Newtonian design, central obstruction and secondary sizing for imaging

[900SL]

I'm looking at a TS ONTC 150mm f5 and have some questions regarding secondary sizing for imaging, using a small sensor camera (533MC or MM pro with 11 mm sensor, or a 178MM with a barlow)

I've only just dipped my toes into this but I understand the basics (I think ), namely fully intercepting the f5 lightcone, the distance from secondary to focal point, size of fully illuminated field/fall off, and avoiding secondary edge issues?

Using Mel Bartels Newtonian Scope Designer here:  http://www.bbastrodesigns.com/NewtDesigner.html#diagonal

I get a 35mm secondary assuming 150mm FL, f5 mirror, assumed 150mm from secondary to FP.

I think the secondary fully captures the lightcone? 

23% central obstruction with a 35mm secondary

Fully illuminated region is 8mm and light fall off is not significant to the edge of a 533 sensor?

Appreciate any feedback on this

 

[vlaiv]

9 minutes ago, 900SL said:

I get a 35mm secondary assuming 150mm FL, f5 mirror, assumed 150mm from secondary to FP.

I think that 150mm is very short distance between FP and secondary.

From optical axis to the surface of the OTA, there is at least 80mm of distance if not more.

Actually - it is 180mm according to TS website. It has 175mm of inner diameter of OTA tube and 2.5mm wall thickness.

So there is total of 90mm "eaten up" by OTA alone. That places focal plane at just 50mm above surface of the OTA, and I don't think that focuser is very low profile.

Focuser that comes with that OTA has 58mm to 2" connection:

and on top of that you need to account 2" to sensor distance for your camera. All in all - I'd put FP to secondary distance to be at least 170mm - 175mm

[900SL]

Thanks Vlaiv, I re-ran the numbers on a 200mm secondary distance and I'm looking at a 50mm secondary I think, which is 33% obstruction.

Is this likely to be an issue with imaging contrast and resolution, and am I heading towards a 200mm diameter newt?  

 

 

[vlaiv]

3 minutes ago, 900SL said:

Is this likely to be an issue with imaging contrast and resolution

What are you going to image? Planets or DSO?

You won't have any issues with contrast as contrast can be adjusted for images. It is only important for visual - where you can "tweak" the image.

Resolution will be minimally impacted for planetary (most people do planetary images with SCTs which have up to 40% central obstruction) and virtually not at all for DSO because resolution is governed by other factors - seeing, mount performance and aperture size.

If you take two 6" scopes - one with large secondary and one with optimized secondary - you might be able to see the difference at the eyepiece when viewing planets in good seeing - but otherwise, scopes will perform the same and you won't be able to distinguish them.

[900SL]

Hopefully both.. lunar with barlow and DSO with coma corrector

[vlaiv]

Well, in that case it might be worth optimizing secondary, but do consider camera upgrade in the future.

[ONIKKINEN]

Mine has a roughly 33% obstruction, and its definitely not too much for imaging. For visual, it has been an issue for a few times although it has been a while since i actually looked through the scope. The central obstruction can be visible at lower powers and in a bright environment such as very light polluted skies or looking at the Moon.

By the way even if you do have a small sensor now, will that be the case in the future? Sizing the secondary upwards might be a smart choice that saves you the headache of refitting one in the future. Also it will help with flats somewhat. Its very easy to have an off-center fully illuminated circle with a newtonian and with a small mirror that will have a greater effect on vignetting and may become an issue.

[900SL]

22 hours ago, ONIKKINEN said:

Mine has a roughly 33% obstruction, and its definitely not too much for imaging. For visual, it has been an issue for a few times although it has been a while since i actually looked through the scope. The central obstruction can be visible at lower powers and in a bright environment such as very light polluted skies or looking at the Moon.

By the way even if you do have a small sensor now, will that be the case in the future? Sizing the secondary upwards might be a smart choice that saves you the headache of refitting one in the future. Also it will help with flats somewhat. Its very easy to have an off-center fully illuminated circle with a newtonian and with a small mirror that will have a greater effect on vignetting and may become an issue.

Kiitos Oskari

The more I look at it, the more an 8" f4.5 ONTC makes sense. But then I add up all the costs...

[ONIKKINEN]

1 hour ago, 900SL said:

Kiitos Oskari

The more I look at it, the more an 8" f4.5 ONTC makes sense. But then I add up all the costs...

My scope is basically the TS f/4.5 ONTC at this point, but it came to be so as a ship of theseus and not a single purchase.

The initial scope (VX8) was around 900€, carbon tube from klaus helmerichs 380€, better secondary spider 200€, Baader diamond steeltrack focuser 400€, CNC rings and losmandy plate maybe 300€, flocking and other miscellaneous stuff lets say 100€ (at least). At this point the price starts to look the same as the TS ONTC version, so i think their price for that scope is quite fair.

Edited June 8, 2023 by ONIKKINEN

[900SL]

On 08/06/2023 at 14:34, ONIKKINEN said:

My scope is basically the TS f/4.5 ONTC at this point, but it came to be so as a ship of theseus and not a single purchase.

The initial scope (VX8) was around 900€, carbon tube from klaus helmerichs 380€, better secondary spider 200€, Baader diamond steeltrack focuser 400€, CNC rings and losmandy plate maybe 300€, flocking and other miscellaneous stuff lets say 100€ (at least). At this point the price starts to look the same as the TS ONTC version, so i think their price for that scope is quite fair.

I know who I'm calling when I need advice on collimation

Do you have a dew shield or heater on the secondary Oskari? 

I'm still not decided on this set up, I might stick with the 90mm refractor for widefield and get a Bresser MC 127/1900 Mak for lunar and planetary ( has good reviews for planetary). The 8" Newt might be a handful on my mount in any wind.

[ONIKKINEN]

7 hours ago, 900SL said:

I know who I'm calling when I need advice on collimation

Do you have a dew shield or heater on the secondary Oskari? 

I'm still not decided on this set up, I might stick with the 90mm refractor for widefield and get a Bresser MC 127/1900 Mak for lunar and planetary ( has good reviews for planetary). The 8" Newt might be a handful on my mount in any wind.

Just a dew shield + i ordered my tube to be a little bit longer than it should for the focal length. The secondary sits maybe around 40cm recessed into the tube. But also, the spider in mine has a 60mm central diameter while the mirror is only 63mm in minor axis diameter so very little pokes out from behind it. So far have only found ice on the edge of the secondary once, but to be fair i only happened to check out of curiosity that time and not because i noticed something was wrong. Could be happening more often but if it is its not a major issue.

If you plan on mostly imaging with the planet scope then you should go for the biggest aperture you can muster. The small maksutovs are good for their size and price and apparently for visual because of their great contrast, but will probably lose to any decent 8'' newtonian when imaging. Actually you could go for a 10'' even, if you plan on only taking on the Moon and planets since tracking accuracy is not that critical for that use.

Wind is a real issue though, no solution for that. Have made a round trip to the Porkkalanniemi site a couple of times without even bothering to set up because it was too windy. Some other times i chose a mosaic target that i could bin x3 or more so that i can "hide" the bad data caused by wind. But in that case you could just skip the headache and use the 90mm that probably doesn't fall apart in a breeze.

Edited June 10, 2023 by ONIKKINEN

[Dan_Paris]

On 07/06/2023 at 11:29, 900SL said:

Using Mel Bartels Newtonian Scope Designer here:  http://www.bbastrodesigns.com/NewtDesigner.html#diagonal

I get a 35mm secondary assuming 150mm FL, f5 mirror, assumed 150mm from secondary to FP.

I think the secondary fully captures the lightcone? 

 

 

You should tell first which coma corrector you plan to use. Why ?

1. Because the corrector contributes to vignetting as well. At some point increasing the secondary size will not improve field illumination, because vignetting by the corrector takes over.
2. Because the best position of the focal plane (which is correlated to the primary to secondary distance) is dictated by the corrector characteristics (length, backfocus, focus shift).

 

 

[900SL]

4 hours ago, Dan_Paris said:

 

You should tell first which coma corrector you plan to use. Why ?

1. Because the corrector contributes to vignetting as well. At some point increasing the secondary size will not improve field illumination, because vignetting by the corrector takes over.
2. Because the best position of the focal plane (which is correlated to the primary to secondary distance) is dictated by the corrector characteristics (length, backfocus, focus shift).

 

 

Hi Dan and thanks. I'm new to newts but this lot seemed to stack up, so to speak:

ONTC 8" F5 (changed from 6" due to availability)

Focuser: TS UNCN2-G2 R&P focuser:  https://www.teleskop-express.de/shop/product_info.php/info/p14361_TS-Optics-2-Inch-UNC-Gen--2-Newtonian-Focuserwith-additional-thread---loads-up-to-5-kg.html

Coma corrector:  TS-Optics NEWTONIAN Coma Corrector 0.95x Maxfield - 3-element - 2" connection  https://www.teleskop-express.de/shop/product_info.php/info/p3822_TS-Optics-NEWTONIAN-Coma-Corrector-0-95x-Maxfield---3-element---2--connection.html

I went for the Maxfield because its shorter and should not project into the tube, and should work well with my 533 sensor (or APS-C)

Regards

 

Richard

[ONIKKINEN]

Have you ordered all of that already? Might want to rethink that corrector as it does not correct coma completely and has generally terrible sharpness across even a small field (see spot diagrams and reported RMS radius). Nowhere near diffraction limited, i am pretty sure your current refractor has better sharpness than the Maxfield would give.

I had one, but then again my scope is a little bit faster than f/5 so in a worse starting position so might not be that bad for yours. But overall would never recommend that to anyone just because of the absolutely butchered spot diagrams it is designed to give.

You can borrow mine to test if you want to, was not planning on ever using it again anyway.

[900SL]

53 minutes ago, ONIKKINEN said:

Have you ordered all of that already? Might want to rethink that corrector as it does not correct coma completely and has generally terrible sharpness across even a small field (see spot diagrams and reported RMS radius). Nowhere near diffraction limited, i am pretty sure your current refractor has better sharpness than the Maxfield would give.

I had one, but then again my scope is a little bit faster than f/5 so in a worse starting position so might not be that bad for yours. But overall would never recommend that to anyone just because of the absolutely butchered spot diagrams it is designed to give.

You can borrow mine to test if you want to, was not planning on ever using it again anyway.

Thanks Oskari, I always appreciate your straight talking

The GPU was the better option as I recall from the other thread about the Bresser Newt?

Not ordered yet, I'm spending these bright northern nights filling on line shopping baskets but trying not to press the button.

Edited June 12, 2023 by 900SL

[ONIKKINEN]

13 minutes ago, 900SL said:

Thanks Oskari, I always appreciate your straight talking

The GPU was the better option as I recall from the other thread about the Bresser Newt?

Not ordered yet, I'm spending these bright northern nights filling on line shopping baskets but trying not to press the button.

I am seeing the same reasoning in your thinking as i had 3 years ago. Only i did not bother actually researching and was ultimately disappointed in the Maxfield but i got one for the same reasons you were thinking of one.

By the way the focuser intrusion thing should be avoidable with the ONTC scopes because they have 3 sets of mounting holes for the primary at different distances so you can mix and match gear as you will. Any corrector should reach focus and intrusion should be minimal if at all present.

The GPU seems like the safe bet judging from the spot diagrams although the ES one is not bad either. Particularly the flatness of the spot sizes looks very good which is important for a newt. This is (not entirely) because small amounts of field curvature result in double diffraction spikes at the edges which is very obvious even if the star sizes are not noticeably different. Collimation is also trickier with a less flat corrector because small amounts of miscollimation lead to large differences in spot sizes as the light cone goes further from center. The GPU at least on paper is more resistant because it is so flat (and they do mention this in the description too).

[Dan_Paris]

I think the Maxfield is OK at f/5 but not at f/4 as it adds some spherical aberration.  But if you can afford the GPU it is certainly a better option.

3 hours ago, 900SL said:

ONTC 8" F5 (changed from 6" due to availability)

 

Is the scope already assembled or is there some possibility of customization, in terms of primary to secondary distance (or equivalently position of the focal plane) and secondary size? Ideally those parameters should be tuned to a specific coma corrector to optimize the illuminated field and the secondary obstruction.

Edited June 12, 2023 by Dan_Paris