Pondering upgrades!

[Rustang]

So I currently have my SW80 ED DS PRO which is awaiting a test run with a new (second hand - Thanks Steve!) focuser to see if it will sort the upset alignment after its cat incident! Its been a great scope but after I sell my wedding photography gear at the end of September I'm going to be looking at equipment upgrades and first in line is the scope and I've already started on the NB filters, 3.5nm Ha here I come!!  . I've known about the TS PhotoLine scopes for a while. I like the idea of the 90mm which I have already turned down before because I wasnt ready at that time and ended up upgrading my camera which was the better idea (Thanks Adam!  ) The 90mm Photoline has a 600mm FL so with the same reducer I would get the same filed of view/FL, the 80mm Photoline is 480mm so would be the same field of view/FL with just a flattener! Why the 80mm though if i already have an 80mm!? I guess to keep things the same but with a better focuser-would it be worth it!?, is it a better quality scope!? I'm also considering a couple of the Williams Optics range, the FLT 91 - more light gathering, similar FL, great focuser and the 81 GT at 478mm FL again is pretty much the same as I currently have with the 85x reducer. I have also considered the SW Esprit 100 but it might just be to much weight for my mount which I'm not necessarily looking to upgrade because of funds! There's also the SharpStar 94 EDPH and the StellaMira 85mm ED2 Triplet 

So I would like to know what other options are out there but first what do I wont!? 

-Something that's going to be worth the up grade and be with me long term, I might buy other smaller widefield scopes for something different but this will stay as my main scope so i would like it to be future proof.

-Similar Focal length because I like what I have currently which is 480mm with the reducer 600mm without.

-A better focuser, i.e stronger and if it has a rotator then even better as I'm finding it a pain to keep the composition the same on targets when I have to keep removing the scope but I am looking to change that!

-Light enough still to keep my SW HEQ-5 PRO running. My mount is old and the guide graph is up and down but the RMS error has always been great so if it aint broke......! I might look to get it professional tuned/belt modified to help it out a little instead of moving up to a HEQ-6 Pro

-More light gathering would be nice, would going up to a 91mm from 80mm be much different? 100mm or there about's would be better I guess?

- I like the idea of having a handle on top, more practical because I have to keep taking it off the mount after each session but i think I would still look to have my guide scope directly on top so that might rule that out.

Budget for the scope, I would prefer a good second hand option to keep more funds back for other upgrades but possibly around £1500-£1800 if new, not sure yet so I',m looking at options first.

Edited August 4, 2021 by Rustang

[ollypenrice]

1 hour ago, Rustang said:

 

-A better focuser, i.e stronger and if it has a rotator then even better as I'm finding it a pain to keep the alignment the same on targets when i have to keep removing the scope but I am looking to change that!

 

One simple trick is to use a spirit level. Set your counterweight bar to horizontal using a normal one. Then, if using a DSLR, hold it against the bottom of the camera and set that to horizontal as well (or vertical if in portrait mode.) If using a dedicated astro camera you can use double sided tape to stick a mini spirit level to the back of the camera. To find horizontal just take a sub of a few seconds while slewing slowly in RA. The star trails show the present camera angle. Adjust till the trails are perfectly horizontal and then stick on the mini spirit level. That will let you get a repeatable orientation very quickly.

1 hour ago, Rustang said:

 

-More light gathering would be nice, would going up to a 91mm from 80mm be much different? 100mm would be better i guess?

 

This is a minefield! The really important aspect of light grasp is not how much light the scope grasps but how much light each pixel grasps. Unfortunately there is no ready made popular unit to describe this. It is determined by sampling rate in arcseconds of sky per pixel, and also by aperture. Sampling rate is determined by focal length and pixel size. Aperture, of course, is decided by the size of the objective. Depending on the specifics, a move from 80mm to 100mm might increase or decrease the amount of light arriving on each pixel.

I think the sanest way to look at telescopes is to start with the focal length because that will decide the framing of your picture. Short FL means wide field, long FL means tight framing of small objects. A move from 80mm to 100mm will, in all probability, bring a small increase in FL.  Do you want that? It probably won't be enough to give great results on small galaxies and planetaries but it may be enough to crop a larger object which you don't want to crop!

Once you have chosen your focal length, consider the aperture. Provided you don't compare F ratios of different focal lengths, F ratio and aperture are effectively synonymous as they are in the camera lens world. At your chosen FL a fast F ratio is to be preferred.

Rather than agonize over the small print of a scope's spec I would look for its results on here, on Astrobin and on other sites to see what you think of them.

£1800 would put you within bargaining range for a Takahashi FSQ106N. This is the old, fluorite version of the FSQ which I use myself and generally prefer over the latest one because it holds focus better during cooldown. It has a 530mm FL, can cover a full frame chip and operates at a speedy F5. A bit heavy for the HEQ5 though.

I'd also consider a TeleVue TV85 with dedicated flattener-reducer. Very well made and seriously under-rated for AP, so prices are attractive.  Check out Frans Kroon who has one of these in his arsenal: http://www.franskroon.nl/ngc2237.htm  You will not have any mechanical issues with one of these.

Olly

Edited August 4, 2021 by ollypenrice
Typo

[david_taurus83]

If your looking at the 100mm refractor range, if it was me, I'd upgrade the mount. You don't want to spend all that cash on a shiny new scope only to find its too much for you current mount. Or at least belt mod your HEQ5 and stick around the 70/80mm range like the WO you mention above. The GT81 with a 1.0 flattener will keep you at your current focal length. A bit cheaper but with the same FPL53 glass and same rack and pinion focuser is the Altair Astro range of scopes. Pretty much the same as the TS versions. I've had 3 from Altair now. Lovely scopes.

[Lee_P]

This doesn't quite match your specification so perhaps isn't helpful, but I upgraded from a SW80 ED DS PRO to an Askar FRA400 and found it to be a big step up.

[Rustang]

2 hours ago, ollypenrice said:

One simple trick is to use a spirit level. Set your counterweight bar to horizontal using a normal one. Then, if using a DSLR, hold it against the bottom of the camera and set that to horizontal as well (or vertical if in portrait mode.) If using a dedicated astro camera you can use double sided tape to stick a mini spirit level to the back of the camera. To find horizontal just take a sub of a few seconds while slewing slowly in RA. The star trails show the present camera angle. Adjust till the trails are perfectly horizontal and then stick on the mini spirit level. That will let you get a repeatable orientation very quickly.

This is a minefield! The really important aspect of light grasp is not how much light the scope grasps but how much light each pixel grasps. Unfortunately there is no ready made popular unit to describe this. It is determined by sampling rate in arcseconds of sky per pixel, and also by aperture. Sampling rate is determined by focal length and pixel size. Aperture, of course, is decided by the size of the objective. Depending on the specifics, a move from 80mm to 100mm might increase or decrease the amount of light arriving on each pixel.

I think the sanest way to look at telescopes is to start with the focal length because that will decide the framing of your picture. Short FL means wide field, long FL means tight framing of small objects. A move from 80mm to 100mm will, in all probability, bring a small increase in FL.  Do you want that? It probably won't be enough to give great results on small galaxies and planetaries but it may be enough to crop a larger object which you don't want to crop!

Once you have chosen your focal length, consider the aperture. Provided you don't compare F ratios of different focal lengths, F ratio and aperture are effectively synonymous as they are in the camera lens world. At your chosen FL a fast F ratio is to be preferred.

Rather than agonize over the small print of a scope's spec I would look for its results on here, on Astrobin and on other sites to see what you think of them.

£1800 would put you within bargaining range for a Takahashi FSQ106N. This is the old, fluorite version of the FSQ which I use myself and generally prefer over the latest one because it holds focus better during cooldown. It has a 530mm FL, can cover a full frame chip and operates at a speedy F5. A bit heavy for the HEQ5 though.

I'd also consider a TeleVue TV85 with dedicated flattener-reducer. Very well made and seriously under-rated for AP, so prices are attractive.  Check out Frans Kroon who has one of these in his arsenal: http://www.franskroon.nl/ngc2237.htm  You will not have any mechanical issues with one of these.

Olly

Thanks for all the advice and tips. You mentioned sample rate which I forgot to mention myself. I have very limited knowledge on this and have only done the calculations for my current set up but don't really understand it. Obviously this is a factor I would need to consider as i don't wont to shoot myself in the foot that way. As you probably know I have the QHY-9 mono CCD camera that I'm not necessarily looking to change just yet either so I would like the new scope to be compatible with that but what I don't wont to happen is if I look to change the camera later down the line I don't then wont to change scopes again! it the most basic of ways (if possible) what are the factors I need to consider then in regards to matching that camera , i.e focal length, aperture etc?  Sample rates and compatibility is honestly something I haven't got my head round properly yet while fumbling my way through this hobby. I do like my focal length, a little more room wouldn't matter but not tighter! If I had endless money I would get a range so I could capture galaxies and planets better but for now I'm happy with Neb's! Apeture wise, I'm not necessarily aperture hunting it more about the scope being a good upgrade in general.

Edited August 4, 2021 by Rustang

[Rustang]

1 hour ago, david_taurus83 said:

If your looking at the 100mm refractor range, if it was me, I'd upgrade the mount. You don't want to spend all that cash on a shiny new scope only to find its too much for you current mount. Or at least belt mod your HEQ5 and stick around the 70/80mm range like the WO you mention above. The GT81 with a 1.0 flattener will keep you at your current focal length. A bit cheaper but with the same FPL53 glass and same rack and pinion focuser is the Altair Astro range of scopes. Pretty much the same as the TS versions. I've had 3 from Altair now. Lovely scopes.

Cheers for the info and advice, in and around 100mm but it will probably end up being somewhere between 80-90mm. I probably should upgrade the mount to future proof myself  but that would then take a big chunk out of the budget for the scope, filters etc. I'am considering modifying my mount though.

[Rustang]

1 hour ago, Lee_P said:

This doesn't quite match your specification so perhaps isn't helpful, but I upgraded from a SW80 ED DS PRO to an Askar FRA400 and found it to be a big step up.

I guess at 400mm it would give me a little more room than 480mm so I could cut off more of my crappy vignetting corners!!  

Edited August 4, 2021 by Rustang

[Lee_P]

13 minutes ago, Rustang said:

I guess at 400mm it would give me a little more room than 480mm so I could cut off more of my crappy vignetting corners!!  

Haha, well I figured it would be light enough so that you wouldn't need to change your mount; and sufficiently below budget that you could afford to invest in other upgrades

[Rustang]

5 minutes ago, Lee_P said:

Haha, well I figured it would be light enough so that you wouldn't need to change your mount; and sufficiently below budget that you could afford to invest in other upgrades

Its a good choice to consider, I just cant get my head around all the tech stuff to see if it would be a good upgrade, my brain just goes, smaller aperture, not a good upgrade but i know that isant the case I just wish I fully understood the in's and out's of why!

Edited August 4, 2021 by Rustang

[Lee_P]

Just now, Rustang said:

Its a good choice to consider, I just cant get my head around all the tech stuff to see if it would be a good upgrade, my brain just goes, smaller aperture, not a good upgrade but i know that isant the case I just wish I fully knew the in's and out's of why!

Lots of example photos on my website. You would be able to get a higher quality instrument if you were to spend your full initial budget though!

[Rustang]

3 minutes ago, Lee_P said:

Lots of example photos on my website. You would be able to get a higher quality instrument if you were to spend your full initial budget though!

Its that difficult situation of buying to future proof yourself but also trying to spread the money you have as far as you can!  

[Lee_P]

7 minutes ago, Rustang said:

Its that difficult situation of buying to future proof yourself but also trying to spread the money you have as far as you can!  

https://astronomy.tools/calculators/ccd_suitability  <--- Scroll to the bottom and put in your potential telescopes and cameras. Then you'll get an idea of what's a good match.

[Rustang]

4 minutes ago, Lee_P said:

https://astronomy.tools/calculators/ccd_suitability  <--- Scroll to the bottom and put in your potential telescopes and cameras. Then you'll get an idea of what's a good match.

My memory often fails me, I forgot there were these kind of calculators 👍

[Rustang]

The CCD calculator seems to show that a longer focal length is better for my camera, the 600mm of the SW 80 ED being the better at 1.86, with my reducer its 2.18 which is slightly under sampling in ok conditions. The Askar 400 is the worst at 2.78 and the WO GT 81 is not as bad but still at 2.33. It appears I'm pretty limited to focal length then with my current camera.

[ollypenrice]

2 hours ago, Rustang said:

The CCD calculator seems to show that a longer focal length is better for my camera, the 600mm of the SW 80 ED being the better at 1.86, with my reducer its 2.18 which is slightly under sampling in ok conditions. The Askar 400 is the worst at 2.78 and the WO GT 81 is not as bad but still at 2.33. It appears I'm pretty limited to focal length then with my current camera.

Here are a couple of images scandalously undersampled   at 3.5 "PP.  

https://www.astrobin.com/383965/

https://www.astrobin.com/301531/?nc=&nce=

On widefield targets like these you can be undersampled and still get results which I think are nice. On small targets, though, the only way to capture fine detail is to use a finer pixel scale, somewhere around 1.0 "PP. You can go for less but will the seeing not blur out the extra detail? It probably will. I wouldn't get hung up over the difference between 1.8 and 2.7"PP.

Nor would I get hung up on the HEQ5/NEQ6 business. There is no evidence that I'm aware of to suggest that one is more accurate than the other.  The six just has a bigger payload. Individual examples vary considerably so either could be more or less accurate than the other. Before making the fatal mistake of fixing something that ain't broke, check your guide stats. If PHD knows your guide cam pixel size and guidescope FL it will give you the RMS in arcseconds. The RMS in arcseconds should be no more than half your sampling rate in arcsecs per pixel. So if you are getting an RMS of 1 arcsecond while imaging at 2"PP you are fine. A good HEQ5 can often manage 0.5".

Olly

 

[Rustang]

56 minutes ago, ollypenrice said:

Here are a couple of images scandalously undersampled   at 3.5 "PP.  

https://www.astrobin.com/383965/

https://www.astrobin.com/301531/?nc=&nce=

On widefield targets like these you can be undersampled and still get results which I think are nice. On small targets, though, the only way to capture fine detail is to use a finer pixel scale, somewhere around 1.0 "PP. You can go for less but will the seeing not blur out the extra detail? It probably will. I wouldn't get hung up over the difference between 1.8 and 2.7"PP.

Nor would I get hung up on the HEQ5/NEQ6 business. There is no evidence that I'm aware of to suggest that one is more accurate than the other.  The six just has a bigger payload. Individual examples vary considerably so either could be more or less accurate than the other. Before making the fatal mistake of fixing something that ain't broke, check your guide stats. If PHD knows your guide cam pixel size and guidescope FL it will give you the RMS in arcseconds. The RMS in arcseconds should be no more than half your sampling rate in arcsecs per pixel. So if you are getting an RMS of 1 arcsecond while imaging at 2"PP you are fine. A good HEQ5 can often manage 0.5".

Olly

 

Thanks again Olly, all good to know. Great images by the way!

 

Although my guide graph looks bad most of time, as I say the total RMS is always under the 0.7 something which I calculated for my set up, the best I've had it is 0.28 so not bad for the un modified old girl, best £400 I've spent so far! Yes a smooth graph might look better and I'm sure I could do some tweaking of settings somewhere but I don't won't to fiddled and make it worst, I would rather get it modified/tuned professionally, call it a service as I've done nothing in the nearly 3 years of owning it. At least if done professionally, they know what they are doing then it would in theory be ready to go, il probably make it worse! The only time I would consider the NEQ6 is for the greater payload as you say. I've got some thinking to do. My main other issue is this vignetting to sort. Would be nice to have an easier run on things when out on a session and the processing after and hopefully that will be the case in time. 

Edited August 4, 2021 by Rustang

[ollypenrice]

1 hour ago, Rustang said:

Thanks again Olly, all good to know. Great images by the way!

 

Although my guide graph looks bad most of time, as I say the total RMS is always under the 0.7 something which I calculated for my set up, the best I've had it is 0.28 so not bad for the un modified old girl, best £400 I've spent so far! Yes a smooth graph might look better and I'm sure I could do some tweaking of settings somewhere but I don't won't to fiddled and make it worst, I would rather get it modified/tuned professionally, call it a service as I've done nothing in the nearly 3 years of owning it. At least if done professionally, they know what they are doing then it would in theory be ready to go, il probably make it worse! The only time I would consider the NEQ6 is for the greater payload as you say. I've got some thinking to do. My main other issue is this vignetting to sort. Would be nice to have an easier run on things when out on a session and the processing after and hopefully that will be the case in time. 

Care needed, here. You can't just eyeball a guide graph and learn anything. Using an off axis guider in a large reflector our guide graph looked like the Himalayan skyline but the true guide performance was stunning, around 0.3 arcseconds.

You say you have an RMS of  0.7 but have you fed your guide camera pixel size and and guidescope focal length into PHD? If you have, then 0.7 is quite good and will support a sampling rate of twice that, so 1.4 arcsecs per pixel.  But (this is very important) if you haven't fed that info into PHD then the 0.7 will mean an RMS of 0.7 of a pixel at the autoguider. Given the short FL of most guiders, that will translate into a much larger error than 0.7 of a pixel at the imaging camera.

So are you dead sure your RMS is being recorded in arcseconds rather than pixels?

Olly

[Rustang]

1 hour ago, ollypenrice said:

Care needed, here. You can't just eyeball a guide graph and learn anything. Using an off axis guider in a large reflector our guide graph looked like the Himalayan skyline but the true guide performance was stunning, around 0.3 arcseconds.

You say you have an RMS of  0.7 but have you fed your guide camera pixel size and and guidescope focal length into PHD? If you have, then 0.7 is quite good and will support a sampling rate of twice that, so 1.4 arcsecs per pixel.  But (this is very important) if you haven't fed that info into PHD then the 0.7 will mean an RMS of 0.7 of a pixel at the autoguider. Given the short FL of most guiders, that will translate into a much larger error than 0.7 of a pixel at the imaging camera.

So are you dead sure your RMS is being recorded in arcseconds rather than pixels?

Olly

There's really still so much I probably dont know but should know and thought I had worked out by now! Its pretty overwhelming this hobby and I'm surprised Ive even got this far!

So I worked out what RMS error I can go up to by the following-

QHY 9 mono - pixel size 5.4 so 5.4 / 480fl x 206.3 = 2.3 arc seconds per pixel-  then 2.3 / 2 = 1.16 so I gathered i can be 1.16 and under at 480mm focal length. I'm pretty sure the correct guide camera pixel size and fl have been entered correctly but looking back at an image of one of my sessions, next to RMS Error at the top, in brackets is (px): so reads RMS Error (px): which I'm guessing is not good then if that pixels and not arceseconds!?

For me this has been an incredibly hard hobby to take on with everything to learn and think about, so I have learnt the minimum which has been hard enough and just used that to get by and never ventured out of that comfort zone, so I have taken the RMS and the fact my images appear ok that all is well with guiding, if its not and that RMS error is not showing the correct reading, would I have not seen something wrong in my images!?

Pictures below of guide camera settings that were originally entered into PHD2

 

Edited August 4, 2021 by Rustang

[ollypenrice]

Perhaps someone else could come in here. I'm not imaging at the moment and can't remember what PHD tells you about the RMS units. From memory I thought it gave the RMS in arcsecs if you'd given it the guider pixel size and FL, as you have. (To be quite honest my guiding has been so good for so long that I take little notice of it and have a rusty memory of troubleshooting. The joys of using Mesu mounts!)

If it's giving you the error in terms of pixels at the guider then you just have to mulitiply the error according to the difference between guider pixel scale and imaging pixel scale. (If your guider runs at 4"PP and your imager at 2"PP then the error 'seen' by the imager is twice that of the guider. In this case you'd be looking for an RMS at the imager of 1 arcsec, requiring an RMS at the guider of half that, so 0.5 pixel at the guider.

In the end, if the stars look tight and round then you're getting decent pictures. However, round stars are not a perfect indicator of optimal guiding because, when the RA and Dec errors are equal, as they may well be with an autoguider, they can be blurred equally in all directions. When calibrating the first Mesu we had here we alwys had round stars but, as we refined the PHD parameters, they became significantly smaller.

Olly

[Rustang]

25 minutes ago, ollypenrice said:

Perhaps someone else could come in here. I'm not imaging at the moment and can't remember what PHD tells you about the RMS units. From memory I thought it gave the RMS in arcsecs if you'd given it the guider pixel size and FL, as you have. (To be quite honest my guiding has been so good for so long that I take little notice of it and have a rusty memory of troubleshooting. The joys of using Mesu mounts!)

If it's giving you the error in terms of pixels at the guider then you just have to mulitiply the error according to the difference between guider pixel scale and imaging pixel scale. (If your guider runs at 4"PP and your imager at 2"PP then the error 'seen' by the imager is twice that of the guider. In this case you'd be looking for an RMS at the imager of 1 arcsec, requiring an RMS at the guider of half that, so 0.5 pixel at the guider.

In the end, if the stars look tight and round then you're getting decent pictures. However, round stars are not a perfect indicator of optimal guiding because, when the RA and Dec errors are equal, as they may well be with an autoguider, they can be blurred equally in all directions. When calibrating the first Mesu we had here we alwys had round stars but, as we refined the PHD parameters, they became significantly smaller.

Olly

Thanks Olly, I'm going to go and bang my head against a wall for a bit then il look into it! If it is reading in pixels then I guess my guiding isant as good as I thought but I also guess that if Ive been happy with my stars then it must either be just on its limit or I'm lucky! To me my stars look round, well in then centre anyway!! fairly crisp but that's about all I know about what a good star should look like.

[Rustang]

So after some reading I think Ive understood it a little better and Ive also only just realised I'm not at 480mm fl Im at 510mm fl 🙈

From from what Ive read online it goes like this:

5.4 / 510 x206.3 = 2.2 arc seconds per pixel for my QHY9. Guide camera = 3.75 / 225 x 206.3 = 3.44

If I have a RMS of say 0.7 its apparently goes 0.7 x 3.44 = 2.41 then 2.41 / 2.2 = 1.09 pixel RMS error on final image so if my QHY-9 's arc seconds per pixel is 2.2 and my RMS calculates to 1.09 then is that well with in tolerance!?

 

this is the online paragraph that I got the above calculations from above-

"– First of all, please bear in mind that you can have a pretty ugly looking guiding graph that produces little or no visible effect on the actual image. The opposite is also true of course! You need to convert the pixel lines on the vertical scale and/or the RMS error to arcseconds of error by performing the guider pixel scale multiplication described above. Now compare that to the pixel scale of your imaging rig. You may find that what looks like a major guiding issue is actually less than a pixel or two of error on your final image.

For example, looking at the RA error in the graph above:

0.2 RMS pixels error x 2.67 arcseconds per pixel = 0.53 arcseconds of error on the guider image

0.53 arcseconds of error / 1.9 arcseconds per pixel imager scale = 0.28 pixel RMS error on final image

Edited August 5, 2021 by Rustang

[Budgie1]

If your 80ED turns out to okay with the new, second hand, focuser, what about a focuser upgrade for that scope?

I upgraded the focuser on my 100ED to one of THESE, which I got from another member on here. It feels much more robust & better quality than the standard focuser, is easier to fine-focus and has a built-in rotator. The only drawback is that it doesn't have a thread on the end, so is push-fit only, but does have a clamping ring inside (rather than relying on just thumbscrews). It feels very secure with my SW Reducer/Flattener & ASI294 on the back of it.

Now I've got an 80ED to go along with the 100ED, I'll probably swap the focusers over later in the season and put this one on the 80ED, as I think that may get more use.

[Rustang]

8 hours ago, Budgie1 said:

If your 80ED turns out to okay with the new, second hand, focuser, what about a focuser upgrade for that scope?

I upgraded the focuser on my 100ED to one of THESE, which I got from another member on here. It feels much more robust & better quality than the standard focuser, is easier to fine-focus and has a built-in rotator. The only drawback is that it doesn't have a thread on the end, so is push-fit only, but does have a clamping ring inside (rather than relying on just thumbscrews). It feels very secure with my SW Reducer/Flattener & ASI294 on the back of it.

Now I've got an 80ED to go along with the 100ED, I'll probably swap the focusers over later in the season and put this one on the 80ED, as I think that may get more use.

Thanks martin, its worth considering, what do you mean by it doesnt have a thread on the end!? I've got other things that need sorting aswell now as it turns out Ive been reading my guide RMS error wrong so actually mu guiding isant that great so looking to get my mount belt modifiedand tuned then I can turn my attention back to the scope to either upgrade or modify if it turns out to be ok!

[Lee_P]

23 hours ago, Rustang said:

So after some reading I think Ive understood it a little better and Ive also only just realised I'm not at 480mm fl Im at 510mm fl 🙈

From from what Ive read online it goes like this:

5.4 / 510 x206.3 = 2.2 arc seconds per pixel for my QHY9. Guide camera = 3.75 / 225 x 206.3 = 3.44

If I have a RMS of say 0.7 its apparently goes 0.7 x 3.44 = 2.41 then 2.41 / 2.2 = 1.09 pixel RMS error on final image so if my QHY-9 's arc seconds per pixel is 2.2 and my RMS calculates to 1.09 then is that well with in tolerance!?

 

this is the online paragraph that I got the above calculations from above-

"– First of all, please bear in mind that you can have a pretty ugly looking guiding graph that produces little or no visible effect on the actual image. The opposite is also true of course! You need to convert the pixel lines on the vertical scale and/or the RMS error to arcseconds of error by performing the guider pixel scale multiplication described above. Now compare that to the pixel scale of your imaging rig. You may find that what looks like a major guiding issue is actually less than a pixel or two of error on your final image.

For example, looking at the RA error in the graph above:

0.2 RMS pixels error x 2.67 arcseconds per pixel = 0.53 arcseconds of error on the guider image

0.53 arcseconds of error / 1.9 arcseconds per pixel imager scale = 0.28 pixel RMS error on final image

I can't help with the technical side of things -- happy to leave that to Olly! -- but in general I'd say that if your stars aren't showing any trailing then don't stress over the mount / guiding too much. Sure, things can always be better, but there's something to be said for appreciating when things are "good enough" -- this allows you to focus your attentions (and money!) on another area to improve upon which may actually have a greater impact overall.

[Rustang]

4 minutes ago, Lee_P said:

I can't help with the technical side of things -- happy to leave that to Olly! -- but in general I'd say that if your stars aren't showing any trailing then don't stress over the mount / guiding too much. Sure, things can always be better, but there's something to be said for appreciating when things are "good enough" -- this allows you to focus your attentions (and money!) on another area to improve upon which may actually have a greater impact overall.

Cheers, totally under stand that and yes to me considering now how bad my mount is tracking, the stars have, to me, seemed ok, no trailing. I would still like to look at the opportunity to do something to aid its guiding though, Ive been reading the RMS wrong and where I should be at 1.1 and thinking I was always at 0.7 or under, I'm actually mostly at 1.49  and above! Its all going into the mix of what's in the pot to spend and I had already decided it's probably likely im keeping the mount so a tune up wont hurt as the mount is getting on now.