Astrophotography scope upgrade?

[twelly27]

Hi all

I'm looking to upgrade my scope to one that is designed and built for astrophotography, a few people on twitter have recommended to skywatcher ED80 would this be a good choice? I will be using a HEQ5 pro mount aswell, is there and other quality scopes like that that are around 6/7 inch? Or doesn't it matter that the ED80 is on 3inch because of how well built the optics are etc?

Thanks

Chris

[drummermandan02]

Hi Chris.

I haven't got much to help you with this decision, there are many other guru's on here that will be able to help. But, I have a HEQ5 mount and a ED80. I have only used it a little, and haven't managed any DSO photography - I'm still trying to get to grips with polar aligning the mount. But maybe this weekend is the time! But the scope is great. It seems highly recommended for astrophotography, and I believe adding a focal reducer helps even further.

[haitch]

If you are moving to astrophotography the issues, whilst similar in some ways, differ significantly from visual astronomy.

I am only a beginner in the dark side myself so I'm sure I will get corrected in some of this (and maybe learn a few things myself in the process).

Aperture is not the be all and end all. What is more important is the ratio of the focal length of the scope to the aperture. This drives how fast your sensor will capture photons. So aperture is important relative to the focal length - too long a ratio and you end up needing very long exposures which are very demanding on the mount.

Focal length gives you magnification i.e. image scale so the longer the focal length the more small objects will fill your sensor but large objects may not fit in your field of view. It's more important to choose the focal length dependant on what you want to image also taking into account the size of the sensor - a smaller sensor is the equivalent of putting a short focal length ep in a scope. Once that is decided you need to get a reasonable focal ratio so that is when you need a bigger aperture to obtain it with a longer focal length scope.

The SW ED80 isn't particularly fast but the other side of that is that it's easier to produce good quality colour free optics at slower ratios (SW also use surprisingly good quality glass) so you do get good optics and you can buy a field flattener/reducer that effectively divides the f ratio by 0.8 or 0.85 can't remember which improves the f ratio and improves the shape of stars in the outer part of the frame.

The ED80 is also fairly light and will fit very nicely on the HEQ5 with all the attachments you'll want to add on.

[haitch]

PS - I owned one for a while but never used it for photography. Very nice optics but I sold it as I wanted a grab and go scope with a retractable dew shield.

If you are looking at SW scopes in particular (they do offer good vfm) They do an ED100 & 120 too but, of course, the price goes up. They also do faster ones called Equinox and if you've got very deep pockets they do some well though of triplets up to around 6" (Esprit I think the range is called).

It just depends how deep your pockets are.

[ronin]

The ED80 is often mentioned for AP however be aware it is still a doublet and there is an amount of CA. May be small and insignificant to the eye but not sure for AP.

Someone here was posting about having to wait for first light for what would be a good AP scope and that was a TMP92SS.

Another option would be one of the newer WO GTF scopes, there is the 81 and the 102, you would have to order either from the US or from the WO site direct so add say 30% on for the assorted charges.

At larger apertures you are looking at Mak newtonians or RC's. Think TS do a GSO RC for AP - someone on SGL has one so a post with a suitable title may get a response.

There manufacturers like APM in Germany that make excellent refractors which would be suitable for AP.

TS do a good selection of triplets but not sure how they perform.

Refractors will need a flattener - except the WO GTF's as they have one built in - or a coma corrector if a reflector..

Hope no-one minds but a PM to Keiren (Mod) may be useful - he had a GT-81 and flattener for sale a few weeks back which he may or may not have around still. Likely dropped off or well down the list now. Equally he could be on his honeymoon or something.

[haitch]

A GT81 went up for sale on AstroB&S earlier - £650 compared to the ed80 going 2nd hand for around £200 to £250.

It is a doublet but with an FPL53 Schott glass element and at f7.5 colour doesn't seem to be an issue.

It has been said to me (although I don't have enough experience to verify) you are better of with a good doublet than a cheap triplet.

Oh and CA isn't an issue with narrow band either.

You can go the newt route but I think there is something purer about refractor imaging (and the lack of hassle achieving adequate collimation and heavy duty mounting requirement - though not so much as a big frac!).

I reckon the ED80 is a great scope for a first try at imaging. Buy one 2nd hand and you are unlikely to lose anything on it if you get the bug and decide to sell it on to upgrade.

I'm only a dabbler at imaging so if you want hardcore opinion on £10k mounts and £5k scopes ignore me