Imaging with the 130pds

[Pitch Black Skies]

16 minutes ago, alacant said:

The earlier European ones, yes. Proper GPUs come with a graph showing the relation between focal length and backfocus and have a chart showing recommended positions for various focal lengths. Beware modem mass produced copies.

This for example is a proper GPU which will give corner to corner over APS-c.

This, although it may still work up to a point, isn't but still way better than the 2 element correctors.

Cheers 

 

 

All this time I have been gutted about buying the Skywatcher Aplanatic instead of the GPU with the brass collar.

Are you saying my Aplanatic is the same as the proper GPU? 😮

I noticed the flange doesn't unthread on the proper GPU just like my Aplanatic. That's really the reason why I wanted the GPU with the brass collar. It can be threaded onto the focuser tube. Do you think that is an advantage or not really necessary? I can see how camera rotation could be a problem.

Cheers 

[Elp]

7 hours ago, Ratlet said:

I'm considering the 130pds as a first scope to try both visual and astrophotography

It's a good upgrade if youre coming from a smaller aperture. I use a GSO coma corrector so the focus draw tube doesn't need altering from standard. The GSO makes a bit of difference, looking visually it's more minimal and find I can do without it.

In terms of visual performance, although my refractor (Z61) is pin sharp, from where I am I cant really see any DSOs through it. With the 130pds I am able to see the faint cluster smudges that litter the sky (they are very faint but visible) and to put it into more context with my Z61 I can comfortably view the moon, with the 130pds it is incredibly bright that it becomes uncomfortable without a moon filter so a testament to its light gathering capability.

I've tried a few image tests but nothing concrete as of yet, there's a few mods which can be done to the scope to improve its function but for the time being I'm happy with how it is from factory other than covering the back of the primary to reduce light leak.

 

[alacant]

13 hours ago, Pitch Black Skies said:

not really necessary

If you go threaded, you then need to introduce a rotator; more to tilt, add to the weight and go wrong.

We'd recommend getting a genuine prescription GPU (such as the one you own already) and using three 120° thumb screws on the original SW focuser collar to secure it.

But please note that, and having tried endless combinations of ccs with numerous 130s in various states of health, that's only what works for us. Over here, where it just HAS to work, pragmatism is the only means of survival!

Cheers 

Edited May 31, 2022 by alacant

[edarter]

15 hours ago, Ratlet said:

It seems the ts GPU is the best option from a 'just works' point of view. Plenty of time to think about things.  For the time being.

I don't mind fiddling with things but I remember a forklift at my work that kept breaking.  By the time the found the fault they had replaced almost everything except the forks and the seat! 

I'll need to have solid think about my next step but I appreciate the advice massively.

There is an alternative route that doesn't necessarily mean the extra expense of the GPU (over the Baader) or cutting the focuser. You can get low profile DSLR adaptors from the likes of FLO. They add 1mm to the image train rather than the normal 11mm. So you are half way to getting that 20mm needed in order to keep the focuser out of the OTA. This is where I'm at currently and I intend to get the other 10mm by moving the primary mirror up the OTA by 10mm and using silicone sealant to secure it to its mount. Trouble is I can't find details on what screws and springs people have upgraded to in order to move the primary!

What I would say is that the GPU seems to be less sensitive to tilt than the Baader from everyones experience on here, but is significantly more money. So there is obviously a choice to be had there! Baader+ roughly £30 on extras to get the focus tube out of the OTA (or of course chop the focus tube), or go with the GPU knowing you won't have the OTA protrusion problem right from the start. I'm sort of stuck with the Baader route as I had already purchased it so its a no brainer to spend a small sum on the mods rather than £250 on the GPU.
 

Ed

[Pitch Black Skies]

23 minutes ago, edarter said:

They add 1mm to the image train rather than the normal 11mm.

Is that 10mm not coming out of your backfocus?

With a normal T ring you can achieve focus too by just using longer screws. No need to get the low profile one.

On an older scope I just replaced the factory screws with ones that were 20mm longer. Using this method reduces the focal ratio of your scope but I've heard it can also reduce the amount of reflected light on to the secondary. A larger secondary would probably solve it, but then you are reducing the amount of light that hits the primary 🤪

Personally, I couldn't notice any difference with just using longer screws, but it worried me that I messed with focal ratio so opted for chopping the tube in the end.

I'd advise just try using longer screws and see what happens. If it has an obvious negative impact on your images you can easily change back and it has only cost you a couple of pounds. I think you might be pleasantly surprised.

[alacant]

15 minutes ago, Pitch Black Skies said:

messed with focal ratio

Given that you'll no longer be using the full diameter of the primary mirror, yes. Bear in mind however that a 130mm mirror with a fl of 650mm will always be f5.

The main disadvantage is that if you move the primary mirror 20mm up the tube, the standard issue secondary will not intercept all the light the primary offers. Visually imperceptible. Imaging wise, losing valuable light; imagers need every microgram of light they can possibly muster;)

Edited May 31, 2022 by alacant

[Bibabutzemann]

2 hours ago, edarter said:

There is an alternative route that doesn't necessarily mean the extra expense of the GPU (over the Baader) or cutting the focuser. You can get low profile DSLR adaptors from the likes of FLO. They add 1mm to the image train rather than the normal 11mm. So you are half way to getting that 20mm needed in order to keep the focuser out of the OTA. This is where I'm at currently and I intend to get the other 10mm by moving the primary mirror up the OTA by 10mm and using silicone sealant to secure it to its mount. Trouble is I can't find details on what screws and springs people have upgraded to in order to move the primary!

What I would say is that the GPU seems to be less sensitive to tilt than the Baader from everyones experience on here, but is significantly more money. So there is obviously a choice to be had there! Baader+ roughly £30 on extras to get the focus tube out of the OTA (or of course chop the focus tube), or go with the GPU knowing you won't have the OTA protrusion problem right from the start. I'm sort of stuck with the Baader route as I had already purchased it so its a no brainer to spend a small sum on the mods rather than £250 on the GPU.
 

Ed

Wouldnt you need an adational adapter to reach the 55mm distance between MPCC and sensor with that 1mm adapter?

[alacant]

35 minutes ago, Bibabutzemann said:

Wouldnt you need an adational adapter

Yes. A 10mm m48 ring.

[Pitch Black Skies]

2 hours ago, alacant said:

Given that you'll no longer be using the full diameter of the primary mirror, yes. Bear in mind however that a 130mm mirror with a fl of 650mm will always be f5.

The main disadvantage is that if you move the primary mirror 20mm up the tube, the standard issue secondary will not intercept all the light the primary offers. Visually imperceptible. Imaging wise, losing valuable light; imagers need every microgram of light they can possibly muster;)

Indeed, but if the primary is pushed up 20mm it shortens the focal length to 630mm.

630/130 = 4.85 focal ratio.

I don't believe this shorter focal ratio helps gather light quicker, as you have already mentioned that now all of the light isn't incepted by the standard secondary.

Still not convinced it would make a drastic difference though 🙂

Definitely worth trying if anything.

[Pitch Black Skies]

42 minutes ago, Bibabutzemann said:

Wouldnt you need an adational adapter to reach the 55mm distance between MPCC and sensor with that 1mm adapter?

That's what I was thinking too. It just subtracts from the backfocus. The cc position doesn't change by using 1mm adapter. 

[ONIKKINEN]

18 minutes ago, Pitch Black Skies said:

Indeed, but if the primary is pushed up 20mm it shortens the focal length to 630mm.

Is this really the case? I thought the focal length comes from the shape of the mirror itself and pushing the mirror up will just push the focal point up (and perhaps cause extra vignetting).

[alacant]

17 minutes ago, Pitch Black Skies said:

but if the primary is pushed up 20mm it shortens the focal length to 630mm

The focal length remains exactly the same. This is not the reason the f ratio changes. 

You are instead reducing the effective diameter of the primary as now, not all of it will be intercepted by the secondary: 130mm is reduced to (guess) 110mm.

[Pitch Black Skies]

21 minutes ago, alacant said:

The focal length remains exactly the same. This is not the reason the f ratio changes. 

My bad, you're right. 

A Newtonian's focal length is measured from the distance of the primary to the secondary, PLUS the distance from the secondary to the point of focus 🙂

By pushing the primary up, we are also pushing the point of focus up so in fact the focal length remains exactly the same.

[Pitch Black Skies]

1 hour ago, ONIKKINEN said:

Is this really the case? I thought the focal length comes from the shape of the mirror itself and pushing the mirror up will just push the focal point up (and perhaps cause extra vignetting).

Yeah you're right mate sorry, don't know what I was thinking there.

[starwatcher_ch]

1 hour ago, alacant said:

The focal length remains exactly the same. This is not the reason the f ratio changes. 

Interesting discussion here! Thanks!

A while ago I've asked myself what happens, if during collimation the primary is pushed inside when using extensively all 3 collimation screws.

Thanks for the answer 🙂

Edited May 31, 2022 by starwatcher_ch

[edarter]

2 hours ago, alacant said:

Yes. A 10mm m48 ring.

Exactly that yes. It has the added advantage of pushing the recessed part of the baader down the focus tube enough to no longer interfere with the securing screws.

[edarter]

5 hours ago, Pitch Black Skies said:

Is that 10mm not coming out of your backfocus?

With a normal T ring you can achieve focus too by just using longer screws. No need to get the low profile one.

On an older scope I just replaced the factory screws with ones that were 20mm longer. Using this method reduces the focal ratio of your scope but I've heard it can also reduce the amount of reflected light on to the secondary. A larger secondary would probably solve it, but then you are reducing the amount of light that hits the primary 🤪

Personally, I couldn't notice any difference with just using longer screws, but it worried me that I messed with focal ratio so opted for chopping the tube in the end.

I'd advise just try using longer screws and see what happens. If it has an obvious negative impact on your images you can easily change back and it has only cost you a couple of pounds. I think you might be pleasantly surprised.

It does come out if the back focus yes, but a 10mm extension ring solves that and moves the recessed part of the baader out of the way of the securing screws on the focus tube. That's one of of the causes of slight tilt with the baader.... So kills 2 birds with one stone.

I'm sure there must be a way of working out how much of the primary you would lose by pushing it up the OTA 10mm but I can't figure out the maths on that.

I'm also researching whether a screw in fixing on the end of the focus tube is lower profile than the original, if so it would further reduce the focus tube protrusion meaning even less requirement to move the primary up the OTA.

Edited May 31, 2022 by edarter

[alacant]

2 hours ago, Pitch Black Skies said:

My bad

Stress as we may, at the end of the day, we're never going to turn a €200 SkyWatcher into anything like a mechanically and optically sound top of the range reflector.

We can have fun trying of course, but far better just to go out with whatever stuff you have and... Use it:)

Anyway, long hot moonless night ahead. Let's make the most of it.

Cheers 

Edited May 31, 2022 by alacant

[edarter]

6 minutes ago, alacant said:

Stress as we may, at the end of the day, we're never going to turn a €200 SkyWatcher into anything like a mechanically and optically sound top of the range reflector.

We can have fun trying of course, but far better just to go out with whatever stuff you have and... Use it:)

There's a long hot moonless night ahead here on the Costa Blanca.... Cue self pity...

Cheers 

As long as it's cloudless as well! Wet here in southern UK tonight. Clear skies forecast tomorrow night though so fingers crossed!

[barbulo]

On 30/05/2022 at 21:55, Space Oddities said:

I hesitated between the TS GPU and the TS MaxField 0.95x, and finally went for the latter. I have yet to test it, but I heard it's as good as the TS GPU.

I’ve asked the guys of Teleskop-Express about the same dilema, and they recommended me the Maxfield over the GPU. Their arguments were the excessive length and the possible subsequent tilt effect of the latter. 
Any thoughts?
@Space Oddities: have you tried the Maxfield?

[Space Oddities]

Haven't had a chance yet!

[barbulo]

A mono one: M101 in luminance, 1.5 hours of usable data captured last night. 

Yesterday I spent a few hours thoughtfully collimating the telescope. Now less astigmatism but still some residual coma (worst areas have been cropped).

Tired of not having round stars in the corners, I've ordered a TS-Optics 0.95x Maxfield 3-Element coma corrector, advised by the folks of Teleskop-Express. We'll see if it works. Someday.

[Pitch Black Skies]

https://www.firstlightoptics.com/offers/offer_skywatcher-explorer-130p-ds-ota_307251.html

[Pitch Black Skies]

In the middle of flocking inside the OTA. It's definitely a lot darker in there. Whether it will make any difference to the contrast in my images is another matter 🤷

Oh well,

Nothing Ventured, Nothing Gained.

[Jim Smith]

15 minutes ago, Pitch Black Skies said:

Whether it will make any difference to the contrast in my images is another matter

It'll be interesting to see if it does. I'm keen to hear your results.