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StellaLyra Classical Cassegrain & Ritchey-Chrétien Telescopes


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10 hours ago, neil phillips said:

Why not a dew sheild increases contrast too. 

I'm in the process of building the power and dew heater boxes ready for this scope and wanted to at least anticipate the need. I am likely to add some  shielding to help ward off the local garden/street lights but that's a quick fix with some camping mat. Another new floodlight has just appeared in one of the local gardens, they must breed! Thanks anyway!

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  • 3 weeks later...

More or less agree although not completely. The scope is a lunar planetary scope anyway. Also fitting the Baader Steeltrack means not having to add or remove the extenstions which I agree is a pain. 

Wouldn’t be my only scope but as Ed says would appeal to those with a scope collectiom where it would fill a niche very well.

Should also be noted that the C6 is a bit variable in optical quality. Ranges for so-so to excellent.

 

Edited by johninderby
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  • 1 month later...

Is there any plan having 5" RC? I am in a hunt for a cheap 5" f~8 telescope for capturing galaxies and planetary nebulas.  RC design seems to be ideal, but 6"is a bit too big for my setup, and m, closet, so I would prefer something smaller. Or some different type of telescope I should watch out for?

Celestron C5 STC  is s bit too high in price. And maks are too slow, and not best solution for galaxies. Any ideas? 

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50 minutes ago, Vulisha said:

Is there any plan having 5" RC? I am in a hunt for a cheap 5" f~8 telescope for capturing galaxies and planetary nebulas.  RC design seems to be ideal, but 6"is a bit too big for my setup, and m, closet, so I would prefer something smaller. Or some different type of telescope I should watch out for?

Celestron C5 STC  is s bit too high in price. And maks are too slow, and not best solution for galaxies. Any ideas? 

I doubt there will be 5" or smaller RC - secondary obstruction will be just too large to get decent sized field for imaging. Even 6" version is F/9 and not F/8 because of this.

If you really want 5" scope with long focal length - then why not Mak127? Too slow?

F/12 is not too slow :D - at least not if you know how to treat your data.

Don't think in terms of F/speed - think in terms of aperture at resolution.

Say you want to image at 1.5"/px with 5" aperture. Many people use for example 4" APOs and image at higher resolution, and although APO will be F/5 or something like this - your scope at F/12 will be faster.

Say you have 3.75um pixel size DSLR sensor. Mak127 has 1500mm of focal length. Pixel size would suggest that you will sample at 3.75*206.3/1500 = 0.515"/px - but why don't you bin your data x3 to get to 1.5"/px or maybe even x4 to get to 2"/px.

Only thing that you need to worry about is single exposure - you need to make it long, like 5 or more minutes long - to overcome read noise.

Alternatively, why don't you get 130PDS? That is also light weight imaging scope. It is also capable of similar resolutions - like 1.5"/px - 2"/px and you don't need to bin it.

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Hi vlaiv, thanks on your response, so I see, too big of a second mirror and it would be like f10 and there would be vignetting on full frame ir even aps c. 

Well I have EQ5 with belt onstep, rms is ~1-2 arcsec. So additional weight, and longer FL is not best combination I beleive. 

Currently I have 130p heritage that has overpolished mirror and I lose some details there. I have nice small 330/61 APO for wide field. So 130 pds would be bigger(optically better) version of heritage but I would need coma corrector, that could be hit and miss as is my OVL FF for APO that is not flatting enough :D, while i do not need anything for RC and it is inexpensive. 

127 mak is a bit heavy and i would need correction as well? together with 1500 fl i do not think my mount would be able to handle those 5 mins. Maybe 102 mak as last resort? 

Regarding cameras I own 550D and Rpi High Quality camera, hardware binned to 2x (1.55->3.1um px)

 

 

Edited by Vulisha
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2 hours ago, Vulisha said:

Well I have EQ5 with belt onstep, rms is ~1-2 arcsec.

Ok, so that there is a bit of a problem.

You already have a scope that will provide you with nice focal length given your pixel size.

2 hours ago, Vulisha said:

Currently I have 130p heritage that has overpolished mirror and I lose some details there

What exactly do you mean by over polished mirror?

Another important point is - what are you trying to achieve? You say you want to capture galaxies and planetary nebulae. Do you have any idea of working resolution (how "close in" do you want to get)?

Or at least what sort of FOV you want for your images?

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11 hours ago, Vulisha said:

so I see, too big of a second mirror and it would be like f10 and there would be vignetting on full frame ir even aps c.

Your comment here seems to be regarding the suitability of using a hypothetical 5" RC with a larger sensor.  From my experience with the 6" RC, this needs a flattener even for an APS-C sensor. I also considered a full-frame flattener but this was impractical for some reason now forgotten.  

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So I got a guy that I took telescope to for colimation as I was not able to get good resolution on planets, so I was sure I was doing bad colimation as I do not have laser, only camera, but he took a look through it and he said that miror is basically hyperbolic and not parabolic and that colimation is good but what i got is maximum I can get out of it as mirror is less than average. 

 

Regarding FoV I am aiming for roughly ~1-1.5° so I can fit M81, Sombrero, Leo triplet and planetary nebulas, small and large. 800mm FL was some "minimum" and 1200mm would be "maximum" I believe. 

 

@fireballxl5 I do have OVL FF for APO, would that be theoretically usable? 

Edited by Vulisha
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2 hours ago, Vulisha said:

So I got a guy that I took telescope to for colimation as I was not able to get good resolution on planets, so I was sure I was doing bad colimation as I do not have laser, only camera, but he took a look through it and he said that miror is basically hyperbolic and not parabolic and that colimation is good but what i got is maximum I can get out of it as mirror is less than average. 

 

Regarding FoV I am aiming for roughly ~1-1.5° so I can fit M81, Sombrero, Leo triplet and planetary nebulas, small and large. 800mm FL was some "minimum" and 1200mm would be "maximum" I believe. 

 

@fireballxl5 I do have OVL FF for APO, would that be theoretically usable? 

If telescope is slightly under or over corrected and is not the best performer on the planets - should not impact much deep sky imaging.

Difference in resolution between DSO imaging and planetary imaging is often x2-x3 and even if image is not that sharp on planets - it does not matter for DSO imaging as atmosphere blurs planetary level detail in long exposure.

RC scope with larger central obstruction has the same performance as telescope with 1/4 of wave spherical aberration of the mirror (which is to say that RC is not diffraction limited / or nearly diffraction limited scope by design) and is rarely used for planets - yet is very fine for DSO imaging.

But if scope is poor - you should probably look to replace it with something.

If your aim is for FOV of 1 to 1.5 degrees that is 60 to 90 arc minutes or 3600 to 5400 arc seconds.

There is rule of the thumb that your guide RMS should really be half of your imaging resolution. This means that with 1-2" RMS guide error - you can't image below 2"/px

FOV of 3600"-5400" then translates into 1800-2700px image size.

Your Canon 550D has 5184 x 3456 pixels so there is plenty of space to capture 1800-2700px in width.

You can sample at each pixel and crop

You can use bin x2 and even bin x3 (this will give you ~1700px in width).

Pixel size of 550D is 4.3um so you have basic focal length for 2"/px of 443mm. From this I'd say to look for telescopes that are either 400-440mm, 800-900mm or 1200-1300mm in focal length.

What is your budget? I'm thinking that you'll be best served with say 80mm F/5-F/6 ED refractor if you want to keep the weight down. You already have OVL FF, right? But that is non reducing, so might only work with 80mm f/6 to give you what you want.

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4 hours ago, Vulisha said:

I do have OVL FF for APO, would that be theoretically usable? 

I don't know though I see that TS offer dedicated flatteners for both refractors and RC's, implying that they are different designs. I have only used the TSRCFlat2, https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4006_TS-Optics-RC-1-0x-Flattener-Bildfeldkorrektor-fuer-Ritchey-Chretien---2--Anschluss.html

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2 hours ago, fireballxl5 said:

I don't know though I see that TS offer dedicated flatteners for both refractors and RC's, implying that they are different designs. I have only used the TSRCFlat2, https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4006_TS-Optics-RC-1-0x-Flattener-Bildfeldkorrektor-fuer-Ritchey-Chretien---2--Anschluss.html

Oh, so they probably are different makes sense if you think about it. 

4 hours ago, vlaiv said:

If telescope is slightly under or over corrected and is not the best performer on the planets - should not impact much deep sky imaging.

Difference in resolution between DSO imaging and planetary imaging is often x2-x3 and even if image is not that sharp on planets - it does not matter for DSO imaging as atmosphere blurs planetary level detail in long exposure.

RC scope with larger central obstruction has the same performance as telescope with 1/4 of wave spherical aberration of the mirror (which is to say that RC is not diffraction limited / or nearly diffraction limited scope by design) and is rarely used for planets - yet is very fine for DSO imaging.

But if scope is poor - you should probably look to replace it with something.

If your aim is for FOV of 1 to 1.5 degrees that is 60 to 90 arc minutes or 3600 to 5400 arc seconds.

There is rule of the thumb that your guide RMS should really be half of your imaging resolution. This means that with 1-2" RMS guide error - you can't image below 2"/px

FOV of 3600"-5400" then translates into 1800-2700px image size.

Your Canon 550D has 5184 x 3456 pixels so there is plenty of space to capture 1800-2700px in width.

You can sample at each pixel and crop

You can use bin x2 and even bin x3 (this will give you ~1700px in width).

Pixel size of 550D is 4.3um so you have basic focal length for 2"/px of 443mm. From this I'd say to look for telescopes that are either 400-440mm, 800-900mm or 1200-1300mm in focal length.

What is your budget? I'm thinking that you'll be best served with say 80mm F/5-F/6 ED refractor if you want to keep the weight down. You already have OVL FF, right? But that is non reducing, so might only work with 80mm f/6 to give you what you want.

Well ED80 is my dream scope but it is way out of my budget even without proper FF. When I saw this RC in 6" I would expect 5" to be even less expensive and somewhere upto 500 eur. And that would be my limit. 

 

So if I understand your calculations correctly mak 102/1250 would be ideal for 3x binning on 550D? 

2 hours ago, fireballxl5 said:

I don't know though I see that TS offer dedicated flatteners for both refractors and RC's, implying that they are different designs. I have only used the TSRCFlat2, https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4006_TS-Optics-RC-1-0x-Flattener-Bildfeldkorrektor-fuer-Ritchey-Chretien---2--Anschluss.html

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1 minute ago, Vulisha said:

So if I understand your calculations correctly mak 102/1250 would be ideal for 3x binning on 550D?

It would certainly work.

There are few things that you should be careful with:

1. Maks have moving primary mirror - and guide scope is not the best way to guide them (much like SCTs) - OAG is better option. It might work for you as scope is small but you could easily get differential flexure - just something to keep in mind.

2. You'll probably get vignetting with APS-C sized sensor on Mak102 (and even Mak127), so flats are a must

3. You will need long subs - like 4-5 minute subs because of long focal length - light is spread over many pixels (which will be binned in software later) and it takes longer to swamp the read noise.

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