6" f4

[alacant]

Hi everyone. Having had hands on with one of these, I'm addicted. Looking at the options they seem to be all the same: ts, gso and this one. Mine too was the same except it had no manufacturer marking and the owner bought it second hand so I'm non the wiser. What I didn't like was the spider connection which distorted the tube as you tightened the vanes; the reinforcement at the top of the tube which prevents this on other designs is too far away from the spider thumbscrews. One other quibble was the the focus position was almost at the end of the focus racked-out position even on my dslr again begging the question on the secondary position.

Does anyone know of a better design where the spider isn't buried so deep into the tube?

Cheers and clear skies

[vlaiv]

Well, one of design optimizations of Newtonian is to place secondary deeper in tube - this is done to minimize stray light hitting the back side of tube - when viewed thru focuser tube, secondary usually does not block full view of main tube, and one should flock that area to prevent reflections, and also longer tube prevents stray light reaching this area in the first place.

I've read somewhere that spider/secondary should be at least x1.5 diameter of OTA down the OTA tube.

Focus position on the other hand depends on size of secondary (it is in relation to primary - secondary distance as well and F/ratio). One wants small secondary with "shallow" focus position - for planetary observation, and big secondary with enough backfocus for photographic applications - field illumination depends on this, and one needs enough backfocus to accommodate for all accessories like coma corrector, OAG, filter wheel, ....

Now, if I understand correctly you have your eyes set on getting one of those? I guess probably best option, depending on budget, would be to go for customized version. You can contact vendors and see if they are willing to tweak design to your specs (and that would depend on your exact needs - what the scope will be for). I know TS does have option for fully customized newtonian (secondary size / position, choice of mirror quality, and extras). Not sure if Orion Optics would be willing to do the same, but I believe you can already specify precision of mirror when you purchase scope.

HTH

[alacant]

2 hours ago, vlaiv said:

customized version

Hi vlaiv and thanks for your reply. Some good pointers.

What I find strange is that I see only one design available off the shelf. I think I could do the stray light with a dew shield. More concerning is the lack of reinforcement for the secondary spider; tightening the adjusters make the tube walls collapse. The example I tried was of flimsy construction akin to the old blue tube telescopes. Collimation changed randomly. What was OK in one position, changed with tube orientation. I'm almost certain this was due to the spider moving under the -heavy- secondary+support.

Unfortunately custom is beyond budget, but I'm not averse to making modifications; this post is helped by the recent price reduction;)

Quote

what the scope will be for

DSOs. When you've used one of these, you realise just how dim your 130pds is!

[vlaiv]

Well, if you are aiming for DSO, and again depending on budget, is SW quattro 8" F/4 out of the question?

Standard version is cheaper but heavier with ~10kg, while carbon fiber tube is more expensive but weights ~ 8.5Kg.

On the other hand, you say tube you tried was blue with no markings, so I suspect that it might have been ATM or something, so tube material is questionable. Don't think that any of scopes you mentioned (TS, GSO ...) would have such problem.

There is however DIY solution to it that might help. I would add stiff ring with holes in right places to reinforce that part of OTA (similar to iron rings used on barrels) to prevent it from bending.

As for question of availability, well, small F/4 newtonians (anything not dob mounted) are only good for photographic applications due to coma, size of secondary, harshness on eyepieces and critical need for precise collimation. So there was simply no market for such instrument being mass produced (well there were couple of larger models like 8" quattro) - but you could certainly get it custom made. Now with raising popularity of astro photography and EAA, they are starting to be interesting / attractive, hence GSO and TS versions.

 

[alacant]

48 minutes ago, vlaiv said:

is SW quattro 8" F/4 out of the question?

I have the ES 208 f3.9. Just looking for something to replace my 130pds; faster and wider...

51 minutes ago, vlaiv said:

tube you tried was blue with no markings, so I suspect that it might have been ATM or something, so tube material is questionable. Don't think that any of scopes you mentioned (TS, GSO ...) would have such problem.

No. I likened it to the old blue skywatcher tubes. The 150 f4 OTA I tried was white and looked identical to the ts and gso, just unbranded.

56 minutes ago, vlaiv said:

I would add stiff ring with holes in right places to reinforce that part of OTA (similar to iron rings used on barrels) to prevent it from bending.

Yeah. I think that's the only realistically economical way forward. I don't understand how designs such as these make it to the market.  Maybe that's why they're not pouplar?

Cheer and thanks again vlaiv for your time.

[Adam J]

4 hours ago, alacant said:

I have the ES 208 f3.9. Just looking for something to replace my 130pds; faster and wider...

No. I likened it to the old blue skywatcher tubes. The 150 f4 OTA I tried was white and looked identical to the ts and gso, just unbranded.

Yeah. I think that's the only realistically economical way forward. I don't understand how designs such as these make it to the market.  Maybe that's why they're not pouplar?

Cheer and thanks again vlaiv for your time.

There are some great f4 150mm newtonians about, you just have to pay for them.

[astrosathya]

I purchased a GSO 6" F4 Newtonian about ten days ago and I've had first light with it. Fine telescope for the price we pay. One can never expect premium stuff for a cheap price. But they are definetly worth the money you pay.

[alacant]

4 hours ago, Adam J said:

some great f4 150mm newtonians about

I've only been able to find one type:(

[alacant]

54 minutes ago, astrosathya said:

GSO 6" F4 Newtonian about ten days ago

Hi. That's good news. If I may, a couple of questions?

How do you find the secondary spider attachment? Does it distort the walls of the tube when tightened?

Where is the focus position with your dslr? A rough estimate of where the draw tube is relative to the focuser would be fine.

TIA and really glad you're enjoying your f4. Hope to join the club soon:)

[astrosathya]

4 minutes ago, alacant said:

Hi. That's good news. If I may, a couple of questions?

How do you find the secondary spider attachment? Does it distort the walls of the tube when tightened?

Where is the focus position with your dslr? A rough estimate of where the draw tube is relative to the focuser would be fine.

TIA and really glad you're enjoying your f4. Hope to join the club soon:)

Hi, I do not tighten the spider at all. It came sufficiently tightened out of the box. The only thing I do is use the three Philip screws for collimation. I do not use a DSLR, though I have a self modded Canon 500D, I haven't tested it yet. I use a QHY9M with filter wheel. Since My QHY9M is at 40mm line on the focuser, and it sticks out about as much, My guess is that to have a 20mm mark on the focuser with a DSLR, you'll need to use the supplied 35mm extension tube. I am using the GSO Coma Corrector. 

[wimvb]

Interesting read.

Custom ts scopes (ontc) are in a completely difderent ballpark, I think.

All three scopes you linked to, are basically the same gso, just rebranded. Ts even write this.

I wouldn't worry too much about the spider vanes distorting the tube. This can be fixed if you're willing to tinker some, as noted previously. But shifting collimation would be a deal breaker for me. What's the use of a fast scope that can't keep collimation? If you have the possibility to test the scope you have been using, try to find out how much of an issue this is.

Btw, burrying the secondary so deep into the tube, is just one way of adding a dew shield and balancing the scope. It's the focal length, desired back focus, and illuminated area that determine where best to put the secondary in relation to the primary.

[alacant]

4 hours ago, astrosathya said:

My guess is that to have a 20mm mark on the focuser with a DSLR, you'll need to use the supplied 35mm extension tube

Yeah, thanks. That confirms what I found; hanging so far from the tube, it's not workable for dslrs:(

[alacant]

2 hours ago, wimvb said:

the focal length, desired back focus, and illuminated area that determine where best to put the secondary in relation to the primary

Hi. Thanks. Yeah, but in this case, the focal point seems to be over 12cm from the tube. The secondary is very close to the primary and seems much larger than necessary. I wonder what camera they had in mind? Why not a smaller secondary further from the primary which would give a more reasonable point of focus closer to the tube? Or had they already made 1000 units -hence the heavy price reduction- before they realised?! Dunno. It looks less likely the more I think about it:(

[vlaiv]

1 minute ago, alacant said:

I wonder what camera they had in mind? Why not a smaller secondary further from the primary which would give a more reasonable point of focus closer to the tube?

I guess one has to account for following scenario:

Coma corrector + OAG + filter wheel + Camera - that is quite a bit of optical path to accommodate. Also, small secondary will not provide enough field illumination.

[alacant]

9 minutes ago, vlaiv said:

Coma corrector + OAG + filter wheel + Camera

Yeah, I hadn't thought of that. So it does effectively eliminate dslr users. But hey, how is the tube gonna cope with all that lot hanging off anyway? Time for a rethink...

[alacant]

OK, this is with an eyepiece, but hey... Half of the light path happens after the light hits the secondary! That just doesn't seem workable....

[vlaiv]

Best way to cope with it is to turn tube so that focuser / DSLR is oriented down, perpendicular to counterweights shaft. That would be the best way to optimize weight distribution and balance everything out - opposite than in given picture where everything is pointing up and at an angle.

This is what I mean:

[wimvb]

3 hours ago, alacant said:

OK, this is with an eyepiece, but hey... Half of the light path happens after the light hits the secondary! That just doesn't seem workable....

Yes, but it's an astrograph = imaging scope.

Compare to this f/2.8

http://www.takahashi-europe.com/en/epsilon-180ED.php

[alacant]

1 minute ago, wimvb said:

Compare to this f/2.8

Exactly; where a dslr comes to focus at a reasonable distance a few mm from the tube. Rather than in a different postal district!

Not to worry. The dream of a €300 f4 was exactly that.

Cheers and clear skies. 

[vlaiv]

Hm, F/2.8 Epsilon is flat field instrument with built in field flattener and focal reducer (such is optical design).

Take F/4 newtonian instrument, add suitable coma corrector / focal reducer and observe how much back focus you have left after that.

For example:

https://www.teleskop-express.de/shop/product_info.php/info/p5768_TS-Optics---Boren-Simon-6--f-2-8-PowerNewton-Astrograph.html

Technical specs state 65mm backfocus - comparable to that of Epsilon 56mm.

Now with F/4 "bare bones" newtonian you have a bit more backfocus (depending on design) to accommodate appropriate coma corrector.

Take for example tech spec of x0.73 reducer / coma corrector:

"Necessary focus distance from the 2" receptacle of the focuser: 85 mm - that is fulfiled by almost all Newtonian telescopes with 2" connection"

 

[alacant]

 

47 minutes ago, vlaiv said:

with F/4 "bare bones" newtonian you have a bit more backfocus

Hi vlaiv. But it's not just a bit more. It's a lot more. So much so that a dslr will not reach focus even with the focuser racked out fully; an extension tube is needed. The turning force this introduces I feel sure contributes to the impossibility of retaining collimation as the tube angle changes. So yeah, you're right. A 6" f4 which works costs €silly:(

Cheers and thanks again for your input.

[Adam J]

10 hours ago, alacant said:

I've only been able to find one type:(

Look for the zwo one. 

[vlaiv]

35 minutes ago, alacant said:

 

Hi vlaiv. But it's not just a bit more. It's a lot more. So much so that a dslr will not reach focus even with the focuser racked out fully; an extension tube is needed. The turning force this introduces I feel sure contributes to the impossibility of retaining collimation as the tube angle changes. So yeah, you're right. A 6" f4 which works costs €silly:(

Cheers and thanks again for your input.

Well, TS site states for GSO version that back focus is 93mm - quite in line with what I would expect if they aim to accommodate variety of accessories.

And that is not such a big back focus. Take for example my RC 8" - back focus is stated as: 254mm from back side or 159mm from focuser 2" receptacle. Yep, one needs to be careful not to get too much tilt with everything in place (and 50mm extension tube on top of it all )

On one occasion I was shooting Jupiter with x2.7 barlow (that pushes focus point still further out). Now that looked a bit ridiculous, I had 80mm and 50mm extension tubes in place, then barlow lens, then 1.25" extension and camera

 

[alacant]

1 hour ago, vlaiv said:

back focus is 93mm

Hi. What does that mean? Can I therefore calculate the focus position for my DSLR? Let's say I have DSLR + cc. I need 55 mm from the cc to the sensor. Can we now calculate how far out the focuser needs to be at focus? 

Cheers.

[vlaiv]

2 minutes ago, alacant said:

Hi. What does that mean? Can I therefore calculate the focus position for my DSLR? Let's say I have DSLR + cc. I need 55 mm from the cc to the sensor. Can we now calculate how far out the focuser needs to be at focus? 

Cheers.

So, CC - Sensor is 55m, now we need mechanical and optical length of CC.

Both of these should be specified for CC, what CC is it?

Mechanical length is just simply distance from 2" focuser connection to appropriate CC thread on camera side (for most CCs this is just a few mm, because most CC bodies go inside focuser tube). Then there is optical length of CC, and this can range quite a bit, it will be more for CCs that are focal reducers as well as correctors.

But let's assume that total length is 10mm (mechanical and optical) so it would be 93 - 10 - 55 = 28mm - this means that you would need to rack out focuser 28mm.

Focuser on GSO scope is monorail 2" - which has 50mm travel, so you would not need any extension tube to reach focus position.

If you assume that mechanical and optical length is ~0, then you get 38mm focus position. This can still be done without extension tube, but I would probably add some sort of spacer, so that focuser tube is not racked out that much (best position for focuser is 1/3 - 1/2, I believe) - but this depends on how good the focuser is. I also have monorail on my 8" RC, and I did not have had any tilt problems with it. Only objection to this focuser is that it has no any sort of thread for threaded application to avoid tilt altogether (My TS80mm apo with 2.5" R&P has M63 thread, so I can screw in flatteners instead of inserting them in 2" - it should help a lot with tilt, still waiting for first light on that )