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Old 28-01-2020, 12:18 PM
Dober (Steve)
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Location: Delaney's Creek Qld Australia
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Changing primary mirror from f6 to f7, any issues


I am looking to get some comments on what effect good or bad would result in changing a f6 to f7.

My scope is an 8", 1200 fl. thus f6. I am wondering what issues would arise if that was to occur. Fixed length of tube and position of secondary would be an issue and slightly narrower field potentially?

My knowledge is limited, I have a few thoughts but I might as well hear it from those with more experience than I have at present. The reason is I might have the opportunity to get a better quality mirror than that supplied with the scope.

This is most likely a no no but thought I would ask, its all learning after all.

Thanks in advance for any comments.

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Old 28-01-2020, 12:25 PM
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You will have to add 20cm to your existing tube (at rear side) as the FL of your new mirror will be 1400mm.
Also, the tube balance will be affected.

All that is doable, but is it worth it?
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Old 28-01-2020, 03:17 PM
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mental4astro (Alexander)

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Adding to Bojan's points.

* An f/7 mirror would mean a smaller secondary mirror is possible to be used.

* % smaller true field of view isn't that significant from f/6

* From fabrication point, it is easier to produce a fine figure in a slower mirror than faster, but th f/ratio difference here is not that great. You will eant to KNOW that you ate getting a FAR BETTER f/7 mirror to justify the trouble. You will want a test report & know how to interpret the report results, or know what to look for in a star test. A mirror with a Strehl ratio of 0.98 is as close to perfect as possible, or a wave ratio of 1/8 or smaller.

* Hanging a bunch of extension tubes off the focuser is not the way to accommodate the longer focal length - you will introduce a huge amount of fles into the optical train. You really need a longer OTA. The new mirror may not actually fit the existing primary cell! I've seen this very often. For instance, Synta mirrors are all slightly over-size and won't fit in a GSO cell. You may need to make a new cell for the primary.

* From a planetary point, f/7 and slower is often considered more desirable than a faster ratio. I know of a 15" f/8 mirror, and damned if it is not tempting for me for making a lunar and planetary scope!!!

* Photographicallly f/7 is not really a big deal with today's cameras for imaging DSO's.

Unless you can determine a significant improvement in quality between the two mirrors, and this means testing both mirrors, really testing and not just hearsay, it isnot worth the change of mirrors. I am assuming your mirror is a GSO. On a whole their mirrors are good. But they can also throw up astonishingly good mirrors too. You need to know how to identify what you have, and not assume that because it is a GSO that it will be ordinary. That would be a mistake. Same with Synta mirrors. You might be surprised.

If you want some pointers on what to look for, just ask. For a novice it can be difficult to interpret star testing images, but there are some tips.

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Old 29-01-2020, 11:59 AM
Dober (Steve)
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Changing primary mirror from f6 to f7, any issues

Thanks you Bojan and Alex,

My scope could not be extended easily as it is a collapsible Skywatcher so although I am confident the other mirror would be better it appears the conversion would be not something I would attempt or worthwhile with my current skill level given other things with the optic train could be affected or require fine tuning.

As you say Alex this might encourage a change in secondary to maximise any benefit the new mirror might give me, and then there is the cell.

I read about primary mirrors from premium makers that some have in their scopes but to date I have not heard people talk of premium secondary's or who supplies them. Is there a premium maker that produces a matched set, is there such a thing, or is it indeed necessary to go that far?

I would have thought that if one can see big improvements with their top of the line primary mirrors and they have quality eyepieces (Televue etc) that a basic secondary supplied with the average off the shelf scope would provide a weak link in the chain, this is of course with good collimation and seeing.

Your thoughts on my thoughts re secondary mirrors please.

In the short time I have been a member of this forum I see you both are regulars in providing advice. The open sharing of hard earned knowledge is a wonderful thing. Thanks
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Old 29-01-2020, 05:21 PM
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See https://www.bbastrodesigns.com/diagonal.htm for secondary sizing.

Note distance from diagonal to focal plane is semi-diameter of the tube plus focuser height plus allow 25mm back focus distance. This will affect secondary size along with how much edge ilumination (magnitude drop) you are prepared to accept and that depends on your observing preferences.

I.e. If you are variable star observer you will be using a low power eyepiece for maximum field and you will want 100% illumination (zero magnitude drop) across the entire field stop diameter of the eyepiece you intend on using. This results in a larger secondary.

If you are a planetary observer you can afford to have a small fully illuminated field resulting in a smaller secondary.

For general purpose observing aim for a magnitude drop of about 0.4, I think this equates to about 70% illumination at the edge of field. (Mel used to talk in % illumination but switched to magnitude drop a few years ago; I'm more used to % illumination).

Anyway I digress here but as you can see one can customise and tweak a Newtonian telescope in the early design stage depending on what interests you.

As for secondary mirror suppliers, try http://www.antaresoptics.com/SEMirrors.php

Buy the highest Wave PV quality you can afford as any difference will show itself under the best seeing conditions. However any of the mirrors offered are first class.

Note don't undersize your secondary too much as you will have a lip around the edge if using a proper holder (if glued you have no lip but I don't like that method of mounting and don't recommend it). Also the edge of a secondary may or may not be as flat as the centre so any lip will cover that.

You will hear advice on reducing the secondary for better contrast and yes this is true but often over stated. Anything under 25% (secondary to primary diameter) is fine, around 20% better and preferred and anything less you really start to lose too much edge illumination. (If planetary only observer on axis it doesn't matter so much).

If you change from f6 to f7 you may or may not be able to reduce the secondary. (I wouldn't bother). Plug in the numbers into Mel's calculator and see for yourself. Use inches as it's easier and multiply by 25.4 to get mm later when needed. (Sec mirrors are sold in inches).
For f6 change first three lines to 8, 48, 7.5 and last line to 0.6 and update.
For f7 change first three lines to 8, 56, 7.5 and the last to 0.6 and update.

If you leave the last at 0.4 it won't offer you the smaller (planetary only) diagonal as an option. The 7.5 is half tube widith plus focuser height (fully racked in) plus one inch back focus. (Change to suit what equipment you have).
The 1.4" is the field stop diameter of the eyepiece you are designing for. This is the only figure you need to multiply by 25.4 and match up against field stop data of an eyepiece. 1.4" equates to approx 35mm and this would fit say a 35mm Panoptic or 21mm Ethos. If using an eyepiece with such a field stop you decide then how much magnitude drop you are willing to accept. (Note I use Tele Vue data because it is readily available; and they are fine eyepieces).
The scale at the bottom of the graph on Mel's page is the field stop radius left and right of centre.

See http://www.televue.com/engine/TV3b_page.asp?id=214 for field stop data.

If you're happy with say a one inch (25.4mm) fully illuminated field then change the 1.4 to 1.

Note also a faster focal ratio has a shallower fall off and a slower focal ratio has a steeper fall off of light.

If you do decide to change your secondary mirror you will more than likely need a new holder and maybe even a spider (or drill the hole out if the new holder has a larger diameter shaft). Note too your existing mirror is most likely a metric size if out of China. These are not simply the imperial size x 25.4. I.e. If you divide the metric mirror size by 25.4 you won't get the imperial equivalent. If this is the case go the larger especially if getting a new holder with a lip. Last thing you want is a very small fully illuminated field.

Anyway lots there for you, hope it's not too confusing.
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Old 29-01-2020, 09:06 PM
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Outcast (Carlton)
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I'm just curious, what objective test have you used to determine that your collapsible tube dobsonian has inferior optics?

What exactly is it that you are hoping to gain by switching to a 'premium' set of optics for this scope?

You may wish to read this...or not; at the end of the day, it's your money (assuming you are purchasing the primary, secondary & will spend money somehow extending the length) but, I suspect you will likely not actually observe any discernible change in the views...


Unless of course, you have objectively measured that your primary just isn't up to scratch, in which case, I would be returning it from whence it came & seeking a refund or replacement... you might just be surprised how good those Chinese 'mass produced' optics actually are (and some of them are actually exceptional)..

My 2 cents...
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Old 03-02-2020, 02:18 PM
Dober (Steve)
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Location: Delaney's Creek Qld Australia
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Changing primary mirror from f6 to f7, any issues

Firstly thanks for all for their comments.


very detailed explanation of the relationship between the primary and secondary mirrors and there application in terms of viewing various objects. So much to consider. Thank you for the links also.

I have abandoned the idea of changing primary mirror due to previous comments received and given the implications for other existing components and tube length, a bit to much trouble for me at present given my lack of experience at his time. It sounds like for that mirror I might as well build a scope from the ground up, something for another day. Your comments will go in my file for future reference. Thanks for making the effort and taking the time to respond.


thanks for your inquiry also. I have to say I haven't conducted any objective tests and for the most part I'm Ok with my scope and think it works well but I am slowly making adjustment in an effort to get the best out of it. The mirror I was thinking about was a Zambuto and your article speaks highly of them.

The link you provided sort of backed up my thoughts and the reason I asked about secondary mirrors and their effect on a premium primary. I surmised that the potential of a top of the line primary may be diminished by a lessor secondary or for that matter a budget eyepiece and maybe the viewers own eyeball. That's with all things being equal in terms of seeing, colimation etc.

The link also touched on an advantage of a premium mirror that also rang true for me and that is they provide better contrast, something that a lot of people talk about with eyepieces and scope improvements. I have that basic understanding and last year flocked my entire scope, blackened screw heads and focuser tube to help with contrast.

After Carlton's inquiry I thought I would explain how my original question came about.

I toyed with getting a bigger scope than read somewhere that with premium optics one may get a view compared to a scope with a bit more aperture due to the quality of a primary given they provided better contrast. I had a thought (misguided maybe) that say a 12" - 14" would cost x$ but present other issues around transport, coma etc. It maybe a stretch to think an 8" with good glass would get the view of a - 12"- 14" but might be good enough of an improvement to justify the cost and still have ease of transport.

After all a 21 Ethos costs $1,349, 31 Nagler $1,050 which from what I understand many have invested in. Both of those by themselves cost more than my 8" Go-To. I don't have either of those but I have purchased other less expensive Tele Vues in hope getting the best out of my scope and viewing experience. It all helps and investing in good glass I think will pay off.

The way I am looking at it is with the $'s you can always get a better scope but while I am starting out I don't mind making some adjustments or strategic purchases on transferable accessories or fit out to get a better or deeper view.

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