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Help choose a good scope to image galaxies


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Hi Guys,

 

I have that equipment itch again and I'm in the market again to buy a new telescope to use for imaging. I would most likely mount it on my EQ6R mount so weight wise there are lots of options. I am looking to fill a gap in my equipment and get something with a longer focal length to take images of galaxies. Currently I have an Esprit 100 and redcat so wide and wider fields of view are covered, now it's time to get up close and personal.

I would be using the telescope with a ZWO 1600 and filter wheel.

I have made a shortlist, with some of my pros and cons by each scope. 

 

Celestron Edge HD 9.25 £2450 (FLO) https://www.firstlightoptics.com/edge-hd-series/celestron-edge-hd-series.html

 

Pros

Good focal length

Tried and tested (lots of advice out there from other owners)

 

Cons 

Top of my budget (All in including accessories is around £3000)

Not sure I like fat looking stars (I love the crisp images you get with refractors)

Cooling time

 

Explore Scientific ED 127 £1330 (RVO) https://www.rothervalleyoptics.co.uk/explore-scientific-ed-apo-127mm-f75-aluminium-essential-triplet-refractor-ota.html

 

Pros

I like the star shape and crisp images

Bottom end of my budget leaves me change to spend elsewhere

 

Cons 

Focal length is not as much as the other options, plus with a reducer it's even less. However, it is still much more than what I have currently.

Apparently the length of the OTA can cause issues with tripod legs 

 

Orion Optics Newtonian 350/1600 £2210 (Astroshop) https://www.astroshop.eu/telescopes/orion-optics-uk-telescope-n-350-1600-vx14-ota/p,57573#tab_bar_1_select

 

Pros

Good focal length

Don't mind the star shape

 

Cons 

Collimation

Weight is heavy at 20 kgs (I'd be pushing my mount pretty much to the limit)

 

1197851015_Telescopecomp.PNG.681f6cbb51026a10d0a95c0e7aff3352.PNG

So what are your thoughts, I'd be happy with any of those three, but I'd be interested to hear any other ideas and suggestions. My all in budget is around £3,000 so there are plenty of really great options. As mentioned this scope is going to image galaxies so focal length is important. 

 

Thanks,

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You might want to consider this from the other end, as it were.

How accurate is your guiding and how is your seeing?  The results might not be any better imaging at 0.33 " per pixel (your first option with the C9.25) if the seeing never gets below 1.25 arc secs or the mount won't guide at better than 1.0 " .   On the other hand, if your mount guides at .5", you have nice stable skies and use a focal reducer then imaging at .5" on the good seeing nights might give very satisfying results?

 

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This is a very interesting topic!

I love the focal length of an SCT, but they have a certain softness and are generally a bit of a pig to use for imaging. I have a Celestron EdgeHD 8" and almost love it...! If I had the cash, I think I would invest in an Esprit 150 and take the hit in focal length, but enjoy the vast increase in quality. If you do go for the SCT, be warned that really the only way to successfully guide it is with an OAG.

I look forward to hearing what people reckon about the long focal length options available these days.

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Given the mount you will be using and camera you will be using, any scope with focal length over 700mm will be either over sampling or will need to bin pixels to get to good sampling rate.

700mm with 3.8um pixel size of ASI1600 will give you 1.12/px. That is really high sampling for most scopes under 8" and in principle with EQ6 class mount - you should consider that to be your upper limit (in fact - it would be closer to 1.2-1.4"/px).

127mm F/7.5 triplet would there fore be ideal scope. You are used to refractors, and with good x0.75 FF/FR (Riccardi reducer / flattener), that scope could operate at 714mm.

If you are willing to get a newtonian - 6" F/5 with good coma corrector will give you 750mm.

Next option would be to double focal length and use x2 binning.

Something in 6"-8" in RC domain would not be a bad choice?

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29 minutes ago, vlaiv said:

Given the mount you will be using and camera you will be using, any scope with focal length over 700mm will be either over sampling or will need to bin pixels to get to good sampling rate.

 

58 minutes ago, almcl said:

You might want to consider this from the other end, as it were.

How accurate is your guiding and how is your seeing?  The results might not be any better imaging at 0.33 " per pixel (your first option with the C9.25) if the seeing never gets below 1.25 arc secs or the mount won't guide at better than 1.0 "

 

This is something to definitely consider, and also frustrating as I have only recently switched to a ZWO 1600 thinking that it would fit for most of what I want to do for now. Is there a more suited camera for longer focal lengths?

 

32 minutes ago, vlaiv said:

127mm F/7.5 triplet would there fore be ideal scope. You are used to refractors, and with good x0.75 FF/FR (Riccardi reducer / flattener), that scope could operate at 714mm.

 

Something in 6"-8" in RC domain would not be a bad choice?

The Explore scientific was high on my list, any suggestions on a RC OTA?

 

34 minutes ago, vlaiv said:

If you are willing to get a newtonian - 6" F/5 with good coma corrector will give you 750mm.

 

This scope was on my radar as well, perhaps something to consider? https://www.365astronomy.com/sharpstar-20032pnt-200mm-super-fast-f-3.2-paraboloid-newtonian-reflector-telescope-red.html It does have a built in reducer that brings the focal length down to 640.  Also I have heard good stuff about the build quality.

 

Best,

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6 minutes ago, 5haan_A said:

 

This is something to definitely consider, and also frustrating as I have only recently switched to a ZWO 1600 thinking that it would fit for most of what I want to do for now. Is there a more suited camera for longer focal lengths?

 

The Explore scientific was high on my list, any suggestions on a RC OTA?

 

This scope was on my radar as well, perhaps something to consider? https://www.365astronomy.com/sharpstar-20032pnt-200mm-super-fast-f-3.2-paraboloid-newtonian-reflector-telescope-red.html It does have a built in reducer that brings the focal length down to 640.  Also I have heard good stuff about the build quality.

 

Best,

Only issue is I don't see a great difference in FOV.

 

504736965_Telescopecomp2.PNG.00bf878b9d14176ea0289737a9375a7f.PNG

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1 hour ago, 5haan_A said:

This is something to definitely consider, and also frustrating as I have only recently switched to a ZWO 1600 thinking that it would fit for most of what I want to do for now. Is there a more suited camera for longer focal lengths?

ASI1600 is perfectly capable of doing what you are asking. There are better cameras but for different reasons (I can explain those as well).

We really need to first understand FOV / pixel scale and galaxy size in the image and how all of those fit together.

Let's take M63 as our case study, although you won't be imaging that galaxy for another half of the year (it will be positioned appropriately come springtime).

I just fired up Stellarium and did very quick measurement of extent of this galaxy - it is about 15 minutes of arc or 900 arc seconds. Given that you have EQ6 mount and that average skies are as they are and so on and so on (we can discuss that as well but it is rather technical discussion) - you need to sample at about 1.2"/px (in reality I would advocate for 1.2-1.4 but you already have scope that samples at 1.43"/px).

Let's say you go for 1.2"/px. How large would that galaxy be when looking at it at 100% zoom level (1:1 or one image pixel to one screen pixel) - only 750px wide. Here it is:

image.png.3a565d7e3a94e90c38b0d9bca07282f2.png

This is it - look at this image at 1:1 (if you have larger computer screen and you are not reading this on your phone - you should be able to see it displayed at 1:1 on web page).

You can't get closer than that. You can - but:

1) there is no point, you wont capture any additional detail

2) you will waste your SNR because light will be spread over more pixels and each pixels will get less signal overall - less signal means lower SNR. This is why you don't want to over sample.

Very important point number one - when you have your pixel scale set - galaxy image at 1:1 will always look the same size regardless of the scope and camera used to capture it.

Now look at this:

image.png.8fcfcd083b7765bf92f5b8931e3af19e.png

This compares field of view between two very different setups. First is ASI290 with 80ED and matching x0.85 FF/FR giving resulting focal length of about 510mm - sampling resolution ~1.2 (I rounded it up it is just a bit shy of 1.2 at 1.17"/px but that is not important for what I'm trying to show here) vs Skywatcher 130PDS newtonian at 650mm focal length and ASI1600 - again sampling at 1.2"/px

We have seen from rule number one that galaxy image will have the same size when viewed at 1:1 - it will be ~750px across. On the other hand, these two setups have very different FOVs.

How come and what does it mean? Explanation is really simple - ASI1600 has 4656px in width while ASI290 has only 1936px. If each pixel covers same area of the sky - ASI1600 will simply cover more of the sky. Remember - galaxy size on the image when viewed at full size / 100% zoom / 1:1 will look the same.

Very important point number two - think of FOV in terms of sky covered by certain number of pixels at given resolution. More pixels you have - more sky is covered. You can crop away empty sky that does not contain target for better framing.

If in above image you crop away empty pixels of ASI1600 and leave only central 1936 pixels - you will get same image as with ASI290! Same FOV and same galaxy size when viewed at 1:1.

Only issue with having a large FOV is when image is viewed at "screen size" - that means whole FOV is scaled so it can fit on screen. If you have more pixels in your image than there are pixels on the screen that you are using to view the image - image will be shrunken down and galaxy will look smaller. More additional / empty FOV you have in this case - galaxy will look smaller and smaller.

Now very important point number three :D

Since galaxy of interest is only 750px wide and most other galaxy targets are of similar size or frequently smaller than that, 4656px that ASI1600 provides is much more than you need. To solve this you can either crop or you can bin.

It is waste of sensor area to capture empty space around galaxy so perhaps better solution is to bin. What is binning? It is taking group of 2x2 pixels and making them act as single pixel. In fact, you can take groups of 2x2 or 3x3 or 4x4 pixels and so on...

Let's examine ASI1600 with pixels binned 2x2 - what does it mean for us? First, you'll no longer have 4656px in width, you'll now have 2328px in width (similarly half of pixels in height as well). Much less wasted space around the galaxy 750px wide. Second thing that happens is that your pixels now "grow". They are no longer with their sides 3.8um - they now have twice as much - 7.6um.

This in turn means that you can use twice as long focal length and still have 1.2"/px. We no longer need 650mm, we can now look at scopes that have 1300mm of focal length. This in turn has very important consequence - longer focal lengths also mean larger apertures - more light gathered and faster imaging. Very good!

Here is another example of galactic imaging setup and its FOV:

image.png.8f2539e1bb77009b6a27203f2692ba45.png

This is 1370mm with ASI1600 and 6" aperture. This is 6" RC scope (also available as GSO, AltairAstro and TS branded scope. This scope is made out of mirrors only and does not require much in terms of correctors / reducers and such. Problem is that it is reflector - needs collimation and many people find collimating RC very intimidating - but it's not that hard really.

Just to address your post from the beginning - could C9.25 EdgeHD be good galaxy imaging scope? Only if you use it correctly. You want to sample at 1.2"/px or there about as I doubt that your mount/guiding and skies will allow for more. What sort of FOV can you expect with ASI1600?

image.png.ec05d6117acf4ad5026e32740bf1cfdc.png

Ok, not bad, but you have 2350mm of focal length, how do you get 1.2"/px with ASI1600?

Well, if you bin your camera 4x4 - you will get 1164 x 880px camera and sample at 1.33"/px - which is to be expected, after all above FOV is pretty tight and if galaxy with its extent is 670px (sampled at 1.33"/px - same 900" but less pixels as pixel covers bigger part of the sky - rule #1) - then there is only something like 250px left on each side of empty sky.

Your images will be small by today's standards when everyone has 1920x1080 or higher res screen, but you will have very fast system that has 9.25" of aperture to collect photons.

In the end, what camera is better than ASI1600 in the light of above rules? - One that is larger - it allows for larger scopes / larger aperture while still retaining same FOV / resolution (if properly binned). For example ASI6200 paired with 16" RC will deliver same FOV as ASI1600 paired with 8" RC.

It has about x2 more pixels in both width and height and binning those pixels can make same resolution as binning ASI1600.

Since all else can be made equal, we are left with 16" of aperture collecting light vs 8" of aperture collecting light - that is x4 increase in light. Which setup do you think is faster :D

Hope this helps a bit when choosing a telescope.

I think I would go for either 1300mm (which means binning x2 and 2300x1700 image size) or 2000mm (which means binning x3 and images that are roughly 1500 x 1100 px in size).

You can go for 650mm scope and crop empty sky - in fact, you already have 550mm scope. Just take one of your images with smaller target and crop away empty sky and see how you like different FOV (this won't change details in the image - target will still be the same size when viewed at 100% - or 1:1).

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Vlaiv gave an exceptional answer. I did try to image with a 12 inch SCT and a Mesu mount. I found it very frustrating, not because of the mount, but because of the scope. The 2500 focal length was too much, minor but evident mirror shift after a merdian flip, minor but evident shift if you focus by moving the mirror. I am going the refractor route myself. If you want a mirror scope, that's just fine, but pick one with a stable mirror like an RC or a CDK. 

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Lots to think about!

Thanks Vlaiv for the explanation it was insightful enough for me to come to some initial conclusions. Firstly that I am not going to go for a massively long focal length just for the sake of it. Secondly that I will bin, I had never considered binning and always kind of viewed it as something  that people do when they are in a rush, but it  makes a lot of sense especially when trying to reduce the amount of sampling. 

Also thanks for the guys giving me their thoughts on SCTs, the celestron edge was a real front runner in my list but after the comments from you guys I think I'd prefer to make my life easier not harder. 

With all that being said, I have done some research and have identified two potential candidates for a longer focal length scope. 

 

Option 1 is to go with the refractor

Explore Scientific ED 127 £1330 (RVO) https://www.rothervalleyoptics.co.uk/explore-scientific-ed-apo-127mm-f75-aluminium-essential-triplet-refractor-ota.html

Option 2 is to go with this RC 

TS-Optics 8" f/8 Ritchey-Chretien Astrograph with Carbon Fiber Tube https://www.365astronomy.com/ts-optics-8-f-8-ritchey-chretien-astrograph-with-carbon-fiber-tube.html

I'd bin at 2x as it's got a focal length of 1624 mm. 

Here is the FOC calculator I put in a 10 inch version of option 2 as well. As you can see the difference isn't massive considering the £1500 and nearly 10 kg weight difference.

1318425909_Telescopecomp3.PNG.6d7cefdcc8385453483671fbfb33f22f.PNG

What do you guys think? I am tending towards the RC as I think it offers a drastically different alternative to what I currently have in the locker. 

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1 hour ago, 5haan_A said:

Option 2 is to go with this RC 

TS-Optics 8" f/8 Ritchey-Chretien Astrograph with Carbon Fiber Tube https://www.365astronomy.com/ts-optics-8-f-8-ritchey-chretien-astrograph-with-carbon-fiber-tube.html

Well, I'm sort of biased on that one :D

IMG_20160730_214040.jpg

Yes, that is my TS RC8" steel OTA version in action side by side to ST102 in guide scope role here.

I like that setup and it works very good for me, but only after I've spent considerable amount of money (almost the price of scope) in improving my mount and some additions for the scope itself.

I tuned it, replaced bunch of bearings, did belt mod on it, changed puck/clamp for vixen/losmandy combo one from Geoptik and finally changed tripod for Berlebach planet short version. Now mount guides at 0.5" RMS if conditions are good and you need them to be good to fully exploit 1"/px that 1624mm and ASI1600 bin x2 give you.

I still believe that I'm oversampling most of the time and I'll be adding reducer / flattener at some point.

You might have a bit less issues with EQ6 but make sure your mount runs smooth and guides good. I use OAG with above scope now.

Scope will need change of focuser for best performance. Stock monorail 2" is decent focuser but does not offer threaded connection. I switched to 2.5" R&P version from TS - this one:

https://www.teleskop-express.de/shop/product_info.php/info/p5769_TS-Optics-2-5--Rack-and-Pinion-Focuser---holds-Acc--up-to-6-kg---travel-95-mm.html

 

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1 hour ago, vlaiv said:

I forgot - it gives rather nice FOV on galaxies with ASI1600:

M51v1.png

Lovely that's pretty much the FOV that I am looking for.

 

How do you find collimating the RC is there an easy method?

I know what you mean about accessories, the good thing is that by going for this scope I do have more budget left to invest in the right accessories.

 

Thanks, 

 

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1 minute ago, 5haan_A said:

How do you find collimating the RC is there an easy method?

I found it rather easy to do - but using particular method that requires CMOS camera (original is with DSLR - whatever is sensitive and has fast download times).

Here is tutorial that I followed - only difference being way I measured defocus in corners - I used FWHM tool in SharpCap with my ASI1600 rather than bahtinov mask (which I do have but find completely useless next to FWHM/HFR readings and even eyeballing star size in the image).

https://deepspaceplace.com/gso8rccollimate.php

 

 

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+1 for the RC8.. I pretty much wasted a whole season messing about with a SCT. My RC is the same as the carbon one you linked to. Got mine from Altair before they stopped doing them. I use it with an Astro Physics CCDT67 0.67 reducer. The stock focuser split and fell to bits early on as partly plastic & not really up to the job of hanging much off it. I replaced with a Moonlite & motor. It will have been on the mount (EQ8) for 5 years next month! I keep thinking of replacing with a large frac just to simplify life even more as I'd like to improve on stars & contrast but having said that.. glad to have anything under UK skies!.. it's mostly operated unattended via ACP

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Nice, just placed an order. Soon to be a member of the RC club. What swayed me in the end was the FOV, the fact I won't have to worry about mirror flop and that NASA used an RC design for the hubble telescope. 

Now its time to add on the various accessories, which will no doubt come to near abouts what I paid for the scope.

 

Best, 

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  • 3 weeks later...
On 20/08/2020 at 06:21, vlaiv said:

I found it rather easy to do - but using particular method that requires CMOS camera (original is with DSLR - whatever is sensitive and has fast download times).

Here is tutorial that I followed - only difference being way I measured defocus in corners - I used FWHM tool in SharpCap with my ASI1600 rather than bahtinov mask (which I do have but find completely useless next to FWHM/HFR readings and even eyeballing star size in the image).

https://deepspaceplace.com/gso8rccollimate.php

 

 

Hi Vlaiv, 

Did you tried this method on a real star?, would it be possible to perform with an artificial star?, at the moment I´m having trouble with my RC 6¨, i think it´s way out of collimation.

 

Thanks!.

Edited by Felipe
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11 hours ago, Felipe said:

Hi Vlaiv, 

Did you tried this method on a real star?, would it be possible to perform with an artificial star?, at the moment I´m having trouble with my RC 6¨, i think it´s way out of collimation.

 

Thanks!.

Yes, I did it on a real star, but doing it on artificial star should work without any problems - it should even be easier since you don't have to worry about atmosphere and changing FWHM values (I did not use a Bahtinov mask but instead looked at FWHM values to see which corner is out of focus) - just place star some distance away - like 10-15 meters (you don't have to go long distances like when checking the optics - as long as artificial star is unresolved you are fine and a bit of spherical won't make the difference).

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