Imaging with the 130pds

[woldsman]

On 17/10/2023 at 19:32, 8472 said:

God, I'm stupid - of course it is! 🤦

Thanks!

Not 100% sure what is being asked here but if the problem is what is the effective aperture of a 130pds allowing for the central obstruction, surely it can’t be 60.5mm. Looking at the scope,  are you really losing over half of the aperture?! You have to take account of mirror reflectivity as well as the obstruction. If the obstruction is 13%  of the primary (ie the ratio of 47/2 squared to 130/2 squared) and reflectivity is 90% (which may be too low), then I think the calculation is 130mm x 0.9 x sqrt(1-0.13) which works out at 109mm.

Edited November 4, 2023 by woldsman

[Mandy D]

On 17/10/2023 at 18:54, AL1 said:

I think it is surface area that gives light gathering power

i.e

(pi X (130/2)^2) - (pi X (47/2)^2)  =  (pi X R^2)

 

R = sqrt((130^2-47^2)/4)

R = 60.6 mm radius for equivalent refractor

Just over 120mm diameter.   I would double check my maths though

 

The equation you give simplifies to D² - d² = D_E²

D = diameter of primary mirror

d = diameter of obstruction

D_E = effective diameter

Pi cancels throughout and for ratios it does not matter whether you use the radius or diameter.

Yes, it is area that gives light gathering capability. Confusion arises here because we use linear measurement (i.e. diameter) to define resolving power, simply because we always talk about linear resolution not areal resolution. If we always talked in square radians for resolution (i.e. areal), then the light gathering and resolution would be equivalent in ratio and things would make more sense.

[edarter]

On 01/11/2023 at 22:46, woldsman said:

Why would a gso coma corrector obviate having to address the issue of focuser protruding? Am thinking about getting a 130pds but the focuser mod is off-putting.

You don't have to do the focusser mod to almost completely eliminate the FT protrusion in to the OTA. Currently available CC's either keep the native Focal length but have the disadvantage of OTA focus tube protrusion, or increasing focal length/ratio which in turn eliminates the protrusion. I too was not keen on taking the hacksaw to my scope, or spending £200+ more on another CC having bought the Baader MPC Mk3 so came up with an alternative. This is for DSLR though so your mileage may vary for a dedicated astro camera! Bear with, its difficult to explain so the sketch may show it more clearly!

The protrusion I had into the OTA was about 25mm at focus with a DSLR and the Baader mk3. A significant amount to hacksaw off! so.....
Basically I bought a low profile DSLR adaptor (1mm thick rather than 11mm). To keep the CC  to camera spacing the same you then need a 10mm extension tube between the DSLR adaptor and the CC. There is an added benefit here of both the CC and the extension tube sitting inside the focus tube and therefore gets the horrible shoulder of the Baader out of the way of the clamp screws (therefore eliminating some of the tilt I was also experiencing). This mod gives you 10mm.

Next I looked at the screws holding the primary mirror to the OTA, they are (on my scope at least) plenty long enough that the mirror can be moved up the OTA to about a 10mm gap from the back plate (they were almost touching the back plate from the factory). So I moved the mirror up the OTA and re-collimated. Just to be sure I also did some maths and worked out that this shouldn't cause light loss around the secondary mirror. That gave me another 10mm off my OTA protrusion, which now is about 5mm and has negligable effect on my images, certainly way less than my rubbish processing skills lol

Here's the sketch showing the difference at the camera end of my setup, the thing to note is how much further out the end of the focus tube is relative to the focus point while keeping the CC to focus point exactly the same. Hth!

[woldsman]

3 hours ago, edarter said:

You don't have to do the focusser mod to almost completely eliminate the FT protrusion in to the OTA. Currently available CC's either keep the native Focal length but have the disadvantage of OTA focus tube protrusion, or increasing focal length/ratio which in turn eliminates the protrusion. I too was not keen on taking the hacksaw to my scope, or spending £200+ more on another CC having bought the Baader MPC Mk3 so came up with an alternative. This is for DSLR though so your mileage may vary for a dedicated astro camera! Bear with, its difficult to explain so the sketch may show it more clearly!

The protrusion I had into the OTA was about 25mm at focus with a DSLR and the Baader mk3. A significant amount to hacksaw off! so.....
Basically I bought a low profile DSLR adaptor (1mm thick rather than 11mm). To keep the CC  to camera spacing the same you then need a 10mm extension tube between the DSLR adaptor and the CC. There is an added benefit here of both the CC and the extension tube sitting inside the focus tube and therefore gets the horrible shoulder of the Baader out of the way of the clamp screws (therefore eliminating some of the tilt I was also experiencing). This mod gives you 10mm.

Next I looked at the screws holding the primary mirror to the OTA, they are (on my scope at least) plenty long enough that the mirror can be moved up the OTA to about a 10mm gap from the back plate (they were almost touching the back plate from the factory). So I moved the mirror up the OTA and re-collimated. Just to be sure I also did some maths and worked out that this shouldn't cause light loss around the secondary mirror. That gave me another 10mm off my OTA protrusion, which now is about 5mm and has negligable effect on my images, certainly way less than my rubbish processing skills lol

Here's the sketch showing the difference at the camera end of my setup, the thing to note is how much further out the end of the focus tube is relative to the focus point while keeping the CC to focus point exactly the same. Hth!

Thanks - very interesting.  Have a date in the diary to pick up a 130pds and interestingly imaging indicates it doesn't have pinched stars so will get my slide rule and callipers out to try and figure out why! (although I've a notion). From first principles, using DSLRs with any reflector is likely to be a worst-case scenario because (for Canon at least) the sensor is 44mm back due to the width of the camera body. With (say) a ZWO asi the distance can be as little as 6.5mm. Quite a difference! With Coma Correctors, there may be varying back distances which explain why some have less protrusion than others. but with implications for focal length, as you say. 

[edarter]

2 minutes ago, woldsman said:

Thanks - very interesting.  Have a date in the diary to pick up a 130pds and interestingly imaging indicates it doesn't have pinched stars so will get my slide rule and callipers out to try and figure out why! (although I've a notion). From first principles, using DSLRs with any reflector is likely to be a worst-case scenario because (for Canon at least) the sensor is 44mm back due to the width of the camera body. With (say) a ZWO asi the distance can be as little as 6.5mm. Quite a difference! With Coma Correctors, there may be varying back distances which explain why some have less protrusion than others. but with implications for focal length, as you say. 

You're absoluely right - that 44mm relative to the 6.5mm you quote for some ZWO cameras means the FT is 37.5mm further towards the OTA in real terms. So its no wonder there is no happy medium that suits both dedicated astro cameras and DSLRs. I think the rule of thumb is to get the CC and any spacers down the FT as much as possible in order to move the FT outwards, after that you have to look at other areas or a different CC to improve things further.

[cmanley134]

Hi all - proud new 130P-DS owner here (yay!) and happy to join the thread. I'm sure this has been answered at some point, but I'm attempting to use the Stellalyra (rebranded GSO) corrector and it seems that in its most basic configuration, the corrector has nothing to keep it from falling down into the tube apart from the screws on the focuser. With the 2" visual adapter or a t-ring attached there is a lip/flange that holds it in the right position, but my planned image train was ZWO ASI 2600MC > 21mm filter drawer > 21mm spacer > 16.5mm spacer > corrector to achieve the quoted backfocus of ~75mm. Am I missing a piece here? Thanks in advance for any assistance!

[alacant]

1 hour ago, cmanley134 said:

corrector has nothing to keep it from falling down into the tube

Hi

Yes. Indeed... Nothing.

Fortunately, it would never be deployed in such configuration.

The GSO has m48 threads, so as long as you have allowed for any adapter 48 to 42, the 75mm would be a good place to begin the positioning trials. IIRC, over aps-c, you'll probably find the best correction is however nearer 80mm; there is usually a 5mm m48 to m42 adapter supplied already fitted.

Cheers and HTH

Edited November 8, 2023 by alacant

[cmanley134]

20 minutes ago, alacant said:

Hi

Yes. Indeed... Nothing.

Fortunately, it would never be employed in such configuration.

The GSO has m48 threads, so as long as you have allowed for any adapter 48 to 42, the 75mm would be a good place to begin the positioning trials. IIRC, over aps-c, you'll probably find the best correction is however nearer 80mm; there is usually a 5mm m48 to m42 adapter supplied already fitted.

Cheers and HTH

Thanks for the response! So, if I am understanding correctly, I am going to simply align the top of the corrector with the top of the focuser and clamp it in that position? So the entire corrector is inserted into the focuser tube and the spacers that comprise my backfocus are exposed.

[alacant]

6 minutes ago, cmanley134 said:

align the top of the corrector with the top of the focuser

We can't advise because we don't know the diameter of your spacers. m48 gives you the option of clamping anywhere along the length. Unfortunately, zwo supply only m42.

The main problem with the gso is retaining stability and balance with the focal plane so far from the tube. 

[cmanley134]

1 hour ago, alacant said:

We can't advise because we don't know the diameter of your spacers. m48 gives you the option of clamping anywhere along the length. Unfortunately, zwo supply only m42.

The main problem with the gso is retaining stability and balance with the focal plane so far from the tube. 

Hmm, I guess I can relocate the filter drawer to attach directly to the corrector - it's moving the filter further away from the sensor but I doubt that'll be an issue. Would solve this particular problem. And we'll see how it goes! I agree that the 75mm is a lot - I may pick up an MPCC at some point.

[alacant]

13 hours ago, cmanley134 said:

I may pick up an MPCC at some point.

JTOL and I'm hoping that YMMV...

Have you checked to see if the focuser is capable of holding the camera/cc/fw assembly at the required distance? Even with a quality 2-1/2" r&p, we gave up with a similar specified ES. Here we are at 80mm with a DSLR at the focal plane:

[cmanley134]

1 hour ago, alacant said:

JTOL and I'm hoping that YMMV...

Have you checked to see if the focuser is capable of holding the camera/cc/fw assembly at the required distance? Even with a quality 2-1/2" r&p, we gave up with a similar specified ES. Here we are at 80mm with a DSLR at the focal plane:

That's my task for today - as you may have seen in my previous post the corrector is filthy! Needs a good cleaning before it goes anywhere near my filters and/or camera sensor.

[AstroMuni]

On 01/11/2023 at 22:46, woldsman said:

Why would a gso coma corrector obviate having to address the issue of focuser protruding? Am thinking about getting a 130pds but the focuser mod is off-putting.

I use the Baader MPCC with the ASI533MC and there is no protrusion. I have MPCC going into the focuser tube getting you those precious cm to prevent protrusion.

[cmanley134]

Tried a test-fit with the corrector and ~75mm backspacing - with the camera rotated vertically where it has the most leverage, the focuser seems to be holding the weight. Definitely needs the third thumb screw though, as I can feel some play currently. I picked up a tap and die set so I'll be adding that this weekend.

Edited November 9, 2023 by cmanley134

[edarter]

21 hours ago, cmanley134 said:

Tried a test-fit with the corrector and ~75mm backspacing - with the camera rotated vertically where it has the most leverage, the focuser seems to be holding the weight. Definitely needs the third thumb screw though, as I can feel some play currently. I picked up a tap and die set so I'll be adding that this weekend.

Thats a very tidy cap you have over the mirror end, where did you source that from?

[cmanley134]

1 hour ago, edarter said:

Thats a very tidy cap you have over the mirror end, where did you source that from?

I was actually lucky enough to receive them with the scope courtesy of the previous owner, but I found them here: https://www.optechusa.com/products/hood-hat?variant=44409884082471. U.S. company so not sure if they ship overseas, but pricing is reasonable. I believe this is the second largest size (XXX-L)

[cmanley134]

Spent some time messing with the new scope - got that third screw tapped with a cheap tap and die set off of Amazon and got everything put together to see if I could reach focus.

It does seem like I need to have the focuser racked almost all the way out to focus on the trees across the street maybe 100ft away. I've had the experience with my refractors that the focus point is actually closer in compared to what I get when I try during the day, hopefully that is the case here. Do others with the GSO corrector find that their focuser is nearly fully extended?

The good news is that the third screw is a major improvement - no more play in the image train. Looking forward to first light, if the clouds ever let up.

[woldsman]

Could someone kindly post pictures of the imaging train they have with the 130pds connecting a Baader MPCC mark 3 coma corrector to a zwo camera using a filter wheel? Became a 130pds owner today and don’t know what to do with the short coma corrector - used to field flatteners and reducers!   UPDATE - issue now resolved.

Edited November 22, 2023 by woldsman

[braybarr]

Does anyone have experience dew heating their coma corrector? I live in a very humid area where my CC has been getting dewed over recently. What can I do?

[8472]

16 hours ago, braybarr said:

Does anyone have experience dew heating their coma corrector? I live in a very humid area where my CC has been getting dewed over recently. What can I do?

Mount the OTA with the focuser at the 12 o'clock position.

Although I don't use one, a dew shield may also help. I've also made a secondary dew heater.

[braybarr]

6 hours ago, 8472 said:

Mount the OTA with the focuser at the 12 o'clock position.

Although I don't use one, a dew shield may also help. I've also made a secondary dew heater.

Interesting, how would rotating the OTA to 12:00 position help? I also have a dew shield arriving today or tomorrow. I’ve seen suggestions to solder resistors together for heating but that is well beyond my skill level. 

[cmanley134]

Well, first light was a success - got everything set up and captured 45 x 120s on M45 as my testing target. Lots of things went great! No light leaks, focuser had plenty of tension to hold the image train and I reached and maintained focus easily. Data was clean and easy to process. Star shapes need some fine tuning but this thing is awesome for the money. 

[sagramore]

On 14/11/2023 at 13:07, braybarr said:

Interesting, how would rotating the OTA to 12:00 position help? I also have a dew shield arriving today or tomorrow. I’ve seen suggestions to solder resistors together for heating but that is well beyond my skill level. 

I've never tried it but a dew heater on the secondary might give just enough warmth at the open end of the scope to reduce humidity inside and help? At least you can buy those off the shelf. Otherwise a nice long dew shield could also help without needing extra power or wires so I'd try that first once it arrives!

[sagramore]

On 15/11/2023 at 18:06, cmanley134 said:

Well, first light was a success - got everything set up and captured 45 x 120s on M45 as my testing target. Lots of things went great! No light leaks, focuser had plenty of tension to hold the image train and I reached and maintained focus easily. Data was clean and easy to process. Star shapes need some fine tuning but this thing is awesome for the money. 

Welcome to the club :)

[Whistlin Bob]

1 hour ago, sagramore said:

a nice long dew shield could also help

Yeah- I had a few problems with misting up. I made a long (300mm) dew shield out of an old foam camping mat and duct tape. It looks a bit agricultural, and the plan was to buy a proper one if it worked, but it actually works so well that I've not replaced it. If you're not worried about appearances, I would definitely recommend as a solution.