Changing from a 1.1" Mirror to a 1.3" mirror in a reconfigured TAL-1 - Good idea?

[marcelteaching]

My wonderful TAL-1 (original), that has had it's primary raised by 28mm to achieve focus with modern eyepieces (no longer the original 32mm eyepieces) in the focuser.  

It is of course, a 110mm f/7.3 primary, with a 27mm/36mm secondary.  Sadly, it has a damaged secondary (scratched, sides degrading).

I would like to replace it, and frankly, as I love this scope so much, I don't mind getting an upgraded quality Antares mirror to do it - but the smallest they come in is 1.3".  

So, I am wondering, will that improve the quality of the view, or is the light gathering going to be poorer? 

To be clear, I want to use this more as a deep sky scope and less of a planetary scope - so a fuller Illumination sounds good.  

I'm not good with maths and am struggling with the instructions on Gary Seronik's page.  Can someone who is familiar with the scope go through the numbers and tell me the minimum secondary size, so I can make an educated guess on whether a 1.3" secondary would be worth it?

Incidentally, I also need a replacement TAL focuser, so if you have one spare, let me know. 

[markse68]

Don’t know the answer to your question except increasing secondary obstruction reduces contrast I think 🤔 

Protostar have 1” quartz mirrors

https://www.fpi-protostar.com/quartz.htm

edit- out of stock 🤦‍♂️

Edited November 7, 2019 by markse68

[Ben the Ignorant]

Teleskop Service has 25mm, 28mm and 31mm diagonal mirrors with 94% reflectivity.

https://www.teleskop-express.de/shop/product_info.php/info/p10193_TS-Optics-Newton-Fangspiegel-elliptisch---D-25-mm-kleine-Achse.html

https://www.teleskop-express.de/shop/product_info.php/info/p1666_TS-Optics-Newton-Fangspiegel-elliptisch---D-28-mm-kleine-Achse.html

https://www.teleskop-express.de/shop/product_info.php/info/p1668_TS-Optics-Newton-Fangspiegel-elliptisch---D-31-mm-kleine-Achse.html

The 94% reflectivity figure is typical of GSO so these are probably made there. The first two letters in their product code, GS, suggests the same origin, so they should be very good.

[Ben the Ignorant]

18 minutes ago, markse68 said:

Don’t know the answer to your question except increasing secondary obstruction reduces contrast I think

Yes, it does.

[NGC 1502]

A simple way to decide is this ( assuming the scope is fully assembled )  :-

Rack the focuser out to where it normally is to achieve focus on a distant object.  Put your eye close to the focuser without an eyepiece in position. You will see the primary mirror’s reflection in the secondary.   Ideally the edge of the primary reflection should be fully within the secondary reflection with a bit of space all around. If that’s what you see then the current secondary is about the correct size.

A replacement secondary a bit larger ( within reason ) won’t cause a problem in practice, even if theory suggests otherwise.  If the secondary is not large enough then not all the primary mirror’s light is being intercepted so reducing the scope’s effective aperture.

Hoping that helps, Ed.

 

 

[Peter Drew]

I have a Tal1, the optics are excellent and I doubt whether you would notice any improvement with an "upgraded" diagonal, specially for lowish power deep sky use. A slightly oversize secondary is preferable to an undersize one. For open clusters, which form the bulk of objects nicely seen in a smallish telescope the improved illuminated field will more than offset any reduction in contrast.    🙂

[DaveL59]

thinking about it simplistically and I may be off the mark, but having moved the primary up the tube (I've done the same), you're effectively intercepting the cone of light earlier at a slightly wider point. So a slightly larger secondary might make a bit more of the image the primary is projecting, no?

Not sure if the later Tal-1's with shorter tubes used a different secondary to allow for that, perhaps someone else here with one would know or be able to measure and let you know. Alternatively you could always get the secondary re-coated.

[Second Time Around]

For easy calculation of diagonal size plus lots of useful info on the subject I use this: https://www.bbastrodesigns.com/diagonal.htm 

Edited November 7, 2019 by Second Time Around
Added "easy"

[marcelteaching]

Thanks for the comments so far, a UK mirror would be good, but I was going to put in something  better than a GSO...

>but having moved the primary up the tube (I've done the same), you're effectively intercepting the cone of light earlier at a slightly wider point. So a slightly larger secondary might make a bit more of the image the primary is projecting, no?

This is exactly what has made me confused... Has my moving it up meant that the secondary could now be bigger or smaller?

[DaveL59]

bigger as the secondary is now lower into the light cone if you see what I mean

if you consider the primary as the base of an isoscelese triangle and the secondary sits near the point where the long sides meet, the line across parallel to the edges forms a plane of a certain length. Moving the primary up is much the same as lowering the intercept point nearer the base, resulting in a wider plane at that new point. Yes I know the secondary isn't parallel but the result is much the same, closer the secondary gets to the primary, the wider the light cone hitting it...

Not a great pic but might illustrate what I mean, if you consider upper and lower red lines as where the mirror was/is tho in the Tal it won't be that big of a difference  

Edited November 7, 2019 by DaveL59
add pic

[Ben the Ignorant]

51 minutes ago, marcelteaching said:

I was going to put in something  better than a GSO

Many people assume GSO is just average or subpar quality because it is mass-produced and "not expensive enough", but that ignores two decades of progress in machinery and the plain good will and competence of GSO and its dealers. Looking at hard data from labs, this is what is found when testing two 16-inch GSO parabolic newtonian mirrors:

A 0.93 Strehl ratio; Astro-Physics, LZOS, Takahashi, Vixen and TEC promise 0.95 Strehl ratio in their "expensive enough" apos but the surfaces they produce are never as large as 400mm, so GSO did an excellent job. The test dating from 2008 the mirror is at least that old, so the tooling and/or the know-how at GSO might well have been improved since. That 1/24th wave surface quality is great work for such a large area to polish.

Now watch this:

Another super-large GSO parabolic newtonian mirror, with 0.98 Strehl quality factor! 1/8th wave peak-to-valley, and a nearly science-grade or military/espionage-grade 1/42th wave overall surface quality! Date is the same, lab is that unmerciful Wolfgang Rohr. The complete page is here:

https://astro-foren.de/index.php?thread/9274-die-neuen-gso-16-zöller-im-vergleich/&postID=76801#post76801

So, if GSO can make such splendid 400mm mirrors, how difficult do you think it is for them to produce an excellent 28mm mirror?

Edited November 7, 2019 by Ben the Ignorant

[Merlin66]

Marcel,

Moving the main mirror means that the focus is moved further from the side of the tube (usually done to allow easier camera access)

Ideally a slightly larger secondary mirror would be required to maintain the same un-vignetted FOV. Usually the secondaries are over sized due to the standard sizes used, so not a major issue..

In general, larger secondaries allow larger field coverage at the cost of blocking more of the aperture and the impact on the Airy Disk.

Many planetary observers actually reduce their secondaries to a minimum, just sufficient to give a small FOV around the planets.

 

[joe aguiar]

3 hours ago, Ben the Ignorant said:

Many people assume GSO is just average or subpar quality because it is mass-produced and "not expensive enough", but that ignores two decades of progress in machinery and the plain good will and competence of GSO and its dealers. Looking at hard data from labs, this is what is found when testing two 16-inch GSO parabolic newtonian mirrors:

A 0.93 Strehl ratio; Astro-Physics, LZOS, Takahashi, Vixen and TEC promise 0.95 Strehl ratio in their "expensive enough" apos but the surfaces they produce are never as large as 400mm, so GSO did an excellent job. The test dating from 2008 the mirror is at least that old, so the tooling and/or the know-how at GSO might well have been improved since. That 1/24th wave surface quality is great work for such a large area to polish.

Now watch this:

Another super-large GSO parabolic newtonian mirror, with 0.98 Strehl quality factor! 1/8th wave peak-to-valley, and a nearly science-grade or military/espionage-grade 1/42th wave overall surface quality! Date is the same, lab is that unmerciful Wolfgang Rohr. The complete page is here:

https://astro-foren.de/index.php?thread/9274-die-neuen-gso-16-zöller-im-vergleich/&postID=76801#post76801

So, if GSO can make such splendid 400mm mirrors, how difficult do you think it is for them to produce an excellent 28mm mirror?

Ben great post

I agree most of my stuff is meade celestron skywatcher stuff cept for 2, even those I only bought them this year, so for 23 years I did without.

I agree most mass produced stuff is fine for most people, altho if u got the money and want the extra % go for it

Joejaguar 

[marcelteaching]

The GSO mirrors are good - but I do still think the Antares is better - I wonder though how silly it is worth going? (They range from 49.95 to 99.95)
I wonder what the original TAL mirrors are like? 
I suppose if the primary is very good, it would be worth putting in a very good secondary?
It sounds like going to a 1.3 (from the TAL’s original 1.1”) will be fine - especially since the main mirror has moved up even though it will slightly reduce the light hitting the secondary.
 

Edited November 8, 2019 by marcelteaching

[Merlin66]

Marcel,

As I said, moving the main mirror will increase the light hitting the secondary.

 

[marcelteaching]

Thanks for clarifying, so Yes, a larger secondary is good given I've moved up the primary and more light is hitting it now.  

Does anyone know what the optical quality of the early TAL is like? 

[marcelteaching]

Also - just thinking... would the original hole in the tube be blocking some of the deflected light from the secondary now that the primary has been moved up?

[NGC 1502]

29 minutes ago, marcelteaching said:

Also - just thinking... would the original hole in the tube be blocking some of the deflected light from the secondary now that the primary has been moved up?

Very unlikely given that the light cone from the secondary is getting ever smaller towards the focuser.    Also consider how relatively small is the field lens on the eyepiece is.......

Ed.

[Ben the Ignorant]

6 hours ago, marcelteaching said:

Does anyone know what the optical quality of the early TAL is like?

No, but if you take pictures of the defocus patterns, close to focus and far from focus, on both sides, and maybe get a 40€ Ronchi tester, it can be shown and discussed.

[DaveL59]

4 hours ago, marcelteaching said:

Also - just thinking... would the original hole in the tube be blocking some of the deflected light from the secondary now that the primary has been moved up?

More likely that the secondary isn't reflecting the whole primary light cone now its closer to the primary, I'd have thought, but it'll be relatively little lost compared to the convenience of using std 1.25 inch EPs. A larger secondary would catch the bit that's lost and then you may want to enlarge the hole for the focuser (I believe astrobaby did that on hers when she overhauled but for other reasons) but since the focuser tube isn't going to be changed for a wider one probably little point in doing so.

[Second Time Around]

I mentioned earlier in this topic about https://www.bbastrodesigns.com/diagonal.htm as a way to calculate diagonal size.

What I should have added that inputting the parameters into the calculator is easy except perhaps for one - the distance from the centre of the secondary to the focal plane.

Apologies if I'm teaching Grandma to suck eggs but the focal plane is where the image from the primary mirror is in focus.  To determine where this point is take the eyepiece out and rack the focusser in as far as it will go.  Then point the scope at the moon.  Take a piece of white card and gradually move it away from the focusser until you can see the moon in focus.  Then measure the distance the card is from the tube of the scope.  Add that to half the diameter of the tube and that gives you the "diagonal to focal plane distance" to enter into the online calculator.

It sounds more difficult than it is.  If I can do it anyone can!