69 replies to this topic

[YeloSub]

Since there is one of these threads for the Paracorr I thought it would be neat idea to have one for the GSO CC as well. If there already is one please let me know as I have searched all over.

I will start. I use the GSO CC with the APM XWAs and an ES82 30mm. Based on the where manufacturer established the focal plane (field stop location/focal point)

Eyepiece / focal plane location / GSO CC setting

ES82 30mm / 3mm / 78mm
XWA 20mm / -9mm / 66mm
XWA 13mm / -5.5mm / 69.5mm
XWA 9mm / 0mm / 75mm
XWA 7mm / 3mm / 78mm
XWA 5mm / 3mm / 78mm
XWA 3.5mm / 3mm / 78mm

A negative focal plane means that the focal point is above the shoulder (up in the eyepiece) of the field stop and a positive number indicates it is below the shoulder.

Based on the manufactures (GSO) suggestion that 75mm is the optimal distance.

As posted before you can achieve the proper setting with various extensions or parafocal rings.

Attached is the spec sheet for APM and ES.

If anyone else has any info on confirming their settings when using the GSO CC please post up and share.

This was difficult to find all this info and wanted to share with others.

Clear Skies!

Jake  XWA Series Eyepiece Focal Point.pdf   183.13KB   62 downloads

Edited by YeloSub, 03 December 2021 - 12:39 PM.

[Starman1]

It has been established that the 20mm 100° focal plane position on that chart is in error.

What it actually is, I cannot remember, but it is nowhere near 15.2mm above the shoulder.

[pal77]

I measured my ES100 20mm and it's focal point is approximately -8mm (8mm above the shoulder). Everything else in the table appears to be correct. 

 

For me the optimum distance for the GSO coma corrector was closer to 81mm (from the top end of the CC to the focal plane). This is based on careful experimentation using my telescope.

 

So my eyepieces look like this:

 

Eyepiece / focal plane location / GSO CC setting

 

ES82 30mm / 3mm / 84mm

ES100 20mm / -8mm / 73mm

ES100 14mm / 3mm / 84mm

ES100 9mm / 3mm / 84mm

XWA100 7mm / 3mm / 84mm

XWA110 5mm / 3mm / 84mm

[YeloSub]

I measured my ES100 20mm and it's focal point is approximately -8mm (8mm above the shoulder). Everything else in the table appears to be correct.

For me the optimum distance for the GSO coma corrector was closer to 81mm (from the top end of the CC to the focal plane). This is based on careful experimentation using my telescope.

So my eyepieces look like this:

Eyepiece / focal plane location / GSO CC setting

ES82 30mm / 3mm / 84mm
ES100 20mm / -8mm / 73mm
ES100 14mm / 3mm / 84mm
ES100 9mm / 3mm / 84mm
XWA100 7mm / 3mm / 84mm
XWA110 5mm / 3mm / 84mm

Thanks! I have yet to try and really dial it in. I have just used the 75mm reference and works good. I will have to experiment with 81mm.

Curious what telescope this is with?

[pal77]

It's a 14.25" f/4.1.

 

To test it I used the double cluster with my 20mm eyepiece and moved one of the 2 clusters to the edge of the view. You can start with the 75mm spacing. Then move the eyepiece out of the focuser 2mm and refocus. Keep doing that until you get the best looking stars at the edge of the field of view.

 

If you do this, please post what you come up with for the optimum spacing.

[Jon Isaacs]

It has been established that the 20mm 100° focal plane position on that chart is in error.

What it actually is, I cannot remember, but it is nowhere near 15.2mm above the shoulder.

 

I had the 20 mm ES 100 degree. It's focal plane was very close to the 31 mm Nagler. They were parfocal.

 

I also recommend reading this thread:

 

https://www.cloudyni...coma +corrector

 

Jon

[Starman1]

I had the 20 mm ES 100 degree. It's focal plane was very close to the 31 mm Nagler. They were parfocal.

 

I also recommend reading this thread:

 

https://www.cloudyni...coma +corrector

 

Jon

That would put its focal plane 9.65mm above the shoulder.

[kongqk]

I just did some tests with my APM with GSO setting on my AD12, the good news for me is that, when I have eyepieces fully insert, they all reach focus (with the 20 mm eyepiece almost reach the limit, only 2 mm left). Currently, I am planning to use it directly by fully insert all the eyepieces, which gives me the following GSO setting (75 mm for the GSO)

 

20 mm - 65 mm

13 mm - 69 mm

9 mm - 78 mm

7 mm - 78 mm

5 mm - 78 mm

 

For the above setting, I think I can see the 95+% of the fields are sharp, only the last 3-5% is a little soft. I can live with this.

 

I also tested pull out the 20 mm 2 mm and refocus and repeat method mentioned by pal77, but I found even I reach the focus limit, I still can not get full field 100% clean. When I replaced the GSO CC top with the 35 mm extension tube, I found the 20 mm has the cleanest edge (but the 20 mm can not fully inserted due to the shallower slot), but I found the magnification increases as well, therefore, I think I will just use the original GSO and fully insert my APM eyepieces as my setting. 

Edited by kongqk, 19 December 2021 - 08:56 PM.

[YeloSub]

I just did some tests with my APM with GSO setting on my AD12, the good news for me is that, when I have eyepieces fully insert, they all reach focus (with the 20 mm eyepiece almost reach the limit, only 2 mm left). Currently, I am planning to use it directly by fully insert all the eyepieces, which gives me the following GSO setting (75 mm for the GSO)

20 mm - 65 mm
13 mm - 69 mm
9 mm - 78 mm
7 mm - 78 mm
5 mm - 78 mm

For the above setting, I think I can see the 95+% of the fields are sharp, only the last 3-5% is a little soft. I can live with this.

I also tested pull out the 20 mm 2 mm and refocus and repeat method mentioned by pal77, but I found even I reach the focus limit, I still can not get full field 100% clean. When I replaced the GSO CC top with the 35 mm extension tube, I found the 20 mm has the cleanest edge (but the 20 mm can not fully inserted due to the shallower slot), but I found the magnification increases as well, therefore, I think I will just use the original GSO and fully insert my APM eyepieces as my setting.

So are you using any spacer on the CC? Or parafocal rings on the eyepieces? How are you achieving the different settings?

[kongqk]

So are you using any spacer on the CC? Or parafocal rings on the eyepieces? How are you achieving the different settings?

No, right now I didn't use any spacer or parfocal rings at this point. I found all the APM XWA and ES82 30 mm reach focus without problems, so I just re-focus for the 20 and 13 mm, the 9, 7, 5 and 3.5 mm is parfocal. I may add some parfocal rings to make them all parfocal. 

[YeloSub]

No, right now I didn't use any spacer or parfocal rings at this point. I found all the APM XWA and ES82 30 mm reach focus without problems, so I just re-focus for the 20 and 13 mm, the 9, 7, 5 and 3.5 mm is parfocal. I may add some parfocal rings to make them all parfocal.

Ah, gotcha.

To really maximize the coma correction though you need 75mm (per GSO) to 80mm (based off other users) distance between the CC lens and the focal plane of the eyepiece. Merely getting them to focus is not necessarily ideal, however still probably gives some coma correction.

[kongqk]

Ah, gotcha.

To really maximize the coma correction though you need 75mm (per GSO) to 80mm (based off other users) distance between the CC lens and the focal plane of the eyepiece. Merely getting them to focus is not necessarily ideal, however still probably gives some coma correction.

Yeah, that's what I found as well. Without increasing the distance between the CC lens and the focal plane of the eyepieces, I can see about 95% of the fields are clean, the last 5% is a little soft. I also tried to increase the distance between the CC lens and the focal plane of the eyepieces by replacing the top of the CC with the 35 mm extension tube, it does can achieve the best results with all fields clean. But I also see it increases the magnifications more, which I didn't use it. Maybe I should try a spacer as well in the future ^_^

[YeloSub]

I found that GSO CC is forgiving with not having to make this an exact science for <82° eyepieces and most of the XWAs even. But the 20mm XWA is picky... maybe it's just me.

I just went off of the recommended 75mm distance for my f/6 and f/5 telescopes which seems for work great, but at f/4.5 I think I need to re-evaluate. Especially after reading through the other thread where others say the telescope focal ratio seems to change the sweet spot (I wonder if the TV P2 folks run into this)

I used parafocal rings on mine so far. The goal would be to maybe just have to swap extension rings on the CC when swapping between telescopes.

We'll see. After I experiment more I will share here.

Jake

Edited by YeloSub, 19 December 2021 - 11:46 PM.

[Starman1]

Yeah, that's what I found as well. Without increasing the distance between the CC lens and the focal plane of the eyepieces, I can see about 95% of the fields are clean, the last 5% is a little soft. I also tried to increase the distance between the CC lens and the focal plane of the eyepieces by replacing the top of the CC with the 35 mm extension tube, it does can achieve the best results with all fields clean. But I also see it increases the magnifications more, which I didn't use it. Maybe I should try a spacer as well in the future ^_^

At its proper setting, the GSO coma corrector increases the magnification by 10%.

[kongqk]

At its proper setting, the GSO coma corrector increases the magnification by 10%.

Thanks Don! Then I will buy a spacer to get the optimum setting. Is there a difference if I use a different length of spacers? I am thinking to get a 8 mm spacer, which seems cover the optimum setting mentioned here. 

[Starman1]

Each eyepiece has its focal plane at a slightly different place in the eyepiece, and what you are trying to do is to bring every eyepiece's focal plane to the

correct distance from the lens, i.e. 75mm.

Whatever spacing is necessary to accomplish that is the spacing you should use.

I attach a chart showing the focal plane positions of the XWA eyepieces.

Attached Files

•  XWA Series Eyepiece Focal Point.pdf   183.13KB   69 downloads

[YeloSub]

Read this thread. It explains it all.

https://www.cloudyni...coma-corrector/

Bottom line you need 75mm spacing, the stock eyepiece holder has 46mm. And then depending on your eyepiece focal plane location, you need spacers or parafocal rings accordingly to reach the sweet spot. (some say 79-80mm is the sweet spot).

[kongqk]

Thanks Don and YeloSub, I think I finally understand what you mean now, my previous understanding was completely wrong. This explains why when I replace the GSO top with the 35 mm extension tube (directly screw the bottom of the CC to the extension tube), the top of the extension tube to the CC len is about 70 mm. From the figure, the 20 mm XWA focal plane at about 9 mm above the shoulder, therefore, the total distance is about 79 - 80 mm range, no wonder I see all the fields are clean this way. 

 

Another question, for the 20 mm XWA, it has different focal plane than the other XWA eyepieces, in order to make them all parfocal, I have to buy the parfocal rings for all the other XWAs to reach the same as 20 mm, right? Are there other ways to make them parfocal?

[YeloSub]

Thanks Don and YeloSub, I think I finally understand what you mean now, my previous understanding was completely wrong. This explains why when I replace the GSO top with the 35 mm extension tube (directly screw the bottom of the CC to the extension tube), the top of the extension tube to the CC len is about 70 mm. From the figure, the 20 mm XWA focal plane at about 9 mm above the shoulder, therefore, the total distance is about 79 - 80 mm range, no wonder I see all the fields are clean this way.

Another question, for the 20 mm XWA, it has different focal plane than the other XWA eyepieces, in order to make them all parfocal, I have to buy the parfocal rings for all the other XWAs to reach the same as 20 mm, right? Are there other ways to make them parfocal?

I used parafocal rings on mine. Some cheap ones I found on Amazon. Some have used PVC pipe spacers or similar that had a 2" ID. The 20mm has the shortest focal distance, so that one is stock. Set up the CC with spacers to get it perfect for the 20mm, then add rings to the other eyepieces.

Makes me want to stick to my ES82 set since they are all parfocal... or just get a paracorr. But experimenting with this thing has helped me learn, and it's fun!

[kongqk]

I used parafocal rings on mine. Some cheap ones I found on Amazon. Some have used PVC pipe spacers or similar that had a 2" ID. The 20mm has the shortest focal distance, so that one is stock. Set up the CC with spacers to get it perfect for the 20mm, then add rings to the other eyepieces.

Makes me want to stick to my ES82 set since they are all parfocal... or just get a paracorr. But experimenting with this thing has helped me learn, and it's fun!

Great, sounds like the parfocal rings are the way to go. 

[kongqk]

Just update: I added a 20 mm spacer to the CC, and now it is 46 + 20 = 66 mm. With the 20 mm XWA, 9 mm from the shoulder, the total distance is 75 mm. I tested it out on some of the clusters, and the edge is mostly clean. Right now, I use parfocal rings for the 13, 9, 7, 5, 3.5 and the ES 30 mm to make them all parfocal. 

[Waynosworld]

I made myself an easy to use setup, my GSO CC is set up at 77mm/78mm.

 

Keep in mind that I have fun doing things like this, it is a challenge, and it took a while to get something I was happy with, that anyone can use.

 

I have a Starlight low profile focus assembly on my 16" F/4.42 dob, it is 1.5" racked in all the way, I also had a JMI low profile focus assembly on my Starsplitter that I replaced with a Moonlight low profile focus assembly, so that left the single speed JMI low profile focus assembly to play with, it also is 1.5" racked all the way in.

 

Here is what I made, it is not light, but neither is the Paracorr T2, it is all the way racked in in the photo below

 

You can see the JMI focus assembly, it is racked in so all you can see is the tube on the bottom side, the long screws are the adjustment for the plate that holds the GSO lens housing in place (77mm/78mm).

 

 

In this photo below with the JMI focus assembly racked all the way out you can see the extension tubes I used to hold the GSO lens housing in place on the bottom plate, I also added enough extensions to almost touch the inserted eyepiece when racked all the way in, these extra extensions are used to keep light out when using higher power eyepieces.

 

 

OK, what I do is insert the assembly above into my starlight focus assembly which is racked all the way in, the JMI focus assembly is also racked in all the way, I insert my TV 35mm Panoptic eyepiece in the JMI focuser, I then bring the 35mm Pan eyepiece into focus using the dob starlight focuser, I then lock it in position, every other eyepiece I use I focus with the JMI focus assembly, if I think the dob focus assembly moved I just put the 35mm Pan back in and check to make sure it is in focus.

 

This setup works great, no rings to make the eyepieces all the same, I just focus it like normal.

 

This is not a light assembly and likely would not work well on the 6" thru 12" tube dobs with them bearings that use a tension knob, but it works great on my 16" F/4.42 truss dob and I don't need to add or remove weight when using or not using it, I do have to add my 35mm extension when not using this CC assembly I made except when using my TV 31mm T5, that eyepiece is different.

 

 

[kongqk]

I made myself an easy to use setup, my GSO CC is set up at 77mm/78mm.

 

Keep in mind that I have fun doing things like this, it is a challenge, and it took a while to get something I was happy with, that anyone can use.

 

I have a Starlight low profile focus assembly on my 16" F/4.42 dob, it is 1.5" racked in all the way, I also had a JMI low profile focus assembly on my Starsplitter that I replaced with a Moonlight low profile focus assembly, so that left the single speed JMI low profile focus assembly to play with, it also is 1.5" racked all the way in.

 

Here is what I made, it is not light, but neither is the Paracorr T2, it is all the way racked in in the photo below

 

You can see the JMI focus assembly, it is racked in so all you can see is the tube on the bottom side, the long screws are the adjustment for the plate that holds the GSO lens housing in place (77mm/78mm).

 

DSC00489.JPG

 

In this photo below with the JMI focus assembly racked all the way out you can see the extension tubes I used to hold the GSO lens housing in place on the bottom plate, I also added enough extensions to almost touch the inserted eyepiece when racked all the way in, these extra extensions are used to keep light out when using higher power eyepieces.

 

DSC00490.JPG

 

OK, what I do is insert the assembly above into my starlight focus assembly which is racked all the way in, the JMI focus assembly is also racked in all the way, I insert my TV 35mm Panoptic eyepiece in the JMI focuser, I then bring the 35mm Pan eyepiece into focus using the dob starlight focuser, I then lock it in position, every other eyepiece I use I focus with the JMI focus assembly, if I think the dob focus assembly moved I just put the 35mm Pan back in and check to make sure it is in focus.

 

This setup works great, no rings to make the eyepieces all the same, I just focus it like normal.

 

This is not a light assembly and likely would not work well on the 6" thru 12" tube dobs with them bearings that use a tension knob, but it works great on my 16" F/4.42 truss dob and I don't need to add or remove weight when using or not using it, I do have to add my 35mm extension when not using this CC assembly I made except when using my TV 31mm T5, that eyepiece is different.

Looks great, good design ^_^ Much better than the static spacer/ring!

[krishnak]

Hi, For the 9mm XWA, I have the following setup by using a 20mm spacer.

 

 

Do I need to use 25mm spacer to get the recommended "XWA 9mm / 0mm / 75mm" spacing? Thanks.

[Starman1]

If the 20mm spacer is optimum for that eyepiece, and I don't know it is, the simple way to decide what spacing is needed for another eyepiece is simply to insert it after the 9mm is focused.

If the next eyepiece needs more out focus, you will have to slide it out of the CC to focus.

You can do that, or insert a thicker spacer in the CC.

Or, you can parfocalize all your eyepieces to the one that needs the most in-focus, figure the spacer needed for that one, and use parfocalizing rings to parfocalize all your other eyepieces.

When the CC is set up correctly, all your eyepieces will be parfocal.

To do that, your in-most focusing eyepiece will be optimized when just dropped into the CC and all your other eyepieces will have different amounts of pulling out of the CC to get them to focus.

 

For the 9mm XWA to be optimized at that setting, which has a 68mm CC lens to CC top spacing, the field stop of the eyepiece would need to be 7mm above the shoulder the eyepiece sits on.

I do not know whether that is correct or not, but I suspect it is not.  If that eyepiece has its focal plane at the shoulder, your spacer would need to be 5-7mm longer.

 

But, that spacer in the CC would work with that eyepiece, but might not work with other eyepieces that have their focal planes above the shoulder.

They would need a thinner spacer.

Rather than change the spacer for every eyepiece, adjust the spacer for the eyepiece that needs the most in focus from the focal plane of your scope

and use parfocalizing rings for the other eyepieces so they are automatically in focus when inserted.

Edited by Starman1, 21 February 2024 - 10:41 AM.