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Best scope for Galaxies and small DSOs

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#1 mehdymo

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Posted 18 June 2019 - 07:06 PM

Hi folks,

I have imaged a bunch of wide field DSO in narrowbands using WO GT71. I also have imaged some galaxies using RC6 in LRGB. My mount is IEQ45 Pro and I constantly get guiding RMS of 0.3-0.7 arcsec. I was thinking about the best option for small objects such as galaxies. Here is what I know:

 

1- long focal length refractors (such as ES 127 FCd100): slow (if reduced will have short focal length), long, and expensive, but no collimation is needed

2- RC: very hard to collimate, cheap, enough focal length even after corrector and reducer

3- Edge HD (such as 8"): cheaper than refractors, can provide a wide range of focal lengths using reducer and hyper star, hard to collimation? a dew magnet ( I am from Florida)

4- Newtonian: very cheap (normal newtonians), enough focal length, faster than all of the above items, easier collimation at F/5 (have heard it is hard at F/4)

 

What am i missing? If you used any of those for imaging, please let me know which one is better practically. I know they all need a kind of reducer\flattener or corrector, and yes, I don't like star spikes too, but I will get along with it. 

 

Thanks

 



#2 RaulTheRat

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Posted 18 June 2019 - 07:15 PM

The Newtonian - you forgot to mention long. Physically it's demanding on the mount, both from the point of view of moment arm, and in terms of catching any wind.

You're missing a mak-newt as well - heavier than the newt, but better corrected, doesn't need a paracorr, easier to balance because of the weight at the front. I wouldn't really call one a galaxy scope though, not long enough FL.

I don't own an RC (yet) but I do think people probably overdo the hard to collimate stuff - I heard a lot of that about mak-newts as well and meh, yes it requires care and understanding of what you're doing, but it's far from an insurmountable problem, so I suspect the RCs are the same and I plan to get one at some point, mainly because all the images I've seen from the edge have a lot of star bloat which the RC don't (but then again the RC has diffraction spikes). Dew I don't really think is a big issue - reflectix and a dew strap and you should be good.

I think the choice between the sct and RC comes down to what you can't tolerate and is not really a case of one being better just individual preference/tolerance for their particular strengths and shortcomings.

On an IEQ45 if you can tolerate the bloat from the edge, I'd seriously consider it as they are a fair bit lighter than an RC and you're going to be hard pushed to get enough image scale within your weight limitations.

Edited by RaulTheRat, 18 June 2019 - 07:19 PM.

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#3 Acer

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Posted 18 June 2019 - 07:37 PM

I was in the same predicament a few months ago and decided to go with a MN190 Mak Newt.  It's 1000mm focal length was perfect for the tiny pixels of the ASI183MM Pro that I have.  The camera also has good cropping ability for tiny galaxies since it has such high resolution, though I haven't cropped my images too much yet.

 

I have been more than happy with the scope and what I've been getting from it.  Easy to collimate, focus stays perfect all night, just an all around great scope.

 

Here's a few of the photos I've taken so far, with my (admittedly) beginner skills.

 

Crescent-Nebula-Small.jpg

 

The-Needle-182-Minutes-Small.jpg

 

M51-128-Minutes-LRGBHa-small.jpg

 

Dumbell-Nebula-Small.jpg

 

Iris-Nebula-Small.jpg


Edited by Acer, 18 June 2019 - 07:38 PM.

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#4 PeteM

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Posted 18 June 2019 - 07:38 PM

Considering your 6"RC is already at around 1370mm focal length, the 8" is not much longer and then you will start bumping into mount limits with bigger RC's. So out of the list, the 8" Edge probably is the best option. Even if the RMS of your mount is good now, adding the extra weight of a bigger OTA and more counterweights go change that.


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#5 mehdymo

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Posted 18 June 2019 - 07:39 PM

Yes, Newtonian are long and heavy as well. But, a 6" F/5 should be still shorter than a 5" F/7.5 refractor and it is a lot faster.

Well, I have an RC6 and It is really hard to collimate it ( a lot harder than Refractors). It has two parabolic mirrors and the distance between mirrors is very importnat. The tube axis sometimes is not the same as optical axis and what you see in collimation is not what is really supposed to be. I can take apart a Newtonian and recollimate it all from the scratch in less than an hour, but RC is a different world.  

The Newtonian - you forgot to mention long. Physically it's demanding on the mount, both from the point of view of moment arm, and in terms of catching any wind.

You're missing a mak-newt as well - heavier than the newt, but better corrected, doesn't need a paracorr, easier to balance because of the weight at the front. I wouldn't really call one a galaxy scope though, not long enough FL.

I don't own an RC (yet) but I do think people probably overdo the hard to collimate stuff - I heard a lot of that about mak-newts as well and meh, yes it requires care and understanding of what you're doing, but it's far from an insurmountable problem, so I suspect the RCs are the same and I plan to get one at some point, mainly because all the images I've seen from the edge have a lot of star bloat which the RC don't (but then again the RC has diffraction spikes). Dew I don't really think is a big issue - reflectix and a dew strap and you should be good.

I think the choice between the sct and RC comes down to what you can't tolerate and is not really a case of one being better just individual preference/tolerance for their particular strengths and shortcomings.

On an IEQ45 if you can tolerate the bloat from the edge, I'd seriously consider it as they are a fair bit lighter than an RC and you're going to be hard pushed to get enough image scale within your weight limitations.



#6 mehdymo

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Posted 18 June 2019 - 07:58 PM

to Acer:

 

I like your images. I didn't consider Mak-Newt as I didn't know much about it. I thought an RC with wide possible range of FL would be a better option. I think I should consider it as well. Thanks for your reply.

Here is M51 that I have taken by my RC6. The elongation comes from misalignment, but during stacking it becomes like tracking error.

Attached Thumbnails

  • Final-M51-resized.jpg


#7 the Elf

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Posted 19 June 2019 - 10:04 AM

The RC6 needs OAG to compensate for the weak rear cell. Here is what I got with the RC6 guided by the ZWO OAG on an AVX (the latter happily sold now):

http://www.elf-of-lo...rlpool2018.html

(click for full screen, link to full res image on the page)

So the question is which scope would give you more resolution or more signal. If you are not troubled collimating the RC6 you might just add the OAG and get great images.

Now I'm using the RC8 carbon on an EQ6-R reduced to 1100mm focal length. I've been considering the 190 mak newt as well. I guess the two are pretty much on the same level. Opinions?

Acer: love your images. Really great.


Edited by the Elf, 19 June 2019 - 10:09 AM.


#8 mehdymo

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Posted 19 June 2019 - 10:44 AM

Thanks for the reply. your image is great :) Well, I have used ZWO off axis guider with ASI120MM. I normally have a few guide stars in the field (1-3). I agree that the RC6 is a very good option, but collimation is a real hassle. I have to spend some good nights to recover the original focal length and then start aligning again. As I said in previous comment, normally their optical axis is not aligned with the mechanical one and it makes it really hard to align it in daytime. I think if I had a 6" F/5 reflector, I could collimate it every night in 5 minutes, but collimating an RC needs multiple nights and still can't be 100% sure if it is collimated. I think I have to buy one of those holographic collimators, if it is worth it! 

The RC6 needs OAG to compensate for the weak rear cell. Here is what I got with the RC6 guided by the ZWO OAG on an AVX (the latter happily sold now):

http://www.elf-of-lo...rlpool2018.html

(click for full screen, link to full res image on the page)

So the question is which scope would give you more resolution or more signal. If you are not troubled collimating the RC6 you might just add the OAG and get great images.

Now I'm using the RC8 carbon on an EQ6-R reduced to 1100mm focal length. I've been considering the 190 mak newt as well. I guess the two are pretty much on the same level. Opinions?

Acer: love your images. Really great.



#9 ImNewHere

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Posted 19 June 2019 - 01:33 PM

I have an 8RC and like it, but if I were going to get a new scope for faint fuzzies and it was going to be on my IEQ45 I'd probably look at the edge 9.25
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#10 Peregrinatum

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Posted 19 June 2019 - 01:44 PM

I have an 8RC and like it, but if I were going to get a new scope for faint fuzzies and it was going to be on my IEQ45 I'd probably look at the edge 9.25

Do you think that mount could support the C9.25 well?



#11 mehdymo

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Posted 19 June 2019 - 01:48 PM

It is 21 lbs and with all other equipments it will be almost 25 lbs. I think it can support it. I have used ES127 on it which is 18 lbs and it was guiding well, but it was too long. C9.25 is more packed. It seems like the EDGE is the best among all other options. Do you know how hard is the collimation of EDGE? I have heard it is easier than RC. 



#12 HH_ASTRO

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Posted 19 June 2019 - 01:51 PM

I chose the Edge 8" for small objects, and I have been pretty happy with my choice so far. Here are some of my images:

https://www.astrobin...7672/B/?nc=user

https://www.astrobin...360033/?nc=user

https://www.astrobin...9843/B/?nc=user
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#13 Peregrinatum

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Posted 19 June 2019 - 01:52 PM

It is 21 lbs and with all other equipments it will be almost 25 lbs. I think it can support it. I have used ES127 on it which is 18 lbs and it was guiding well, but it was too long. C9.25 is more packed. It seems like the EDGE is the best among all other options. Do you know how hard is the collimation of EDGE? I have heard it is easier than RC. 

 

I have read that to make the Edge play nice you need to add a focuser, Moonlite makes one just for the Edge.  You need an OAG as well, so those two together would add another 3 or 4 lbs.


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#14 mehdymo

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Posted 19 June 2019 - 02:02 PM

Great images. they are excelent. I have both IEQ45 Pro and Skywatcher AZ-EQ6. I have not tested the AZ-EQ6 yet, but they both have the same load capacity and I guess they have the same guiding results (IEQ45 Pro guides below 1" RMS easily. I get 0.3"-0.77 constantly, how is your AZ_EQ Pro)

I chose the Edge 8" for small objects, and I have been pretty happy with my choice so far. Here are some of my images:

https://www.astrobin...7672/B/?nc=user

https://www.astrobin...360033/?nc=user

https://www.astrobin...9843/B/?nc=user



#15 HH_ASTRO

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Posted 19 June 2019 - 04:53 PM

Great images. they are excelent. I have both IEQ45 Pro and Skywatcher AZ-EQ6. I have not tested the AZ-EQ6 yet, but they both have the same load capacity and I guess they have the same guiding results (IEQ45 Pro guides below 1" RMS easily. I get 0.3"-0.77 constantly, how is your AZ_EQ Pro)

Yes, my AZ-EQ Pro is excellent, I always get below 1" rms. Even 600" exposures at 1400mm focal length get perfect round stars.


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#16 ImNewHere

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Posted 19 June 2019 - 05:31 PM

Do you think that mount could support the C9.25 well?

Easily. I've ran mine with my SN10 multiple times unguided and had no problems. In fact until I got my LX850 mount that was the IEQ45 was the mount for my SN10 (it is now the CEM60 scope) with a Celestron 80mm f/7.5 guidescope, and the scope has aftermarket top and bottom dovetails, a Moonlite focuser, I attach a DSLR to it, and guide with an MGEN II so total load I was running is maybe 40ish pounds. Never had a problem other than in wind, but then again I tend to run heavier than everyone tells me I can.



#17 james7ca

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Posted 19 June 2019 - 10:12 PM

One thing to remember in discussions like this is that it is IMAGE SCALE and not just focal length that determines the final size of the object. So, it's a combination of both pixel size and focal length. That said, a larger aperture will almost always give you a "faster" system but you can trade off speed for a greater image scale with a smaller scope by switching to a camera with smaller pixels (like the IMX178 or IMX183).

 

Also, a guiding performance of between 0.3 to 0.7 arc seconds RMS is a pretty big range (and is that a TOTAL range or just for each axis?). Your guiding will obviously vary with the seeing conditions and there isn't a lot you can do about that, but an RMS of 0.7 arc seconds will limit what kind of image scale you can use.

 

In fact, 0.3 arc seconds RMS (on each axis) is nearing what I would call necessary for dependable, high-quality work at image scale scales starting about at 0.6 arc seconds per pixel. You can drift above that limit (0.3 arc seconds) but it will probably give you slightly less detail, non-round stars, and/or fewer acceptable subs. This will also depend upon the characteristics of the guiding errors (beyond simple RMS and factoring in the differences between the R.A. and Dec. axes) and the length of your exposures.

 

However, a WO GT71 is going to struggle a bit with small objects (regardless of what camera and pixel size you use), but even a 90mm refractor with something like the IMX183 should give you enough image scale for a good range of additional targets and a well-performing RC6 should be more than good enough for practically any target (except for some of the smaller planetary nebulae which have almost no limit on size).

 

Lastly, your seeing conditions can limit your image scale more than practically anything else. So, even with a larger scope or with smaller pixels you may find that your images just get larger and blurrier, not more detailed (because of the seeing). In this case, what a larger scope (aperture) gives you is a faster system, not necessarily more detail.


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#18 mehdymo

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Posted 19 June 2019 - 10:40 PM

Yes, I agree. My camera would be ASI1600 and I will keep my image scale over one (800mm max). At this focal length, I would be able to get good details in most galexies. I have ES102 with field flattener which gives me 714mm, but it if F/7 i.e. slow. However, it is very easy to work with. I have RC6, but I am always struggling with collimation. I think a 6" F/5 newtonian (750mm), can be a good option as it is easy to collimate, has a large enough focal length, keeps my image scale over one, and still small enough to manage by IEQ45 Pro. But, I have not tried it and have no idea how good would be a 6" Newtonian, practically.

One thing to remember in discussions like this is that it is IMAGE SCALE and not just focal length that determines the final size of the object. So, it's a combination of both pixel size and focal length. That said, a larger aperture will almost always give you a "faster" system but you can trade off speed for a greater image scale with a smaller scope by switching to a camera with smaller pixels (like the IMX178 or IMX183).

 

Also, a guiding performance of between 0.3 to 0.7 arc seconds RMS is a pretty big range (and is that a TOTAL range or just for each axis?). Your guiding will obviously vary with the seeing conditions and there isn't a lot you can do about that, but an RMS of 0.7 arc seconds will limit what kind of image scale you can use.

 

In fact, 0.3 arc seconds RMS (on each axis) is nearing what I would call necessary for dependable, high-quality work at image scale scales starting about at 0.6 arc seconds per pixel. You can drift above that limit (0.3 arc seconds) but it will probably give you slightly less detail, non-round stars, and/or fewer acceptable subs. This will also depend upon the characteristics of the guiding errors (beyond simple RMS and factoring in the differences between the R.A. and Dec. axes) and the length of your exposures.

 

However, a WO GT71 is going to struggle a bit with small objects (regardless of what camera and pixel size you use), but even a 90mm refractor with something like the IMX183 should give you enough image scale for a good range of additional targets and a well-performing RC6 should be more than good enough for practically any target (except for some of the smaller planetary nebulae which have almost no limit on size).

 

Lastly, your seeing conditions can limit your image scale more than practically anything else. So, even with a larger scope or with smaller pixels you may find that your images just get larger and blurrier, not more detailed (because of the seeing). In this case, what a larger scope (aperture) gives you is a faster system, not necessarily more detail.



#19 mehdymo

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Posted 19 June 2019 - 10:45 PM

I was also thinking of using bin 2x2 and use my RC6 at its native focal length of 1370mm or buying an EDGE HD 8" and using it with the 0.7X reducer. But, before making any decision, I would like to hear from people that have gone the same route and have some experience of these scopes.



#20 Stelios

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Posted 20 June 2019 - 03:28 AM

Collimating an Edge is very easy--just use Ed's guide to SCT collimation. It holds collimation extremely well. 

 

I do highly recommend the Moonlite focuser, and an OAG is, IMO, essential. So those add to the cost, but make life much easier.


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#21 mehdymo

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Posted 20 June 2019 - 11:11 AM

Thanks for your good information. Do you prefer EDGE over RCs? both of EDGE 8" and RC8 can be bought for $800 used. They both need a focuser and a reducer. Probably RC becomes a little cheaper finally. But, EDGE can be used with hyperstar as well and it has a flat field (RC is not completely flat).

Collimating an Edge is very easy--just use Ed's guide to SCT collimation. It holds collimation extremely well. 

 

I do highly recommend the Moonlite focuser, and an OAG is, IMO, essential. So those add to the cost, but make life much easier.



#22 rkayakr

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Posted 20 June 2019 - 11:41 AM

Wrong question. You want better resolution. Think about upgrading your camera. I have an MN190 with 1000 mm FL. I have been using an ASI1600mm-c and recently added an ASI183mmPro. Going from 3.7 um to 2.4 um gave me a 50% better resolution with the same scope, filter wheel and filters. I could have upgraded to some other scope system that gave me about 1500 mm FL, but that would have more cost, particularly after adding another stepper motor focuser, probably not been as fast as the MN190 at F5.2 and been a bigger pain to learn and swap/rebalance on the mount. It's simple to swap cameras and change FOV and resolution.


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#23 mehdymo

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Posted 20 June 2019 - 12:34 PM

Thanks. I have thought about this, but replacing ASI1600 with ASI183 still does not giving me enough resolution if I use the same scope i.e. GT71 (1.48" with ASI183 and 2.34" for ASI1600). Actually, replacing the scope while keeping ASI1600 gives better resolution. Anything around 800 mm would be easy to manage and gives lower image scale (0.98"). Also, ASI1600 is better than ASI183 in term of noise and sensor size. Probably, using a mid-range scope and ASI183 gives excellent results, but that would need too much money. Longer focal length is the best option for me now.

Wrong question. You want better resolution. Think about upgrading your camera. I have an MN190 with 1000 mm FL. I have been using an ASI1600mm-c and recently added an ASI183mmPro. Going from 3.7 um to 2.4 um gave me a 50% better resolution with the same scope, filter wheel and filters. I could have upgraded to some other scope system that gave me about 1500 mm FL, but that would have more cost, particularly after adding another stepper motor focuser, probably not been as fast as the MN190 at F5.2 and been a bigger pain to learn and swap/rebalance on the mount. It's simple to swap cameras and change FOV and resolution.



#24 WadeH237

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Posted 20 June 2019 - 01:13 PM

Personally, I think that the EdgeHD 8 is the sweet spot for longish focal length imaging on a reasonable budget.  Its native 2000mm focal length will be seeing limited in most scenarios and with most cameras.  For the price, the optical and mechanical quality is very good.  It has a large corrected field.  It has an optional F/7 reducer that maintains a decent sized corrected field.  Hyperstar is an option.  It is far (really, really far...no, even farther) easier to get and maintain collimation than your RC6.  Dew is easily managed with a dew shield and heater.

 

The biggest downside to the scope is that if you want to maintain optimal correction, it has a pretty limited back focus.  For me, that rules out many external focusers.  I use a FeatherTouch focus knob and Optec FocusLynx to automate mine.  Off-axis guiding is a must.  ONAG will work at F/10, but there is insufficient back focus with the reducer to fit it in the image train.

 

I suspect that your iEQ45 can handle it, but it would be about the biggest combination of weight and focal length that I would consider without a serious mount upgrade.  


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#25 mehdymo

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Posted 20 June 2019 - 01:22 PM

Thanks, I think I will finally go that direction. But, for now, without upgrading the mount, I will have to chose between RC6, Newtonian 6" F/5, and a 102mm refractor. However, it seems like EDGE HD is the way to go. 

Personally, I think that the EdgeHD 8 is the sweet spot for longish focal length imaging on a reasonable budget.  Its native 2000mm focal length will be seeing limited in most scenarios and with most cameras.  For the price, the optical and mechanical quality is very good.  It has a large corrected field.  It has an optional F/7 reducer that maintains a decent sized corrected field.  Hyperstar is an option.  It is far (really, really far...no, even farther) easier to get and maintain collimation than your RC6.  Dew is easily managed with a dew shield and heater.

 

The biggest downside to the scope is that if you want to maintain optimal correction, it has a pretty limited back focus.  For me, that rules out many external focusers.  I use a FeatherTouch focus knob and Optec FocusLynx to automate mine.  Off-axis guiding is a must.  ONAG will work at F/10, but there is insufficient back focus with the reducer to fit it in the image train.

 

I suspect that your iEQ45 can handle it, but it would be about the biggest combination of weight and focal length that I would consider without a serious mount upgrade.  




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