StellaLyra 6" F/4 or SW 150PDS F/5?

[Grant93]

Hello everyone,

I'll try to keep it short and sweet, often get carried away and write essays when trying to ask a question. Just trying to decide on which one between these two to purchase, as I understand these are about the biggest scopes possible for imaging on a HEQ5, I hear some people get away with an 8" imaging on a HEQ5, but I heard its not recommended, as by the time the camera is attatched with guidescope and guidecam, you would be very close to hitting the 11kg max payload for imaging, and by how I understand it, its recommended against. If most people think this is a non issue, let me know, because I could be tempted to get the 8" versions instead.

Anyhow I'm looking at one of these as I want a bit more reach, both for DSO imaging and moon/planetry. Would both these scopes be able to handle planetry with my ASI224mc + a 3x barlow?

Conclusion - StellaLyra for that little less reach of 600mm, but the faster optics of F/4. Or the 150PDS for that extra 150mm of reach, and slower (but still nice and fast) optics of F/5.

If theres anything else I need to think about, let me know Also let me know if you would recommend any other scopes

Cheers

Grant

[vlaiv]

I'd go with 150PDS for number of reasons.

It will be better for planetary with somewhat smaller secondary obstruction. x3 barlow and ASI224 will be excellent match with that scope.

(ideally, if you want best planetary performance - I'd look at 6" F/8 newtonian for that role with x2 barlow - or getting a larger aperture scope - but that is probably not your primary interest).

F/4 is very hard to collimate - and if instrument is not well tuned - it is hard to keep collimation. More than once I've heard that people need to mod their scopes in order to make them more rigid - less mirror shift and that sort of thing.

It will need more expensive coma corrector to get good definition on APS-C sized cameras you seem to have in your signature.

I have a feeling that 150PDS will be much more hassle free for intended purposes and will give you better results (F/4 can perform better on DSO but it will require much more skill, investment and care when handling in order to do so).

[Grant93]

8 minutes ago, vlaiv said:

I'd go with 150PDS for number of reasons.

It will be better for planetary with somewhat smaller secondary obstruction. x3 barlow and ASI224 will be excellent match with that scope.

(ideally, if you want best planetary performance - I'd look at 6" F/8 newtonian for that role with x2 barlow - or getting a larger aperture scope - but that is probably not your primary interest).

Yes DSO is the primary interest, Lunar/Planetary for them short June/July nights

Thank you for the answer, makes it a lot easier to make a decision

Grant

[Zermelo]

Co-incidentally ...

 

[alacant]

1 hour ago, Grant93 said:

StellaLyra for that little less reach

Hi

F4 telescopes used to have a bad press but even bottom of the range telescopes such as the ones you cite  these days are easily brought up to imaging standard.

The main problem with the re-badged GSO f4 is not the difficulty in collimation, nor the larger central obstruction, rather the silly focal plane position which is further from the secondary than the primary to secondary distance. This not only makes balance almost impossible, but leads to tilt which no amount of fiddling will solve.

Hence, reluctantly, of the two, we have to recommend the f5. The GSO f5, which has a far more reasonable focal plane around 7cm from the mouth of the focuser, even better.

Just our hands on experience, but HTH anyway.

Cheers

 

Edited March 23, 2022 by alacant

[vlaiv]

4 minutes ago, alacant said:

rather the silly focal plane position which is further from the secondary than the primary to secondary distance.

I find this hard to believe - this would mean that scope is effectively stopped down to below 140mm as secondary minor axis is only 70mm.

Simple similarity of triangles shows that for single point of 100% illumination if secondary is placed half way between primary and focal plane - it must have half of diameter of primary.

Secondary mirror with minor axis of 75mm sitting half way between primary and focal point would produce single point of 100% illumination (everything else would be vignetted).

[alacant]

33 minutes ago, vlaiv said:

hard to believe

Maybe. According to this guy, the primary to secondary distance is estimated to be 330mm. We guesstimated less.

Whatever it is, it was far too far out from the tube to reliably mount even a lightweight dslr, hence our recommendation.

 

 

 

[Grant93]

59 minutes ago, alacant said:

Hence, reluctantly, of the two, we have to recommend the f5. The GSO f5, which has a far more reasonable focal plane around 7cm from the mouth of the focuser, even better.

Is that recommendation the same as the SW150PDS F/5?

Cheers

[Grant93]

1 hour ago, Zermelo said:

Co-incidentally ...

 

Haha yes I saw that too! Also I think there is a returned coma corrector I assume buying from a shop like FLO I'm not gonna get bitten in the ass by buying a return, I assume they check them rigorously when they come back.

Edited March 23, 2022 by Grant93

[alacant]

We'd choose the gso over the SW. TS and omegon resell them. Better focuser, build quality...

But remember that at this end of the market, both would need -albeit simple- modifications  to bring them up to reliable imaging standard. 

HTH

[Grant93]

1 minute ago, alacant said:

We'd choose the gso over the SW. TS and omegon resell them. Better focuser, build quality...

But remember that at this end of the market, both would need -albeit simple- modifications  to bring them up to reliable imaging standard. 

HTH

https://www.firstlightoptics.com/telescopes-in-stock/ts-photon-6-f5-advanced-newtonian-telescope-with-metal-tube.html This I assume?

Looks more fancy too

Cheers

[vlaiv]

4 minutes ago, alacant said:

Maybe. According to this guy, the primary to secondary distance is estimated to be 330mm. We guesstimated less.

Whatever it is, it was far too far out from the tube to reliably mount even a lightweight dslr, hence our recommendation.

 

 

 

You seem to be right in your estimation.

I was intrigued by this question and did some more research - and I came to two very different conclusions.

1. Based on tube:

Looking at this image and other images online - focuser seems to sit roughly at 1/3 from the beginning of the tube. Tube is 530mm long, so that would make distance from focuser to back side roughly 353mm. Given that primary is at least 15-20mm thick and that there is cell - it must be inside tube at least 40-50mm

That still leaves primary to secondary distance of about 300mm - or about 1/2 of focal length.

2. Based on published data.

TS quotes backfocus of about 98mm.

Focuser is listed at about 75mm height, and tube thickness at 178.

This would make secondary to focal plane distance of about 98 + 75 + 178/2 = 262mm

Still quite a bit of room for vignetting, although 70mm is enough not to stop down scope if there is 262mm between secondary and focus position.

[alacant]

32 minutes ago, Grant93 said:

more fancy too

Yes. Apart from the 63mm secondary, that's it. Check to see if that is a misprint. That's the one fitted to the 8" f5 and even there it's oversized. Well, unless you're 36x24 full frame I suppose.

HTH

[Grant93]

3 minutes ago, alacant said:

Yes. Apart from the 63mm secondary, that's it. Check to see if that is a misprint. That's the one fitted to the 8" f5 and even there it's oversized. Well, unless you're 36x24 full frame I suppose.

HTH

Would that have any negative effects even if it is oversized?

[alacant]

On 23/03/2022 at 17:31, vlaiv said:

focuser seems to sit roughly at 1/3 from the beginning of the tube

Yes, that's it. That gives rise to another problem in that being so far from the tube end strengthening collar, tightening the secondary spider causes the tube to distort.

The only route to a sensible retail 6" f4 seems to be the €1000 TS UNC or an eyewateringly priced made-to-your-specification ONTC. On the former, note the reasonable positioning of the focuser, still however with an enormous 70mm secondary.

But hey, tell us otherwise.

Edited March 30, 2022 by alacant

[alacant]

14 minutes ago, Grant93 said:

negative effects

Lower contrast and unecessary loss of light.

I think it's an error. The 6" gso normally ships with a 50mm secondary.

Cheers

Edited March 23, 2022 by alacant

[Clarkey]

FWIW I have the GSO (TS) F4 150mm and I have had to do quite a bit to get it up to imaging standard. Optically it is OK but tricky to collimate. The main negatives were the focuser was simply not good enough and also the cost of the F4 coma corrector. I tried the Baader MPCC as I already had one but this gave poor stars at the edges of the 1600mm pro. Additionally I have had major problems with reflections from the filters. I will try it with a DSLR to see if it is better, but it may not be. The other issue is trying to get the balance right - it took a lot of adjustment to get it right.

If I was to go down this route again I would buy the F5 or something completely different. (I did - I got a Stellamira 90mm ED triplet).

[alacant]

12 minutes ago, Clarkey said:

tricky to collimate

We found that the springs needed upgrading and primary mirror lateral movement fixed as a very minimum.

Ours too went back from whence it came.

[malftobe]

4 hours ago, alacant said:

We'd choose the gso over the SW. TS and omegon resell them. Better focuser, build quality...

But remember that at this end of the market, both would need -albeit simple- modifications  to bring them up to reliable imaging standard. 

HTH

@alacant Do you happen to know if the focus tube would need cutting on the GSO version when adding a coma corrector?

[alacant]

36 minutes ago, malftobe said:

tube would need cutting

Hi

With a non reducing cc, no. There are however other modifications which are necessary/recommended.

[Grant93]

2 hours ago, Clarkey said:

If I was to go down this route again I would buy the F5 or something completely different. (I did - I got a Stellamira 90mm ED triplet).

I have decided on the f5.. hopefully I don't come across too many problems. 

1 hour ago, alacant said:

Hi

With a non reducing cc, no. There are however other modifications which are necessary/recommended.

What modifications would you recommend?

[alacant]

On 24/03/2022 at 00:07, Grant93 said:

modifications would you recommend

• Spread the tube rings further apart on a ~40cm Losmandy plate. Turn the rings 180º so you can use readily available m6 bolts.
• Tie the top of the rings with a similar length of rigid aluminium box section.
• Mount the guide telescope to the latter.
• 1º mirror. Seal to the cell (to prevent lateral movement) using generous blobs of neutral silicone sealant to coincide with GSO's cork. Leave 24 hours on a level surface before refitting...
• 6 x 1.4mm wire springs. Three replacements and three passive over the locking screws. Leave the latter loose. 
• Do not replace the mirror clips unless you are shipping.
• Replace the solid metal secondary support with a lightweight plastic equivalent.
• I think the 2 speed gso focuser is free of flex and rubber 'O' rings. If you go for the sw, you'll need to address that.
• A black shower cap.

Conclusion. An astrograph worthy of the name which will hold collimation at all tube angles. 

Here is an example of a modified gso 203 f5. Without, we would not be able to use a separate guide telescope.

HTH

 

Edited March 25, 2022 by alacant

[900SL]

On 23/03/2022 at 19:20, alacant said:

Yes, that's it. That gives rise to another problem in that being so far from the tube end strengthening collar, tightening the secondary spider causes the tube to distort.

The only route to a sensible retail 6" f4 seems to be the €1000 TS UNC or an eyewateringly priced made-to-your-specification ONTC. On the former, note the reasonable positioning of the focuser, still however with an enormous 70mm secondary.

But hey, tell us otherwise.

I'm thinking of getting a second hand UNC 150 f6 900mm fl  for lunar/planetary/galaxy imaging. Would  I need a coma corrector with small sensor cameras like an ASI 178 or 533?

Edit: coma corrector required outside of center field https://www.cloudynights.com/topic/761192-coma-at-f6-with-newtonian-for-lunar/

Edited February 25 by 900SL

[Stu Wilson]

On 24/03/2022 at 11:08, alacant said:

• Spread the tube rings further apart on a ~40cm Losmandy plate. Turn the rings 180º so you can use readily available m6 bolts.
• Tie the top of the rings with a similar length of rigid aluminium box section.
• Mount the guide telescope to the latter.
• 1º mirror. Seal to the cell (to prevent lateral movement) using generous blobs of neutral silicone sealant to coincide with GSO's cork. Leave 24 hours on a level surface before refitting...
• 6 x 1.4mm wire springs. Three replacements and three passive over the locking screws. Leave the latter loose. 
• Do not replace the mirror clips unless you are shipping.
• Replace the solid metal secondary support with a lightweight plastic equivalent.
• I think the 2 speed gso focuser is free of flex and rubber 'O' rings. If you go for the sw, you'll need to address that.
• A black shower cap.

Conclusion. An astrograph worthy of the name which will hold collimation at all tube angles. 

Here is an example of a modified gso 203 f5. Without, we would not be able to use a separate guide telescope.

HTH

 

Looks very similar to my f/4 TS Optics which Alicante guided me through a couple of years ago lol.

If your at ease with collimation go for f/4 with all these mods including new springs etc, if not go for f/5 but youll still have to collimate alot.

HTH

[Rallemikken]

On 23/03/2022 at 15:19, Grant93 said:

If most people think this is a non issue, let me know, because I could be tempted to get the 8" versions instead.

Soon ending my third full season with a HEQ5 and a 200PDS. Finderscope with guidecamera and DSLr's. Among them a hefty 5D MkII. No problems, as long as you shield it from the wind. And the tripod stands firm. Best is a fixed pillar. I have a StellaLyra 6" f/4 with the 0.9 cc as a supplement for those wide angles. Use the 200PDS most. You need this focal lenght for galaxies and small stuff.