Which Mount ?

[Jkulin]

Thanks Vlaiv, I'm running at 5 sec exposures, as you say the longer then better as you aren't chasing the seeing.

[vlaiv]

1 minute ago, Brian28 said:

Guys what is imaging scale ? Sorry for my ignorance ?

Ratio of sky covered to pixel size. Expressed in arc seconds per pixel, calculated by following formula:

scale = 206.3 * (pixel size in um) / (focal length in mm)

We could roughly divide it into "three" categories - low being over 3"/pixel, medium 2-3"/pixel, high resolution being 1-2"/pixel.

 

[jjosefsen]

1 hour ago, Rob63 said:

Having used an EQ6R for a year now I can say that in terms of bang for buck its a pretty good mount.
The QC from skywatcher must have improved a lot because the mount was immaculate, well adjusted belts with little backlash no black gunk to be seen and no swarf inside the casing.

Carrying handle makes it easy to move outside if you are on non-permanent base.
Guiding averages 0.7” RMS (on tripod ST80 guide scope) and on a night with good seeing (rare) I have guided over an evening at under 0.6" RMS
My PEC curve (5 full rotations of the worm) was ±6.5” 

I have no doubt with a hypertune it could get to 0.5" RMS

You are covering a pretty wide price range so it really comes down to how much you want to spend, EQ6R is good for the price, an EQ8 (too big to lug around) or a CEM60EC would be fantastic (but then again so would a Mesu ?)

 

I'm honestly a little surprised that you aren't getting a lower RMS with the EQ6R.. I have a bog standard EQ6 Pro and often get 0.5-0.7 RMS with my ED80 and 50mm finder guider. I have problems with Dec backlash being pretty bad, and the mount is nearly always way of in dec on the first slew, but it guides well.

On one occasion late summer with really good conditions I had around 0.45-0.5 RMS.. Can't remember if it was on both sides of the meridian though.. Makes me wonder how precisely that RMS is calculated with my gear.

[vlaiv]

43 minutes ago, jjosefsen said:

I'm honestly a little surprised that you aren't getting a lower RMS with the EQ6R.. I have a bog standard EQ6 Pro and often get 0.5-0.7 RMS with my ED80 and 50mm finder guider. I have problems with Dec backlash being pretty bad, and the mount is nearly always way of in dec on the first slew, but it guides well.

On one occasion late summer with really good conditions I had around 0.45-0.5 RMS.. Can't remember if it was on both sides of the meridian though.. Makes me wonder how precisely that RMS is calculated with my gear.

Probably not very precise. 50mm finder will have about 160-170mm focal length. Most guide cameras have pixel size about 3.75um, and that gives guide resolution of about 4.5"/pixel. 1/20 of that is ~0.23" and I would say that 3 times that is the most precision in RMS that you can get - or 0.67".

1/20 being highest precision of star position measurement (often seen figures 1/16 and 1/20 as the most you can get in star position measurement in regard to pixel size). And I would say that if you have ruler being 0.23" long, you can precisely measure RMS that is 3 times that length or higher - if you try to measure 1.5 meters with 1m stick you will either get 1meter or 2meters - nothing in between. If you measure value of 3.5m with one meter stick, and get 3meters, your measurement error is (3.5-3)/3 = ~16.7%.

In case of 50mm finder, I would not trust RMS results below 1" as being accurate. Mind you, that is still sufficient guide precision for most resolutions down to about 1.8"-2"/pixel.

[jjosefsen]

12 minutes ago, vlaiv said:

Probably not very precise. 50mm finder will have about 160-170mm focal length. Most guide cameras have pixel size about 3.75um, and that gives guide resolution of about 4.5"/pixel. 1/20 of that is ~0.23" and I would say that 3 times that is the most precision in RMS that you can get - or 0.67".

1/20 being highest precision of star position measurement (often seen figures 1/16 and 1/20 as the most you can get in star position measurement in regard to pixel size). And I would say that if you have ruler being 0.23" long, you can precisely measure RMS that is 3 times that length or higher - if you try to measure 1.5 meters with 1m stick you will either get 1meter or 2meters - nothing in between. If you measure value of 3.5m with one meter stick, and get 3meters, your measurement error is (3.5-3)/3 = ~16.7%.

In case of 50mm finder, I would not trust RMS results below 1" as being accurate. Mind you, that is still sufficient guide precision for most resolutions down to about 1.8"-2"/pixel.

I was hoping you would chime in on this! ?

I'm shooting hi-res (too high really) of 0.8"/px in Ha and Lum.. I have long been wanting to get another guide scope, but held of due to $ for other projects.. I might bump that up the list now.

I have had my eye on the Zwo 60mm at a fl of 280mm as opposed to the 180mm I am at now. But I wonder if I should just go ahead and get the st80 while I'm at it.. price is not that much more.

 

Edit: Worth mentioning that I have the QHY5-II mono guide camera - 5.2um pixels..

[vlaiv]

How about OAG? I find OAG really good, and also cost effective solution. Easy to adopt and does not suffer flexure issues - just make sure you can accommodate it in optical train (9-16mm optical path depending on model)

Something like this:

https://www.teleskop-express.de/shop/product_info.php/info/p8319_TS-Optics-Off-Axis-Guider-TSOAG16---stable---length-16-mm.html

with appropriate adapter will cost you around 110-120 euro. Depending on connection type (T2 or M48) you will need to choose one of these:

https://www.teleskop-express.de/shop/product_info.php/info/p1649_T2-Ring-for-TS-Off-Axis-Guider-TSOAG9-and-TSOAG16.html

https://www.teleskop-express.de/shop/product_info.php/info/p1650_M48-Ring-for-TS-Off-Axis-Guider-TSOAG9-and-TSOAG16.html

(there is model for Canon EOS also)

Most guide scopes + mounting bits will cost you higher than this.

ST80 will certainly have appropriate guide resolution with QHY5 - 2.68"/pixel, or precision of about ~0.15" - that is enough for 0.5"- 0.6" RMS.

 

 

[Rob63]

My figures for my EQ6-R are an average of the last few months, guiding with an ST80 + ASI120MM  at 1.93" and imaging with an ED120 + 0.85x reducer (1.025").

This is from a Bortle 6 back garden and I set up & tear down every session (sharpcap pro to polar align)

I have seen it as low as 0.48" for brief periods but I think it's more realistic and informative for potential purchasers to give the average RMS over a decent period not a best ever "score".

 

Rob.

[jjosefsen]

12 minutes ago, Rob63 said:

My figures for my EQ6-R are an average of the last few months, guiding with an ST80 + ASI120MM  at 1.93" and imaging with an ED120 + 0.85x reducer (1.025").

This is from a Bortle 6 back garden and I set up & tear down every session (sharpcap pro to polar align)

I have seen it as low as 0.48" for brief periods but I think it's more realistic and informative for potential purchasers to give the average RMS over a decent period not a best ever "score".

 

Rob.

Thanks for explaining that, makes sense!

How do you find PA with SharpCap with the ST80? Is the "narrower" FOV a problem?

[jjosefsen]

38 minutes ago, vlaiv said:

How about OAG? I find OAG really good, and also cost effective solution. Easy to adopt and does not suffer flexure issues - just make sure you can accommodate it in optical train (9-16mm optical path depending on model)

Something like this:

https://www.teleskop-express.de/shop/product_info.php/info/p8319_TS-Optics-Off-Axis-Guider-TSOAG16---stable---length-16-mm.html

with appropriate adapter will cost you around 110-120 euro. Depending on connection type (T2 or M48) you will need to choose one of these:

https://www.teleskop-express.de/shop/product_info.php/info/p1649_T2-Ring-for-TS-Off-Axis-Guider-TSOAG9-and-TSOAG16.html

https://www.teleskop-express.de/shop/product_info.php/info/p1650_M48-Ring-for-TS-Off-Axis-Guider-TSOAG9-and-TSOAG16.html

(there is model for Canon EOS also)

Most guide scopes + mounting bits will cost you higher than this.

ST80 will certainly have appropriate guide resolution with QHY5 - 2.68"/pixel, or precision of about ~0.15" - that is enough for 0.5"- 0.6" RMS.

That will play further havoc with the balancing of my scope.. :S

It could make sense, but I must say I am more partial to a guidescope setup for various reasons.

 

 

[Brian28]

2 hours ago, vlaiv said:

Ratio of sky covered to pixel size. Expressed in arc seconds per pixel, calculated by following formula:

scale = 206.3 * (pixel size in um) / (focal length in mm)

We could roughly divide it into "three" categories - low being over 3"/pixel, medium 2-3"/pixel, high resolution being 1-2"/pixel.

 

Hi Vlaiv .. ok so my scopes

CT8 with QHY163C.          206.3*3.8um / 900 = 0.871  

8" Edge with 0.7 reducer .  QHY163 C      206.3 *3.8/ 1422. = 0.55

So what does this mean in reality..   

[pete_l]

8 minutes ago, Brian28 said:

So what does this mean in reality..   

Personally, I think that in reality almost any astro CCD / CMOS camera with almost any telescope from 400mm focal length up to 3000 will give perfectly acceptable images. While there are theoretical "best" figures, the degree to which the final image is improved by adhering to them is very small. To the extent that a person's experience at processing the data they collect is probably the factor that has the greatest effect.

[spillage]

I am definitely they type of person to just give it a go rather than go by the maths. If I like the results I get then its a winner.

[vlaiv]

23 minutes ago, Brian28 said:

Hi Vlaiv .. ok so my scopes

CT8 with QHY163C.          206.3*3.8um / 900 = 0.871  

8" Edge with 0.7 reducer .  QHY163 C      206.3 *3.8/ 1422. = 0.55

So what does this mean in reality..   

That simply means that it's best that you bin your subs in software 2x2 prior to stacking (after calibration).

This will improve total SNR by factor of 2 over unbinned version, images will be smaller (x2 in width and x2 in height), but we tend to look at images on computer monitor and often images are scaled down for viewing so this will not be an issue.

Only benefit of binning your subs is when looking at images at 1:1 scale. Some people don't bother with this and look at image as presented, while some like to "zoom" in and look at details (faint background galaxies, or structure of objects) - and with binning you will not have blurred look in 1:1 zoom and noise levels will be less. Better SNR also let's you process image deeper - apply sharpening and such without bringing too much noise out.

[Brian28]

14 minutes ago, vlaiv said:

That simply means that it's best that you bin your subs in software 2x2 prior to stacking (after calibration).

This will improve total SNR by factor of 2 over unbinned version, images will be smaller (x2 in width and x2 in height), but we tend to look at images on computer monitor and often images are scaled down for viewing so this will not be an issue.

Only benefit of binning your subs is when looking at images at 1:1 scale. Some people don't bother with this and look at image as presented, while some like to "zoom" in and look at details (faint background galaxies, or structure of objects) - and with binning you will not have blurred look in 1:1 zoom and noise levels will be less. Better SNR also let's you process image deeper - apply sharpening and such without bringing too much noise out.

Do those figures ( my figures in previous post 0.87 & 0.55) have any revalvence to mount capabilities ?

[Rob63]

1 hour ago, jjosefsen said:

Thanks for explaining that, makes sense!

How do you find PA with SharpCap with the ST80? Is the "narrower" FOV a problem?

My ST80 is bolted down hard (not using guide rings) and pretty accurately aligned with the main scope so sharpcap PA is fine.
I adjust until it claims < 10" in both Az and Dec. This results in PHD2 logs saying PA error is anywhere from 0" to 35" which will happily guide out.
 

[vlaiv]

14 minutes ago, Brian28 said:

Do those figures ( my figures in previous post 0.87 & 0.55) have any revalvence to mount capabilities ?

In general they do, but it's a complicated affair.

I'll give a general overview of the impact. Suppose one decides on a certain resolution for imaging based on FOV requirements, selected sensor/scope combination and average sky conditions - let it be 1.1"/pixel. It is generally recommended (this is really rule of thumb and better performance certainly won't hurt, but one can settle for lesser performance as well) that guide RMS for such setup be half of imaging resolution. So, going by this rule, one needs mount that is capable of sustained guiding of 0.55" RMS. If one uses mount that will provide 0.7" RMS - it's not the end of the world - images will be very slightly blurred (in reality hard to perceive, but can be measured as star FWHM difference in subs). If one can afford mount that does 0.3" RMS with ease - again gains will not be "stellar", and might be perceived by eye in final image, but again - star FWHM measurement will be indication of finer setup (a bit tighter stars in final image might be perceived by keen pixel peeper ).

Since your resolutions are 0.87 and 0.55 - both really a bit higher then they should be, you should concentrate on bin x2 versions, being 1.74"/pixel and 1.1"/pixel - one being very standard - close to medium "resolution" (no hard division, but let's say that 2-3" is medium sampling rate and over 3" is low sampling rate, while 1-2" being high sampling rate) while other definitively qualifies as high sampling - you should be aiming for mount capable of 0.5" RMS or better (sustained).

Again, all of this is important if you plan to present / view your images at 1:1 scale (100% zoom in software, or one image pixel being mapped to one screen pixel), and one should bare in mind that good sky conditions are needed for such resolutions anyway, so some of the time if seeing is not very good - even images at this resolutions will look blurred.

Btw, we are treading the "perfectionist" territory here. To many people this high precision of mount and "perfectly" matched resolution for best SNR obtainable simply won't matter - as fine images can be captured with less than optimized setups, and there is value to be had from ease of use and reliability of equipment. Matching everything "perfectly" does not need to be priority for everyone if it means sacrificing other aspect that might matter more to that individual.

[Brian28]

Thanks Vlaiv for your explanation, it makes sense now ..  thank you .. 

[Brian28]

4 hours ago, pete_l said:

Personally, I think that in reality almost any astro CCD / CMOS camera with almost any telescope from 400mm focal length up to 3000 will give perfectly acceptable images. While there are theoretical "best" figures, the degree to which the final image is improved by adhering to them is very small. To the extent that a person's experience at processing the data they collect is probably the factor that has the greatest effect.

Hi Pete , yes I see your point ..and totally understand ..  thank you 

[Brian28]

Has anyone got any experience with the Celestron CGE Pro mount ? . I've seen the size of the mount and I know it's heavy, but quality and accuracy ? 

 

 

[cotak]

6 hours ago, Brian28 said:

Has anyone got any experience with the Celestron CGE Pro mount ? . I've seen the size of the mount and I know it's heavy, but quality and accuracy ? 

 

 

CGE Pro is discontinued and it appears from some stories that getting parts are now no longer possible from Celestron. There are also issues with the cable connections on the CGE Pro such that most folks recommends modification to solve the problem.

I considered it at one point as a used copy was available locally. When I realize that with it's tripod in retracted position it would be as tall as I am (5' 6") and the weight of it I decided against it. 

I still think the CEM60 likely hits a sweet spot for you. 

 

 

[cotak]

5 minutes ago, cotak said:

CGE Pro is discontinued and it appears from some stories that getting parts are now no longer possible from Celestron. There are also issues with the cable connections on the CGE Pro such that most folks recommends modification to solve the problem.

I considered it at one point as a used copy was available locally. When I realize that with it's tripod in retracted position it would be as tall as I am (5' 6") and the weight of it I decided against it. 

I still think the CEM60 likely hits a sweet spot for you. A lot of people will say it's a 1/2 capacity imaging mount but there has been at least one example of a CEM60 carrying a Meade 12" SCT at native focal length and achieving guiding RMS of 0.45". Something that so called premium american mounts of similar payload class are advised to not attempt. Now, for sake of sanity I would advice not to use such a payload at native focal length, but clearly the mass was not a problem for the mount.

 

 

 

[vlaiv]

7 hours ago, Brian28 said:

Has anyone got any experience with the Celestron CGE Pro mount ? . I've seen the size of the mount and I know it's heavy, but quality and accuracy ? 

 

 

You might be thinking CGX and / or CGX-L models?

Looks like both are decent, but I would not know any specific details on their performance

[cotak]

For the new CGX they had a bunch of early mechanical issues but the noise has died down. Performance is pretty much with the pack it seems. Not a bad choice if you stick with Celestron.

[Brian28]

No it was just the Pro I was looking at , there is one available that has had little use and still comes with boxes , from what I've read it's a large mount with a very large payload capacity so I was thinking that in the future I could install it permanently.. and never have to worry about overloading ! 

[Brian28]

emm done it !    Gulp  ordered the iOptron CEM60EC ....  

 

thanks for your views guys ..