Advice needed on slowish fracs

[domstar]

So I want an upgrade to my budget Bresser 70/700. For space and practicality reasons I've narrowed it down to an Omegon 102/600  az-3 with a 660mm focal length or a Bresser Messier 102s/600 with a separate az-3 mount. Both are similar refractors.

My question is whether the extra 60mm focal length will make any difference at all. After finding the crab nebula on Thursday I realised I'd rather go for a scope like this than say a 90/900 and a 102/100 is out of the question due to my balcony, washing line and door. On the other hand I'd still like to look at doubles and planets. So I wanted to know if such a small increase in focal length would be kinder to higher magnification or if I wouldn't notice. Also I'd like to know if it would be a problem to use a 4mm eyepiece in those scopes in order to get higher mag on planets and doubles or if that would be a waste of time.

I realise that these scopes are probably nobody's idea of ideal but I have very specific issues which limit what I should buy.(Also, although I'm in awe of some of the photographers on this forum, I have no inclination to join their ranks)

Thanks

[John]

The additional aperture of the 102mm scopes will undoubtedly help a bit with deep sky objects such as the Crab Nebula although it's not a spectacular object even with my 300mm aperture scope. With a focal ratio of around F/6 the 102mm refractors (presumably achromats rather than ED doublets ?) will show a fair amount of chromatic abberration (CA) but should still show better views of the moon and planets than your 70mm does and you should get better splits of double stars.

Have you thought about something like the 127mm maksutov-cassegrain if space is tight on your balcony ?.

 

[spaceman_spiff]

Hi there,

Have a look at the astronomy.tools website to see what difference 60mm in focal length makes.

https://astronomy.tools/calculators/field_of_view/

You can add your own eyepieces in and compare the FoV of the two scopes. 

Also, I agree with the previous poster that more aperture and shorter FL are better for dim objects but you will see more chromatic aberration in a achromatic refractor telescope. 

Dan 

[domstar]

7 minutes ago, John said:

Have you thought about something like the 127mm maksutov-cassegrain if space is tight on your balcony ?.

Mmm tempting. Why are they always on EQ mounts usually with goto? Is it because it's hard to find anything with the long focal length?

Alas I think budget constraints will do for me.

[domstar]

5 minutes ago, spaceman_spiff said:

Have a look at the astronomy.tools website to see what difference 60mm in focal length makes.

Thanks for that. I was also wondering about CA. Would it be less in a very slightly slower scope? Anybody who admits to having it on these boards has said it's not a problem for them so I don't imagine I'm fussier than anyone here. Also, what about just using an eyepiece with fewer mm in a slowish frac? I mean a 4mm or 6mm. Is it just a case of ramping up to the mag you want? I guess it's not as simple as that. Is there a problem with 150x in those scopes given the aperture is  big enough to support it?(I don't wear glasses)

[Ricochet]

With respect to the extra 60mm focal length I only see that referenced in the specifications. The description (and name!) say 600mm so the 660 might just be a typo.

[andrew63]

I use a fast wide field refractor but i mainly view deep sky stuff - it is not great for planets etc. You ideally want a achro. at F10 and above for planets/lunar.  Given also your limitations of space that's why a Mak as suggested is a good alternative  - even something as small as a skymak 102 would be ok and quite easy to mount.  The 4" F6 is a nice scope, but not ideal for high magnification use.

 

andrew

[John]

1 hour ago, domstar said:

Mmm tempting. Why are they always on EQ mounts usually with goto? Is it because it's hard to find anything with the long focal length?

Alas I think budget constraints will do for me.

You can buy a 127mm mak-cassegrain as an optical tube only. The cost is around £280 new or I have seen them sell for around £150 on the used market.

[domstar]

2 hours ago, John said:

With a focal ratio of around F/6 the 102mm refractors (presumably achromats rather than ED doublets ?) will show a fair amount of chromatic abberration (CA) but should still show better views of the moon and planets than your 70mm does and you should get better splits of double stars.

Thanks. That is the big question for me. As for magnification, is there a practical limit due to lack of focal length or is it just a case of twice the aperture of any scope (depending on conditions)? (Achromat of course- I feel like I'm asking a group of F1 engineers how to get my Skoda to start in the mornings)

[John]

With faster achromat refractors (eg: F/7, F/6 F/5) high magnifications don't really seem to deliver really good results. As well as the CA, such scopes usually have a touch of spherical abberration as well, either over or (more usually) under corrected. They are simply not optomised for high magnification viewing. When used for their strengths, low to medium wide field observations, they are very effective though. Stick to 100x or less and you should have some good views.

 

 

[Alfian]

When you speak of your balcony and a 90/900 or 102/1000 I get the impression it must be quite a small balcony.  A useful "little" scope which punches well above its weight is the Heritage 130 reflector which stores away to next to nothing but needs a small but sturdy table or some similar (homemade) support to bring it up to a usable height. It can even be mounted on a tripod. A quick search on the forum will bring up plenty of references to it. The simple helical focuser is sometimes criticised (myself included) but with a little bit of tinkering it works surprisingly well. The Heritage will do many things reasonably well, moon, planets, brighter DSOs and is easy to use. It is also easy to collimate though it does to tend to keep its collimation well enough.

https://www.firstlightoptics.com/heritage/skywatcher-heritage-130p-flextube.html

That said,If moon, planets, doubles are definitely your thing, with no thoughts of wider field observation then I would definitely be looking at a small Mak like the Skywatcher 102. These are brilliant little (and I mean small) scopes and though small are surprisingly chunky,heavier and well made, more so than they might first appear. A 32 mm plossl EP gives about 40x and about 1.25 degrees fov.  Pushing the mag. up though will give lovely sharp detailed views of the moon with no CA and good views of Jupiter/Saturn within the limitation of its 102 aperture. At higher magnifications objects move out of view surprisingly quickly and with such a narrow fov, something with slomo controls is useful. A 102 Mak will work well on an EQ2 mount. and with the EP being situated so close to the top of the mount you are not breaking your back in terms of viewing position which is always a big help. You can pick an EQ2 up on Astroboot at the moment for £55.

https://www.firstlightoptics.com/maksutov/skywatcher-skymax-102-ota.html

As to a refractor, I have a TS optics 80mm f7.5 scope which sells ridiculously cheaply for what it is optically. A real jack of all trades that does most things acceptably well for its 80mm aperture, even the CA is pretty well controlled. It does needs a diagonal and a finder but even so its still good value. I have fitted a 2" focuser so my 28mm 2" EP gives me 3 degrees fov! At f7.5  it is easy on cheaper EPs too. I  use it on an AZ4 which  is nice but a bit of an overkill for this scope. It would sit easily on an AZ3 or Eq2 quite well.  Though only 80mm I will not be parting with this scope any time soon as its a great grab 'n go scope.

http://www.teleskop-express.de/shop/product_info.php/info/p7935_TS-Optics-Starscope-806-OTA---Wide-Field-Refractor-80-600mm---tube-with-optics.html

Regarding magnification, in my still relatively limited experience, the guideline of twice the aperture in  mm can be misleading.  For much of the time, in the UK, what seems to  limit magnification is not so much aperture as weather conditions and light/chemical pollution. ( ie seeing conditions) On the occasional night when it all comes together or you are visiting a dark site then its good to be able to push it. That said for Luna observation you can often push it quite a bit!

(PS: I'm no F1 engineer, I just used to tinker with old motorbikes.)

[domstar]

Thanks a lot everyone. What I really need now is less choice and someone to tell me to go for it.. 

 

[Alfian]

Ah, decisions decisions. Giving advice is the easy bit, making the decision and living with the outcome  is the hard bit! Inevitably it's all part of the learning curve. 

[mikeDnight]

I was using a 127 Maksutov to look at a few dso's a few nights ago. It gave great views! It's field of view is limited, but with a 32mm plossl both components of the glorious double cluster were in the field, with just a few stars around the periphery excluded. It's star images were sharp and the colours were true. It's a good scope for viewing binaries, and it packs a punch on the moon and planets. Being only 5" it cools rapidly, so by the time you're dark adapted the scope is working full out. 

It would be a good choice given your circumstances!

Mike 

[nightfisher]

On 1/31/2017 at 18:23, Alfian said:

Ah, decisions decisions. Giving advice is the easy bit, making the decision and living with the outcome  is the hard bit! Inevitably it's all part of the learning curve. 

Sometimes we take advice and make a purchase, then as time starts to drift by we start to question said purchase, learning curve!

@mikeDnight lot to be said for 127 Maksutov, Skywatcher are making a new version with 2" back but we have yet to see it

http://www.skywatcher.com/product/bk-mak127-otaw/

[domstar]

3 hours ago, mikeDnight said:

 

I was using a 127 Maksutov to look at a few dso's a few nights ago.

 

Just a couple of questions. Do you think it would work on an az mount or is it too much of a pain to keep things in the field of view? They are usually advertised with a goto or at least an eq. I'm a little bit scared of not being able to find what I want. Also, I've seen a vixen 110/1035 which has a perhaps more manageable focal length. Would the aperture compare favourably with a 102mm frac or would any obstructions mean that a 102mm frac would actually let in more light? 

Thanks for your help. If only there was a club anywhere near me.

[John]

3 hours ago, nightfisher said:

Sometimes we take advice and make a purchase, then as time starts to drift by we start to question said purchase, learning curve!

@mikeDnight lot to be said for 127 Maksutov, Skywatcher are making a new version with 2" back but we have yet to see it

http://www.skywatcher.com/product/bk-mak127-otaw/

It would be nice if they also increased the diameter of the optics so that the full working aperture of 127mm is achieved.

[domstar]

1 minute ago, John said:

It would be nice if they also increased the diameter of the optics so that the full working aperture of 127mm is achieved.

Hi John. What sort of percentage of the aperture gets obstructed by a mac?

[John]

9 minutes ago, domstar said:

Hi John. What sort of percentage of the aperture gets obstructed by a mac?

The Skywatcher 127mm seems to operate at around 120mm effective aperture.

[nightfisher]

20 hours ago, domstar said:

Just a couple of questions. Do you think it would work on an az mount or is it too much of a pain to keep things in the field of view? They are usually advertised with a goto or at least an eq. I'm a little bit scared of not being able to find what I want. Also, I've seen a vixen 110/1035 which has a perhaps more manageable focal length. Would the aperture compare favourably with a 102mm frac or would any obstructions mean that a 102mm frac would actually let in more light? 

Thanks for your help. If only there was a club anywhere near me.

I tried the VMC110 and it failed to impress me, it looked a nice scope but the views seemed to really lack contrast 

[Stu]

12 minutes ago, nightfisher said:

I tried the VMC110 and it failed to impress me, it looked a nice scope but the views seemed to really lack contrast 

I guess the VMC110 is more of an imaging scope, has quite a big central obstruction so the visual contrast is likely to be lower.