Astro-Tech AT102ED or SVBONY SV503 102ED + LS50PT?

[_RA_]

Hi,

I searched, but didn't find anything on these 4" F7 telescopes here. Any thoughts, or actual experience, about their viability for modding to pair with a Lunt50 PT? ERF size and placement?

TIA,

RA

[marktownley]

I've seen a few of these and variant badged otas being use for this purpose. I think the svbony might have a m68 fitting on it which means you just screw the lunt etalon straight on the back.

[_RA_]

Mark, thanks for the quick reply.

That sounds too easy... why aren't people all over that? Or maybe they are, and I just blew it to where now they'll sell out faster and raise the prices?

Would an internal ERF work with that aperture?

[marktownley]

Yes, you could use an internal ERF ahead of the etalon. I think there's a few of these mods been documented on here.

[_RA_]

Yes, you could use an internal ERF ahead of the etalon. I think there's a few of these mods been documented on here.

[Weird. I was sure I responded to this earlier.]

My search skills are generally decent, but I can't find anything here or elsewhere. If it's no trouble, are you able to provide direct links?

[Ivan]

Hi,

I searched, but didn't find anything on these 4" F7 telescopes here. Any thoughts, or actual experience, about their viability for modding to pair with a Lunt50 PT? ERF size and placement?

TIA,

RA

Here is the test of the svbony 503 telescope but 80 ED : https://star-hunter.ru/svbony-sv503-80ed-f7-doublet- refractor-review/ The result is spherical aberration in the red channel. Maybe this is a feature of a particular instance, or maybe the whole series.

Ivan

[_RA_]

Here is the test of the svbony 503 telescope but 80 ED : https://star-hunter.ru/svbony-sv503-80ed-f7-doublet- refractor-review/ The result is spherical aberration in the red channel. Maybe this is a feature of a particular instance, or maybe the whole series.

Thank you for this. I've been following reviews of this series elsewhere, but this provided information I hadn't seen. In particular, the red channel analysis may be relevant for purposes here.

Still hoping for some input/links from those who have used this for solar mods, esp. with the Lunt50PT.

~RA

[Ivan]

Here is the test of the svbony 503 telescope but 80 ED : https://star-hunter.ru/svbony-sv503-80ed-f7-doublet- refractor-review/ The result is spherical aberration in the red channel. Maybe this is a feature of a particular instance, or maybe the whole series.

Thank you for this. I've been following reviews of this series elsewhere, but this provided information I hadn't seen. In particular, the red channel analysis may be relevant for purposes here.

Still hoping for some input/links from those who have used this for solar mods, esp. with the Lunt50PT.

~RA

How to make a mod with those telescopes you are talking about, I do not know. But the principle itself is very well described by Filip here:
viewtopic.php?f=9&t=19238
Based on this information, I made my mod.

Ivan

[_RA_]

How to make a mod with those telescopes you are talking about, I do not know. But the principle itself is very well described by Filip here:
viewtopic.php?f=9&t=19238
Based on this information, I made my mod.

Ivan

That's the very post that got me going. In fact, you can see in that same thread that I bought a Celestron SkyScout90 to follow in the footsteps of GreatAttractor.

But now I'm thinking I'd like somewhat better optics and a slightly larger aperture. Plus, Mark's tantalizing suggestion that the SV503 102ED could have threads ready to go for the LS50PT would seem too good to pass up, if true... no need for custom machining. So I'm still holding out hope for confirmation.

I've asked elsewhere for input from owners, but that got passed over.

~ RA

[Ivan]

That's the very post that got me going. In fact, you can see in that same thread that I bought a Celestron SkyScout90 to follow in the footsteps of GreatAttractor.

But now I'm thinking I'd like somewhat better optics and a slightly larger aperture. Plus, Mark's tantalizing suggestion that the SV503 102ED could have threads ready to go for the LS50PT would seem too good to pass up, if true... no need for custom machining. So I'm still holding out hope for confirmation.

I've asked elsewhere for input from owners, but that got passed over.

~ RA

It seems to me that the difference between 100mm and 90mm will be subtle.
About what kind of thread the sv503 has, you can try to ask the seller by writing to him. If you need, I can send you (in a personal message) their phone number for communication via WhatsApp.

Ivan

[_RA_]

It seems to me that the difference between 100mm and 90mm will be subtle.
About what kind of thread the sv503 has, you can try to ask the seller by writing to him. If you need, I can send you (in a personal message) their phone number for communication via WhatsApp.

Ivan

I agree it's not a big difference. My quick and sloppy math shows 90mm to be about 78% area of 102mm.
Astronomy.tools shows a Dawes' limit of 1.29 vs 1.14; a Rayleigh of 1.53 vs 1.35; and a light grasp ratio of (predictably) .78.
So modest, but not negligible.

Mainly though, I'd want to upgrade the Celestron with a better focuser and other things, so I found myself inclined to put the money into a better scope all around, and the Astrotech and SVbony 102s seem like a really good value.

Yes, please send contact info, and thank you very much! I'd considered contacting them directly via their website, but there have been comments elsewhere about SVbony not responding with reliable and knowledgeable info. Certainly worth a shot, though.

I really appreciate all your help.

[DeepSolar64]

While I am not familiar with the Astro-Tech AT102ED I do have it's smaller sibling the AT72EDII. It's a truly impressive scope for the money. A doublet with two ED glasses, FPL53 mated to a Lanthanum element. It has no residual color that I have been able to see. The scope gives nice sharp images and is good for both low power wide-field views and high power using a 3-5mm eyepiece or a barlow or powermate. It does well with the Altair solar wedge I have.

The Astro-Tech scopes are well rated.

I may eventually get a AT130EDT.

James

[Ivan]

Hey,
I have sent you a private message. If they have anything interesting to say, please post here.
Thank you,

Ivan

[_RA_]

Sorry for the slow reply... just now back from travel transition, and settling in.

I will reach out to SVbony and report anything useful here.

[_RA_]

Well, I still don't have much in the way of solid data, but I thought I'd go ahead and update on the progress so far.

I went ahead and ordered the SV503 102ED today. While I couldn't get all the info I wanted for solar purposes, I decided this is a good value on its own as a night scope. As of this writing, it is on sale at the SVBONY site for US $539.00 with the finder dovetail (not normally included) as part of the package, and I also ordered their 2" to 1.25" adapter for $12.99. Shipping to my area was $87.18 with 3 - 7 day arrival estimate. I was also able to leverage a coupon, so all in was a little over $600.

Since my last post, I've been working with a very diligent customer service rep. She provided me with photos that were clearly not stock, and aimed specifically at my questions. Despite this, I was never able to obtain the thread size for where the focuser screws into the flange (possible mutual language gap). For what it's worth, she did come up with specs for the other side of the flange, and should they be of any use to anyone, I share them here:

70ED：M82X1
80ED：M86x1
102ED：M104.5X1

When the scope arrives, I will assess the metrics as best as I can, and test fit the LS50PT to see if there's a match, and will report back. Finding back focus may take a while longer, though I understand the principle. Then, even if it's a ready-made, I don't yet have an ERF on hand to make it sun-safe for actual view testing.

I'm unlikely to be willing to shorten the OTA if that's what is called for. So like I say, I'm reconciled that this may be my new night scope.

More to come in a week or so.

~ RA

[Ivan]

Hello,
did you receive your telescope? Is there any progress in building the mod?

Ivan

[_RA_]

Hi, and sorry for the latency. I'd hoped to have everything together for a fuller report by now, but no such luck.

The scope did arrive within a week of purchase. As many have noted elsewhere, the build is solid. I have no experience with premier telescopes, but it's hard for me to imagine what more I could want aside from premium glass.

Upon arrival, I did a quick tear-down on the focuser end and could see immediately that no threads match the LS50 m68, so it's definitely not a ready-made.

As for viewing, timing has been terrible. The moon is new, the planets are hibernating, and even simple star tests were fraught with shaky air. Freezing rain and clouds are now in the forecast for a while yet.

I'm still investigating potential for solar mod. Better calipers than what I had arrived yesterday, so I'll come back with some measurements for anyone following along.

Stay tuned.

[_RA_]

What follows are some metrics on the SV503 102ED flange to focuser assembly. Measurements were made with cheap digital calipers, and I'm not a machinist, so expect some imperfections in the data.

None of this is useful for our purposes here without an empirically measured focal point, but that's still to come. I don't have a good distant object nearby, so I'm waiting for a bright moon and clear skies for the projection method.

Until then, there's this for your consideration:

Tube ==> Focuser Assembly
Tube ♂︎ | ♀︎ Flange 1 ♀︎ | ♂︎ Sleeve ♀︎U | ♂︎U Flange 2 ♀︎ | ♂︎ Focuser ♀︎ | ♂︎ Ocular Assembly ♀︎

♀︎ = Female Part
♂︎ = Male Part
U = Unthreaded

Element ------- Connection 1 ------- Connection 2 ------- Length
Tube ---------- N/A ------------- ♂︎ 105.0mm ---------- 0.0mm
Flange 1 ------ ♀︎ 103.4mm -------- ♀︎ 85.0mm ---------- 19.0mm
Sleeve -------- ♂︎ 85.7mm --------- ♀︎U 79.0mm --------- 14.0mm
Flange 2 ------ ♂︎U 79.0mm -------- ♀︎ 73.0mm ---------- 21.3mm
Focuser 2" ---- ♂︎ 73.7mm ----------♀︎ 53.2mm ---------- 48.3mm
EP Ring ------- ♂︎ 53.8mm --------- ♀︎ Standard 2" -------- 15.0mm
1.25 Adapter -- ♂︎ Standard 2" -------♀︎ Standard 1.25" -----10.0mm

Dimensions are either ID or OD according to gender. Length is assembled element's contribution to total assembly length.
Total measured length of assembled fixed elements (no EP Ring, Adapter) is 103.0mm.
Total length of individually measured elements is 102.6mm, so 0.4mm measuring error.
All measurements may include rounding within < 1mm.

The flanges are step-downs from tube diameter to focuser. The sleeve between the flanges is what affords the full 360˚ rotation of the focuser. There are three set-screws which secure this in a recessed track, plus a thumb-screw for locking it in position.

On the front end, the retractable dew shield has an OD of 121mm, in case anyone is considering putting a filter there.

Hope this will be useful to someone, and if it sparks any ideas, don't be shy.

~ RA

[Trying to reformat the tabular data to line up correctly here was a nightmare, and may not even convey across browsers. Let me know if there's a better way to handle tables in the posts, please.]

[marktownley]

Useful to know the metrics thanks!

[_RA_]

So the other night I got the opportunity to measure the focal point, and arrived at a distance of 110mm from the last piece of hardware.

This places the etalon assembly's 168mm position from the focal plane at 15mm from Flange 2, or an additional 21mm from the Rotator Sleeve. Therefore a gap of either 15mm or 36mm will need to be accounted for, not including internals such as threads or smooth fittings.

The method was using lunar projection of a just-past first quarter moon. For hardware I borrowed my old Tasco solar projection kit, which is a rod supporting two moveable plates. The first plate has a centering hole, and the second plate can be moved to received the image at focus.

Normally an eyepiece is used to project the image, but in this case I wanted to locate prime focus, so an empty Barlow tube served to anchor the first plate into the 1.25" EP receptacle. The space between the plates when focused is then measured, and we get the distance from focal point/plane to the first surface on the telescope body -- in this case, the 1.25" EP receptacle.

As an exercise, I performed the same operation on the LS50 with the etalon and focuser removed. I had to tape the first plate to the tube, which was messy and fiddly, but arrived at the predicted 168mm within a reasonable margin of error.

I can see a couple of paths forward with the mod. I'll need to span the space from PT front to reach either Flange 2 or Sleeve, and of course account for the discrepancy in diameters. Seems doable, but suggestions are welcome.

~ RA

[_RA_]

... Though I do have questions.

I understand that 168mm is where Lunt chose to place the PT ahead of focus in an F7 cone, despite the 20mm etalon size. But the collimating lens is ~40mm, which would place it for maximum illumination at 40 x 7 = 280mm. If the negative lens is configured for an exact placement at 168mm, why the extra glass?

I've read and re-read this thread numerous times, and catch something new with each pass. But my understanding of optics is probably less than rudimentary, so whatever conclusions are settled there still escape me.

From what I can gather, placement at 168mm serves as a field stop, so presumably a useful reduction in brightness and a tighter FOV, plus a high percentage of the image inside the sweet spot. I'm less sure about the geometry of the telecentric path vs the converging F7 cone. I can make guesses about effects on illumination and FOV with forward or backward displacement, but less confident about the sweet spot.

In short, how much latitude is there with the 168mm, and how much deviance becomes truly deviant, and in what ways?

I come seeking the Wisdom of the Elders.

~ RA