it out to be. 

 

Whether or not your telescope holds collimation well depends more significantly on the build of your particular OTA. I used a GSO and it does not hold collimation particularly well, even after I replaced the stock springs with stiffer ones. It is fine over the course of the night, and I recollimate before every session, so I didn't find it particularly bothersome. 

 

I'd strongly recommend you to got an f/4 over an f/5 if collimation is your only concern. You'd be happier with the speed and I wouldn't think it's a big issue. 



#7 Eddgie

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Posted 07 December 2019 - 05:55 PM

I have heard such things too, which kind of unsettles me. Which laser and which Newtonian are you using?

I am using a 6" f/2.8 PowerNewt. The base scope is f/4, but with the reducer, it is f/2.8.



#8 KLWalsh

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Posted 07 December 2019 - 08:30 PM

The quality of the mechanical parts are more important (imho) than the f/ratio of the optics. In particular you need a focuser that has no (or minimal) shift as the drawtube moves in/out. You could be perfectly collimated at one distance, then insert a filter, refocus, and be out of collimation - if you have a cheap focuser.
Similarly you need primary and secondary mirror mounts that do not shift as the position of the scope changes.
So - don’t worry about f/4 vs f/5; worry about the mechanicals that support the optics.
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#9 CCD-Freak

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Posted 08 December 2019 - 04:40 PM

You can get an 8" f5 Newt for a good price and it is easier to collimate and has 1000 mm FL.

 

Here is a shot of M83 made with a 4/3 ASI-1600MC-Cool camera with an 8" f5

 

M83-Cal-Sigma-DN-crop-Curves-Sat-mts.jpg

 

 

John

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Edited by CCD-Freak, 08 December 2019 - 04:41 PM.

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#10 StrStrck

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Posted 08 December 2019 - 04:58 PM

The “faster” the scope, the more precise your collimation has to be. And then the collimation hardware gets more expensive. 

Weight also comes into play, an 8” will be heavier, and you’ll want a mount that can handle it. What mount do you have, or have planned?



#11 DCWZ

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Posted 15 December 2019 - 11:49 PM

You can get an 8" f5 Newt for a good price and it is easier to collimate and has 1000 mm FL.

Here is a shot of M83 made with a 4/3 ASI-1600MC-Cool camera with an 8" f5

M83-Cal-Sigma-DN-crop-Curves-Sat-mts.jpg


John
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Excellent image! I intend to use it with a QHY183C/ASI183MC, and I am a little worried that the resolution would be too high for the seeing given the very large focal length and small pixel size of the camera. Do you think my concerns are valid?

#12 DCWZ

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Posted 15 December 2019 - 11:49 PM

The “faster” the scope, the more precise your collimation has to be. And then the collimation hardware gets more expensive.
Weight also comes into play, an 8” will be heavier, and you’ll want a mount that can handle it. What mount do you have, or have planned?


I intend to use it on a iOptron CEM40 (which I will purchase). Do you have any advice?

#13 CCD-Freak

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Posted 16 December 2019 - 09:35 AM

The small 2.4 µ size of the pixels of the ASI183 will mean you will be over sampled with most scopes.  If you go with 800 mm FL you will be at .62 "/pixel and typical seeing is around 2" on  many nights an every once in a while you may be treated to 1" seeing.  The M83 shot was at .78 "/pixel and the seeing was around 2" so don't be too worried about oversampling. 

 

This is the setup for the M83 image.  8" F5 on an AP900 with an ASI-1600MC-Cool

 

8F5-Imaging Newt.JPG

 

Get a good mount......it will save a lot of frustration.

 

 

John

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Edited by CCD-Freak, 16 December 2019 - 09:36 AM.


#14 StrStrck

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Posted 16 December 2019 - 04:41 PM

I intend to use it on a iOptron CEM40 (which I will purchase). Do you have any advice?

That seems to be a capable mount, but remember to do the math, add up all the weight, and try to stay near half (isn’t it?) the max payload weight. But seems like a nice mount, I’d want one for Christmas, but I may just have to settle for a lump of cole instead lol.gif . 

Going 8” f/4 I’d concider a carbon tube, to ensure temperature change won’t make it expand or shrink, and affect collimation and whatnot. How well it holds collimation is related to build quality, and it’s a thing to practise. There are different approaches. I’ve settled for the Catseye, see sig. It’s actually quite rewarding to achieve that “stack”, it’s when you use a certain center marker on the primary mirror, the Catseye lets you stack multiple reflections of that marker. 


Edited by StrStrck, 16 December 2019 - 04:41 PM.


#15 mikefulb

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Posted 16 December 2019 - 04:51 PM

I use a ONTC carbon fiber 8 inch f/4 Newtonian on a Mach1 and it feels just right.  I used to use an Atlas and it would work but you are going to be at the mercy of even light wind with that setup.  With a CMOS camera and using short exposures you can mitigate the risk versus when I was taking 10+ minute exposures with a noisy old KAF-8300 camera.




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